1 Introduction

Hydrochar is defined as a solid carbon product with rich oxygen-containing functional groups (OFGs) produced by the hydrothermal decomposition of biomass (e.g., residues and wastes of plants and animals) in a reactor with water as the solvent and at moderate temperatures (Parshetti et al. 2013; Sevilla and Fuertes 2009a). Hydrochar exhibits superior performance relative to raw biomass in terms of its mass and energy density, dehydration, and combustion performance. In recent years, hydrochar has drawn increasing attention for its economic and environmental benefits in various agricultural and environmental applications (Kambo and Dutta 2015; Khosravi et al. 2022; Wu et al. 2021a).

The term “hydrochar” was coined by Spanish researchers Marta Sevilla and Antonio B. Fuertes in 2009 to describe a hydrothermal carbonization sample used as an intermediate in synthesizing highly crystalline graphitic carbon nano coils (Sevilla and Fuertes 2009a). Further investigations have revealed that hydrochar is derived from dehydration condensation, polymerization, and aromatization reactions, including those of highly aromatic and reactive oxygen functional groups (Sevilla and Fuertes 2009b). The first documented research on hydrochar for environmental applications appeared in 2011; it showed that the hydrothermal carbonization of biomass increases its carbon turnover time; thus, this technique is a potential way to sequester atmospheric carbon dioxide (CO2) (Sevilla et al. 2011).

Hydrochar has recently attracted much attention in environmental applications because of its potential benefits and the scarcity of natural resources. The number of publications and review articles on the environmental applications of hydrochar has risen rapidly since 2011. For example, Goel et al. (2021) reviewed hydrochar adsorption to capture CO2. Luutu et al. (2021) used mate analysis to review the effects of hydrochar on plant growth when utilized as a soil conditioner. Lan et al. (2021) reviewed the progress of the application of hydrochar as a product to improve and remediate the soil environment. Zhang et al. (2020b) reviewed the application of hydrochar as an adsorbent in water environment remediation. Even though most of these reviews looked at hydrochar in various environmental applications, only a few included bibliometric analyses.

It is worth noting that the review studies in the above examples are mainly reviews of a specific area of hydrochar, which are usually based on the author’s specific research area and background and are highly subjective. On the other hand, bibliometrics is based on literature and uses quantitative methods to analyze and process literature characteristics to obtain the necessary data, find out the patterns of change through data analysis, and predict future trends. Compared with traditional review methods, bibliometrics has significant macroscopic research advantages of objectivity, quantification, and modeling (Li et al. 2017). Bibliometric methodologies are knowledge-mapping tools for quantitative analyses of the structure, characteristics, and linkages within a research field. CiteSpace, the primary visualization tool for bibliometrics, is based on “co-occurrence clustering”. The literature is analyzed first by extracting information units (including references at the literature level, keywords at the topic level, authors, institutions, countries, and journals) and reconstructing them based on the type and strength of links between them to generate a network structure with different meanings (e.g., keyword co-occurrence, author collaboration, literature co-citation). Network nodes represent literature information units, whereas links represent connections between nodes (co-occurrences). Finally, through measurement, statistical analysis, and visualization of the network structure, implicit patterns and laws are discovered about the knowledge structure of specific disciplines and fields (Chen 2006). Bibliometric analysis can be presented straightforwardly and comprehensively with easy-to-understand graphical formats and precise representations of relationships between sources through software such as CiteSpace. In this way, researchers can uncover topics likely to be the focus of future research. For example, Wu et al. (2019, 2020, 2021b) conducted a bibliometric analysis of biochar research using CiteSpace to reveal its history, current status, and future trends. Based on this, the goals of this study are to explore recent popular research topics and trends in the environmental applications of hydrochar using CiteSpace and to anticipate future trends by conducting visual scientometric analyses.

The objectives of this study are to perform (1) a systematic assessment of the level of scientific development of environmental applications of hydrochar, (2) an analysis of the present state of environmental applications of hydrochar, and (3) an assessment of potential shortcomings of these applications, which will aid scholars in anticipating future research directions in this field. This study provided a concise scientometric analysis of the environmental applications of hydrochar and provided insight into the field’s developmental history and possible future trends of hydrochar applications in different sectors.

2 Data sources and research methods

2.1 Data collection and processing

Data collection for this study was based on the Web of Science (WoS) core collection database. The search formula [TI = (“hydrochar” or “hydro-char”) or TI = (“biochar” and “hydrothermal carbonization”) or TI = (“biochar” and “hydrothermal liquefaction”)] were used in the data collection process with a search period from January 1, 2011 through December 31, 2021. The retrieved literature types, subject categories, journals, highly cited literature, countries, institutions, authors, and keywords were analyzed statistically. CiteSpace was used for data visualization and analysis, and synonyms were created based on the collected data.

2.2 Scientometrics analysis methods

The JAVA-based quantitative bibliometric visualization tool CiteSpace (version: 6.1. R3), developed by Prof. Chaomei Chen, was used for analyzing the data in this study, and the results were highly objective (Chen 2006). CiteSpace provided the authors, co-keyword, and cluster analysis functions for understanding the relationships between authors and keywords and identifying emerging trends, hot spots, and gaps in hydrochar research. The size of the node in the network mapping graph indicated the frequency or number of occurrences of each entry (e.g., keyword or author). When there were two connected nodes, a correlation was found between them.

3 Results

3.1 Characteristics of publication type and outputs

A total of 2403 papers on hydrochar were included in the WoS core collection from January 1, 2011 to December 31, 2021. The main types of literature were papers, conference proceedings, review papers, online publications, conference abstracts, and letters. Among these types, the proportion of dissertation articles was the largest. An analysis of the wide range of the composition of publication types indicated the importance of hydrochar research. Generally, the number of publications suggests the level of attention given to a field by scientists and, in some ways, it reflects the pace and course of development of the field. Thus, the number of publications in hydrochar research is a good indicator of the growth trend. Hydrochar publications were expected to continue to receive academic attention for some time, as shown in Fig. 1, based on the increasing number of annual hydrochar publications and their cumulative numbers. A literature analysis provided evidence that researchers were increasingly paying attention to hydrochar research due to its importance in producing bioenergy, fertilizer, and waste biomass. There have been two distinct periods of research on hydrochar in environmental applications: a “logarithmic development” period with an average annual growth rate of 67.49% (2011–2016) and a “steady development” period with an average annual growth rate of 35.77% (2017–2021).

Fig. 1
figure 1

The number of published documents on hydrochar in environmental applications each year (the inserted figure is the cumulative number of publications from 2011 to 2021). Hydrochar has been published in environmental applications in 2403 papers between 2011 and 2021. The number of publications has steadily increased over time. As research progresses, it is gaining increasing attention in environmental applications. Research on hydrochar in environmental applications has been divided into two distinct periods: a “logarithmic development” period (2011–2016) and a “steady development” period (2017–2021)

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3.2 Subject categories and journal distribution

The hydrochar research involved 72 subjects (Additional file 1: Fig. S1), of which 10 were selected for analysis, as shown in Table 1. The most popular subjects were “Energy Fuels”, followed by “Environmental sciences”, “Engineering chemical”, “Engineering environmental”, “Biotechnology applied microbiology”, “Agricultural engineering”, “Green sustainable science technology”, “Chemistry multidisciplinary”, “Chemistry physical”, and “Materials science multidisciplinary”. It is generally recognized that hydrochar research is a multidisciplinary field that has received attention from various perspectives. The research on hydrochar was more often centered on environmental categories, suggesting that hydrochar is a popular topic for environmental studies and has attracted widespread interest. Additionally, applications of hydrochar in green and sustainable science and technology have become a popular research topic.

Table 1 Top 10 subject categories, top 10 journals with the largest number of publications, top 10 authors of publications, and the top 10 cited authors of publications of hydrochar in environmental applications
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It was possible to identify the major journals that strongly influence the field by counting the number of related publications published by specific journals (Additional file 1: Fig. S2). This finding helps researchers to select key journals for submissions and reading based on their subject categories. In this study, the publication outputs of 401 journals in the field of hydrochar were determined, and the 10 journals with the most publications were selected for analysis (Table 1). Bioresource Technology had the highest number of publications, followed by Science of the Total Environment. In terms of journals, most of them have published a relatively high number of manuscripts on the environmental applications of hydrochar, thus strongly influencing this field.

Double-map overlay analysis reveals the internal relationship between disciplines by evaluating the subject distribution, citation track and research focus of the paper. A double-map overlay is produced by superimposing and simplifying disciplines using a Z-score. According to the color-coded lines, there is an association between a citing journal and a different field; the thickness of the lines indicates how similar the fields are. Fig. 2 shows a double-map overlay analysis for hydrochar research. On the left is the subject distribution of the research literature. This section of the figure shows that the research on hydrochar is mainly applied to zoology, physics, chemistry, and materials science. On the right is the subject distribution of cited research literature. This section shows that hydrochar researchers are more involved in environmental, toxicology, nutrition, chemistry, physics, and materials science, indicating that the research on hydrochar is multidisciplinary. The thickness of a connection indicates how closely disciplines are related, while the color indicates the path of their relationship. For example, a rose line connects physics to environmental science; a yellow line connects zoology to environmental science. Researchers from other disciplines would explore the diverse characteristics of this field.

Fig. 2
figure 2

A double-map overlay analysis for hydrochar research. On the left is the subject distribution of research literature. It can be seen that the research on hydrochar is mainly applied to zoology, physics, chemistry, and materials science. On the right is the subject distribution of cited research literature. Research on hydrochar is more involved in environmental, toxicology, nutrition, chemistry, physics, and materials science, indicating that the research on hydrochar is multidisciplinary

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3.3 Analyses of countries and issuing institutions

Visual representations of the overall strength of scientific research and the influence of the specific countries in a given research field were based on the statistics of the author’s countries of related articles. Overall, hydrochar research was conducted in 90 countries worldwide. Hydrochar research varied from country to country. For readers to have more direct information, we summarized the main countries (top 10) in hydrochar research (top 3). Researchers can obtain a more thorough understanding of research dynamics in a particular field by analyzing the geographical distribution of research activities and the cooperation between institutions. According to the regional heatmap for hydrochar research (Fig. 3 and Additional file 1: Table S1), China has the most publications (1009, 41.99%, mainly in the fields of energy fuels, soils, and water treatment), followed by the United States (319, 13.28%, energy fuels, soils, and water treatment), Germany (253, 10.53%, soils, energy fuels, and catalysts), Spain (141, 5.87%, energy fuels, soils, and electrochemistry), Italy (120, 4.99%, energy fuels, soils, and catalysts), England (101, 4.20%, energy fuels, soils, and water treatment), Australia (100, 4.16%, energy fuels, and water treatment), Korea (99, 4.12%, energy fuels, soils, and water treatment), India (94, 3.91%, soils, and water treatment), and Canada (92, 3.83%, energy fuels, soil science, and water treatment). Lines between countries and regions indicate their cooperation in Fig. 3, indicating the bilateral cooperative relationship between various countries in hydrochar research. Accordingly, China has the highest productivity and the most nodes compared to other nations and regions. Despite this, it is crucial to strengthen collaborations with other high-yield countries in hydrochar research. Generally, the number of articles published by an institution in a field represents its level of influence and research (Additional file 1: Fig. S3). China was shown to be the high-yield hydrochar research institution, as seven of the ten institutions with the most publications were based in China (Additional file 1: Table S2). The Chinese Academy of Sciences posted the most articles (116 publications) and the University of Hohenheim (Germany) occupied the second highest (87 publications) ranking. The University of Trento (Italy) was ranked eighth (35 publications) and the University of Florida (United States) was ranked tenth (32 publications).

Fig. 3
figure 3

Contributions of various countries worldwide in the published documents for hydrochar in environmental applications. There has been a worldwide push to study hydrochar by governments, as indicated. Comparatively, China is the most productive country in the world. The line indicates a high degree of cooperation between countries and regions

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3.4 Analyses of authors and highly cited publications

Table 1 summarizes the top 10 authors with the highest number of publications and co-citations in the field based on the author publication statistics in the WoS core collection database (Fig. 4) and CiteSpace-based cited author analysis (Additional file 1: Fig. S4). Generally, the number of publications of an author reflects their ability to conduct research in a particular field. In contrast, co-citations reflect the recognition of the author’s knowledge and expertise by peers. Therefore, the number of publications and co-citations is generally regarded as indicators of a scholar’s academic contribution and influence in a particular research field. Author analyses allow readers to learn about the researchers in the field and the researchers’ topics of interest to help readers communicate appropriately in academia according to their research needs; these analyses reflect the research capabilities of the researchers’ countries. From 2011 to 2021, a total of 6649 authors contributed to hydrochar research from their respective fields. According to Fig. 4, many hydrochar application research groups have formed, with close cooperation among the researchers within the groups among several core authors. As shown in Fig. 4 and Table 1, we found that Prof. Kruse Andrea from Germany had the most publications, and his research encompasses several disciplines, as evidenced by his articles. Meanwhile, the author is highly interested in the environmental applications of hydrochar, particularly in improving soils and growing plants. Spanish scholar Prof. Sevilla Marta had the highest co-citation frequency, as shown in Additional file 1: Fig. S4 and Table 1, and proposed the name “hydrochar” in early 2009 as a new method of CO2 sequestration, laying a foundation for subsequent research. In China, Prof. Shicheng Zhang of Fudan University had the highest number of publications, ranking second in total, indicating that this author had conducted in-depth research on hydrochar for environmental remediation applications. Among Chinese scientists, Prof. Zhengang Liu ranked fourth in both publications and total citations, indicating a thorough research background in hydrochar. The total cited frequency of literature provides insight into influential authors and essential topics over time in a field, as high total cited pieces of literature are usually of high quality. The top 10 citations for environmental applications of hydrochar are shown in Fig. 5 and Additional file 1: Table S3. According to Kambo et al. (University of Guelph, Canada), the most frequently cited article compared the production, physicochemical properties, and environmental applications of hydrochar and concluded that the reaction temperature controls these properties.

Fig. 4
figure 4

A scheme of the author and their cooperation relationship contributed to hydrochar in environmental applications. Collaborating with researchers can help research progress. Researchers collaborated on hydrochar in environmental applications between 2011 and 2021

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Fig. 5
figure 5

A scheme of highly cited literature on hydrochar in environmental applications

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3.5 Popular research topics

An analysis of keywords in a research field can accurately pinpoint popular research topics and possible future research directions based on their correlations with research themes in the literature. Keyword view analysis (Additional file 1: Fig. S5 and Fig. 6), a keyword clustering of cited authors (Fig. 7a), and timeline view analysis (Fig. 7b) were performed using CiteSpace. Keyword views provide a good visualization of present and past popular research topics in a field. In keyword views, nodes represent the analyzed objects, with larger nodes indicating more frequent appearances or citations. The color and thickness of the inner circles of the nodes represent the frequencies of occurrences or citations in different periods. Lines between nodes indicate co-occurrences or co-citations, and the thicknesses of the lines represent the strength of the co-occurrences. Cited authors keyword clustering views are based on cited authors, and they display the main research focus of the most cited authors; the automatic clustering label is generated by the spectral clustering algorithm, which then extracts the labeled words from the relevant cited literature. The clustering timeline view is implemented on the keyword clustering analysis of cited authors, focusing mainly on outlining the relationships between the clusters and the historical span of literature in a particular cluster and relationships between the clusters and a segment of popular topic outbreak time.

Fig. 6
figure 6

The keyword network map of hydrochar in environmental applications during 2017–2021. The red circles represent hydrochar in environmental applications, and the black circles represent hydrochar in other applications

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Fig. 7
figure 7

a Cluster view of the total cited author keywords of hydrochar in environmental applications; b cluster timeline view of seven major clusters of co-citation of hydrochar in environmental applications in recent years through fisheye view to show more details toward 2021

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To more intuitively reflect the popular trends and development history of environmental applications of hydrochar, a keyword view analysis from two time periods, 2011–2016 and 2017–2021, was adopted in this study. Hydrothermal carbonization was the subject of 202 papers published between 2011 and 2016. The keywords associated with the raw materials or products of hydrochar were “biomass”, “activated carbon”, “carbon”, “black carbon”, and “biochar”. According to the literature analysis, this observation reflected the focus on hydrochar preparation during this period. However, the keywords “soil” and “adsorption” during this period indicated that researchers began to pay attention to the roles of hydrochar as a soil additive and an adsorbent. The keywords included “carbon sequestration”, “greenhouse gas emission reduction”, “soil quality improvement”, and “remediation of organic pollutants”; however, their relevance was limited, and the exploration of these topics was in its early stages.

A significant increase in the literature on hydrochar was observed between 2017 and 2021 (Fig. 1). As displayed in Fig. 6, the hydrochar research in 2017–2021 showed a more significant diversity and development trend than that in 2011–2016, with a significant increase in keywords, nodes, and intersecting lines. According to the visualization and statistical analyses, the research on the environmental applications of hydrochar from 2017 to 2021 could be divided into “soil quality and plant growth”, “carbon capture and greenhouse gas emission”, “organic pollutant removal”, “heavy metal adsorption and its bioavailability” and “anaerobic digestion”.

The Additional file 1: Table S4 contains clustering information derived from the keyword clustering analysis of cited authors in Fig. 7a and a clustering timeline view in Fig. 7b. Since CiteSpace clusters closely related keywords, the keyword with the largest value within each cluster was selected as the representative of that category and assigned a tag. A good clustering result was characterized by two values, Q and S. A clustering module value of Q > 0.3 indicated a significant clustering structure; a clustering average profile value of S > 0.5 indicated a reasonable clustering class, and a clustering average profile value of S > 0.7 indicated a convincing clustering. Therefore, the clustering Q = 0.84 and S = 0.95 in this study demonstrated convincing clustering results. We can quickly grasp the hotspots of research on hydrochar using Additional file 1: Table S4 and Fig. 7, as well as the history of their development and the research directions of the disciplines involved. The largest cluster is #0 “nanoparticle”, and the clustering timeline is from 2011 to the present. Based on the keywords included in #0 in Additional file 1: Table S4, we could roughly conclude that the most prominent research area regarding hydrochar involves the creation of nanomaterials by carbonization technology and studying their physicochemical properties. Next is cluster #1 “porous carbon”; the cluster timeline is from 2015 to the present. Based on the keywords included in #1 in Additional file 1: Table S4, we could approximate that most researchers use biomass wastes as raw materials and modify them by nitrogen (N) doping or potassium hydroxide (KOH) activation to increase the specific surface area and pore size of hydrochar in order to create porous carbon for energy production/storage, capacitor/fuel cell preparation, and CO2 absorption/capture for environmental use. Cluster #2 is “temperature” and the clustering timeline is 2011 to the present. Based on the keywords included in #2 in Additional file 1: Table S4, we found that the research hotspot of this cluster should be about hydrochar preparation and properties. We speculated that temperature might be the most critical factor affecting hydrochar properties based on the cluster tag words. Based on other clustering keywords, we identified different categories of raw materials used to prepare hydrochar, including but not limited to “sewage sludge”, “agricultural waste”, “food waste”, “plant lignin”, “algae”, “poultry”, and “livestock manure”. Cluster #3 is “adsorption” and clustering timeline is from 2013 to 2021. Based on the keywords included in #3 in Additional file 1: Table S4, we could roughly conclude that researchers usually use agricultural waste and livestock manure as raw materials to prepare hydrochar for environmental applications such as soil improvement. Because hydrochar research is multidisciplinary, one cluster contains multiple research directions, and a single research direction occurs in multiple clusters. According to the cluster keywords, hydrochar as an adsorbent covers clusters #4 “methylene blue”, #8 “hexavalent chromium”, #12 “carbon sequestration”, and #13 “adsorption mechanism”. Hydrochar is a product of the efficient conversion of biomass resources with economic and environmentally friendly applications. Therefore, it has a wide range of applications in removing heavy metals and organic pollutants from soil/water environments and carbon fixation. Several tag words, including cluster #5 “hydrothermal treatment”, cluster #6 “reactivity”, cluster #7 “conversion”, cluster #9 “hydrothermal liquefaction”, cluster #10 “hydrothermal carbonization”, cluster #11 “wet torrefaction” and cluster #14 “anaerobic digestion”, appear to describe hydrochar research, mainly pertaining to the preparation and characterization of hydrochar for energy and fuel applications.

4 Discussion

As described in Sect. 3.2, research on hydrochar is a multifaceted and multidisciplinary field that has received a wide variety of attention. Taking the keyword clustering discussion in Sect. 3.5 as an example, researchers have typically used biomass waste as a raw material and modified it by N doping or KOH activation to increase the specific surface area of hydrochar in terms of pore size, forming porous carbon for applications in energy production/storage, preparation of capacitors/fuel cells, and CO2 adsorption/capture for environmental application. The same research hotspot is involved in multiple disciplinary areas. Moreover, there are some other applications such as hydrochar preparation, energy production, and others. However, according to the distribution of publishing journals and subject categories, it can be seen that the research on hydrochar is more focused on environmental disciplines. Therefore, this study was mainly intended to describe hydrochar in environmental applications. According to Fig. 6, the popular hydrochar research topics in environmental application fields were “soil quality and plant growth”, “carbon capture and greenhouse gas emissions”, “organic pollutant removal”, “heavy metal adsorption and its bioavailability”, and “anaerobic digestion”. Table 2 summarizes the main influencing results, influencing factors, and mechanisms of hydrochar in different environmental application fields.

Table 2 The main results, functions, and mechanisms of hydrochar in different environmental application fields
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4.1 Effects of hydrochar on soil properties and plant growth

Hydrochar showed great potential in improving the structure, nutrient cycle, and water retention abilities of soil due to its developed pore structure, abundant OFGs, rich organic matter content, high aromaticity, and large O/C and H/C ratios (Zhang et al. 2020c). Therefore, hydrochar has been studied extensively as a cost-effective method for soil improvement in both pot experiments and field experiments (Adjuik et al. 2020; Malghani et al. 2014). Soil aggregation was positively correlated with O/C and H/C ratios. A study by George et al. (2012) showed that the addition of hydrochar has a positive effect on soil aggregation, which is presumably related to the organic matter in hydrochar. Additionally, the formation of cation bridges between soil and hydrochar surface active functional groups promoted soil agglomerate stability (Heikkinen et al. 2019). Studies have shown that applying poultry litter hydrochar to sandy soils can increase the soil ion exchangeability, water retention capacity, and soil porosity while reducing soil bulk density, and these positive effects may be controlled by the hydrophilic surface and well-developed pore structure of poultry fecal hydrochar (Mau et al. 2020). Furthermore, hydrochar contains high levels of dissolved organic carbon (DOC), which can be degraded and utilized by microorganisms and used as a slow-release organic fertilizer (Song et al. 2020). The application of hydrochar leads to an increase in the conversion rate of nitrate (NO3−), which reduces the rate of soil nitrification and facilitates denitrification and mineralization. This phenomenon is particularly evident when hydrochar with high C/N ratios, high DOC, and low mineral N content is applied. Plants do not use the fertilizer-based N, but they mineralize NH4+ from hydrochar to meet their N needs (Bargmann et al. 2013b; Egamberdieva et al. 2016; Thuille et al. 2015). The application of hydrochar to soils can promote microorganisms to fix mineral N and reduce N leaching, especially for the case of NO3−, which positively impacts soil N utilization and soil ammonia volatilization (Chu et al. 2020). Thus, it is possible to apply hydrochar instead of N fertilizer, which affects N partitioning between soil microorganisms.

Moreover, the addition of hydrochar can change microbial activity and microbial community composition in soil, thereby improving soil characteristics. Studies have shown that when hydrochar is applied to soil, it has a positive effect on the root colonization of arbuscular mycorrhizal fungi, possibly due to changes in pH that occur after hydrochar addition to soil and because the porous structure of hydrochar can protect fungal hyphae from herbivores (Salem et al. 2013). The composition of the archaeal community of soils changes significantly after the application of hydrochar, possibly because the dissolved organic matter (DOM) released from the hydrochar provides an additional matrix for archaeal growth, resulting in its enrichment (Ji et al. 2020). Sun et al. (2020) found that hydrochar plays a negative and positive role in bacterial and fungal richness and diversity, respectively, because hydrochar is acidic and promotes fungal growth, whereas bacteria prefer neutral conditions. A positive correlation exists between soil rhizobia and soil sulfur (S), and a negative correlation was found between soil rhizobia and soil N. The study of Scheifele et al. (2017) compared the effects of hydrochar and pyrochar on soil rhizobia and found that the effect of hydrochar on rhizobia was more pronounced because hydrochar provided more usable S than pyrochar, and hydrochar led to a decrease in the availability of N in soil solutions, which had a positive effect on soil rhizobia. In addition, hydrochar contains higher contents of nutrients (P, K, Ca, and Mg) than those present in agricultural soils, making it a beneficial agricultural fertilizer for improving soil nutrient balance and enhancing crop growth (Fei et al. 2019; Melo et al. 2016). For example, sludge hydrochar provides plant-available P over time, primarily as Al-associated and Ca-associated P (Shi et al. 2019).

However, excessive application of hydrochar was reported to negatively affect plant growth. Hydrochar has been shown to have initial phytotoxicity (Hitzl et al. 2018), which may be attributed to its low pH value, salinity, organic pollutant contents (Mumme et al. 2018), polyphenol contents, volatile fatty acid contents (Puccini et al. 2018), polycyclic aromatic hydrocarbons (PAHs) contents (Lang et al. 2019b), furan contents (Celletti et al. 2021), and to the presence of other toxic compounds. Moreover, hydrochar could enrich heavy metals and volatile organic compounds (Wang et al. 2016). As hydrochar decomposes, heavy metals and organic contaminants are released and adsorbed by components of soil matrices, such as carbonates, iron oxides, and clays (Lang et al. 2019a). It is concerning that the hydrochar application ratio has a direct effect on plant growth, especially during the seed germination and seedling root development stages (Vozhdayev et al. 2015). Low application ratios promote plant growth (de Jager and Giani 2021). In contrast, high application ratios inhibit plant growth (Bargmann et al. 2013a). Additionally, the high water retention capacity of hydrochar results in poor soil aeration, and the production of CO2 can cause root hypoxia, adversely impacting plant growth (Fornes et al. 2017).

Soil improvement is intended to increase crop yields; therefore, the long-term effects of hydrochar should be considered for soil improvement initiatives to reduce the toxic effects of harmful substances on crops and decrease risks for soil contamination. In some studies, modifications to hydrochar have been shown to reduce its biotoxicity and alleviate its inhibitory effects on plant growth. For example, nitric acid-modified hydrochar did not exhibit phytotoxic properties and stimulated seedling growth; this phenomenon occurred because dilute nitric acid treatment reduced the biotoxicity, increased the N content, and stimulated the seedling growth of hydrochar (Fornes and Belda 2017). Calcium oxide (CaO) modification to hydrochar also reduced the biotoxicity of hydrochar and enhanced crop germination. This result may occur because CaO modification increases the pH of hydrochar and reduces the amount of PAHs precursors in hydrochar; additionally, CaO adsorbs metals and organic compounds that inhibit crop growth (Lang et al. 2019b; Mumme et al. 2018). Furthermore, heat treatment of hydrochar can reduce phytotoxicity associated with hydrochar because the aromaticity and thermal stability of hydrochar can be improved, and the phytotoxic and genotoxic substances present in it can be biodegraded (Bahcivanji et al. 2020; Hitzl et al. 2018).

4.2 Effects of hydrochar on carbon capture and greenhouse gas emissions

Global population growth and greenhouse gas (GHG) emissions have led to global warming, which has caused scholars to focus their environmental research on global change and the realization of “peak carbon dioxide emissions” and “carbon neutrality”. Early published studies have shown the positive impact of hydrochar on carbon sequestration and GHG emission reduction (Malghani et al. 2014; Mestre et al. 2014; Sevilla et al. 2011). The high specific surface area and rich mesoporosity of hydrochar make it an effective technology for capturing CO2 and reducing CO2 emissions (Balahmar et al. 2015; Coromina et al. 2016; Sangchoom and Mokaya 2015; Sevilla et al. 2016). Therefore, hydrochar modified with alkali activation and magnetization has improved adsorption and capture capacity for CO2 (Hao et al. 2017; Liu et al. 2018). A porous activated carbon obtained by activating hydrochar with potassium salts and glucosamine hydrochloric acid has the highest CO2 adsorption capacity among the reviewed literature reports (Durán et al. 2018; Gallucci et al. 2020; Kishibayev et al. 2021; Lu et al. 2021; Shi et al. 2021b), attaining 26.24 mmol/g at 0 °C and 20 bar (Cui et al. 2021). The addition of N-containing functional groups to hydrochar can enhance its porous properties, and acid–base interactions can occur between acidic CO2 and basic N-containing functional groups (Cui et al. 2021; Jiang et al. 2020; Shi et al. 2021a). The metal oxide nanoparticle load can promote the physical adsorption of CO2 to hydrochar at metal sites surrounded by N atoms and hydroxyl groups (Vieillard et al. 2018, 2019). Therefore, metal (hydrogen) oxide nanoparticle-loaded hydrochar materials are used as inexpensive large-scale CO2 trapping materials to reduce GHG emissions and mitigate global warming (Jaberi et al. 2020; Vieillard et al. 2019). It is necessary to understand the available active sites and their nature as they are affected by the activation temperature as adsorption proceeds.

The impact of hydrochar on GHG emissions has also attracted much attention. In some studies, the application of hydrochar to soils has been shown to increase CO2 emissions (Andert and Mumme 2015; Yue et al. 2016), possibly due to the abundant DOM present in hydrochar, which results in an excitation effect on microbial activity and stimulates soil decomposition and mineralization of natural organic matter (Yue et al. 2016). It is also possible that the labile carbon (C) in hydrochar can provide an additional matrix for soil microbes, such as actinomycetes, fungi, and methanogens (Ji et al. 2020). A study by Breulmann et al. (2017) found that the hydrothermal carbonization temperature was associated with hydrochar stability, affecting CO2 emissions. Hydrochar produced at high temperatures contains lower quantities of labile C and higher amounts of aromatic C and may release less CO2 than hydrochar produced at low temperatures (Liu et al. 2017). Hydrochar can affect soil enzymes and microbial activity through the presence of toxic substances, which reduce the mineralization in the soil and reduce CO2 emissions (Niu et al. 2021). Furthermore, results have shown that hydrochar can significantly reduce CO2 emissions when applied to soils containing low water contents, and moisture variability can affect the soil’s rate of microbial C degradation (Adjuik et al. 2020). According to existing studies, differences in conditional parameters and application environments for hydrochar may affect CO2 emissions.

For the effect of hydrochar on methane (CH4) emissions, the research results are not consistent; both inhibitory effects (Chen et al. 2021; Li et al. 2021a; Wu et al. 2021c) and promoting effects (Ji et al. 2020; Zhou et al. 2018) have been observed. A study by Li et al. (2021a) has shown that clay-hydrochar introduction can increase the porosity and aeration of paddy soils, which is beneficial for inhibiting the activity of methanotrophs. Simultaneously, methanotrophs can serve as aerobic microbes that utilize CH4 and oxygen (O2) for nutrition, which is conducive to CH4 consumption and mitigating CH4 emissions. However, it is worth noting that high applications suppress the emission ratio of CH4 less than low applications (Wu et al. 2021c; Zhou et al. 2018). Though a high amount of hydrochar application increases the inhibition of methanotrophs, at the same time, large amounts of DOM released from hydrochar cause an increase in CH4 emission (Cheng et al. 2021; Sun et al. 2020), which is because the decomposition of DOM by microorganisms in anaerobic and partially aerobic environments can produce a large amount of CH4. A study also found that removing a part of the DOM from hydrochar by water-washing can reduce CH4 emissions (Chen et al. 2021). Compared with hydrochar, biochar reduces CH4 emissions by 37.5% (Kammann et al. 2012). Furthermore, a study by Cervera-Mata et al. (2021) showed that hydrochar at different hydrothermal carbonization temperatures has different effects on CH4 emissions, which is a result of the fact that an increase in temperature can reduce the amount of easily decomposed DOM present, and CH4 emissions caused by DOM decomposition are further reduced (Ji et al. 2020). Overall, the DOM in hydrochar may be an essential factor in determining soil CH4 emissions and shows a positive correlation. Therefore, the application level and preparation temperature of hydrochar should be considered when applying hydrochar to soil environment.

Soil nitrous oxide (N2O) emissions from hydrochar have been documented in several studies (Chen et al. 2021; Li et al. 2021a; Xu et al. 2020). The acidic surface functional mass of hydrochar helps reduce soil pH and improve the adsorption capacity of hydrochar for NH4+, and N2O reductase is negatively correlated with pH so that N2O emissions can be reduced (Hou et al. 2020; Li et al. 2021a). Hou et al. (2020) found that the effects of the application of sawdust hydrochar (pH = 3.71) and aging hydrochar (pH = 7.04) on N2O in low-fertility soils were consistent, and it was speculated that there might be a soil matrix effect. Xu et al. (2020) found that although hydrochar can increase the abundance of denitrifying bacteria, it can still reduce N2O emissions. Zhou et al. (2018) found that an essential factor that regulates soil N2O emissions is the availability of soil labile organic C (referred to as DOC) and inorganic N, which serve as the C source and energy for heterotrophic denitrifiers. The high content of DOC in hydrochar could favor the reduction of N2O to N2, resulting in a reduction in N2O emissions (Hou et al. 2020). Therefore, it can be assumed that the substrate (C/N) supply of C and N is the main factor that affects N2O emissions. Hydrochar application can increase the C content of soils, and while a higher C/N ratio in hydrochar correlates with a higher fixation of organic N in soils, thereby reducing mineralization and nitrification, this can lead to inhibition of denitrification in hydrochar-treated soils (Adjuik et al. 2020). At the same time, hydrochar contains humus (containing an amount of DOC), which can improve soil porosity, increase oxygen flux, and reduce N2O emissions (Li et al. 2021a).

In general, hydrochar has shown substantial promise in reducing GHG emissions, and future research should focus on the effects of hydrochar on GHG emissions. More research is needed to elucidate the mechanism of interaction between hydrochar and GHG in the future to clarify how the relationships affect GHG emissions and to optimize the stability of hydrochar.

4.3 Effects of hydrochar on organic pollutant removal

Currently, hydrochar is mainly used in environmental sciences, especially for environmental remediation applications, due to the low material cost and desirable properties such as specific pore structure, abundant OFGs, and N-containing functional groups. Hydrochar is effective in adsorbing organic pollutants, such as pesticides (Eibisch et al. 2015; Liu et al. 2019), antibiotics (He et al. 2016; Nogueira et al. 2018), dyes (Islam et al. 2017b; Vozhdayev et al. 2015), PAHs (de Jager and Giani 2021; Li et al. 2021b), and estrogen (Bargmann et al. 2013a; Yu et al. 2019). Due to its limited porosity and specific surface area, hydrochar is usually activated (Qian et al. 2016), acidized (Jiang et al. 2019; Nguyen et al. 2019), or magnetized after alkali activation to enhance adsorption (Liu et al. 2014; Zhu et al. 2014a, b, c). Thermal activation of hydrochar can enhance the sorption of methylene blue (MB) (Buapeth et al. 2019) and bisphenol A (Yu et al. 2020) because of the increased specific surface area and porosity of hydrochar. The nitric acid modification process enhances OFGs and N-containing functional groups and unsaturated bonds on the surface of hydrochar, improving their adsorption capacity for MB (Nguyen et al. 2019). Alkali activation can increase the specific surface area and porosity of hydrochar and its adsorption performance (Islam et al. 2017a, b). The alkali activation of a magnetic hydrochar composite with iron oxide enhances the adsorption capacity of hydrochar for triclosan, tetracycline, and malachite green (Liu et al. 2014; Zhu et al. 2014a, b, c). It is common to use iron oxide or zero-valent iron loadings to modify hydrochar to improve its removal of organic pollutants (Kermani et al. 2019; Rahmi et al. 2019; Ye et al. 2020a). Moreover, urea oxidation of hydrochar is an effective method for doping heteroatoms to form N-containing functional groups; the increased numbers of pyridinic-N, pyridonic-N, and graphitic-N are believed to help anchor organic molecules via a coupling effect between electron configuration and binding energy to improve adsorption capacity (Hou et al. 2021; Xiao et al. 2020).

Organic pollutants can be removed by adsorption to hydrochar through various mechanisms, including hydrophobic interactions, electrostatic interactions, partitioning interactions, π-π interactions, pore filling, and hydrogen bonding as shown in Fig. 8 (Ning et al. 2017; Tian et al. 2018, 2019; Wei et al. 2020). Eibisch et al. (2015) studied the removal of isoproturon in agricultural soils using hydrochar and attributed adsorption to hydrogen bonding due to the low surface acidity and the abundant OFGs of hydrochar. A study by Liu et al. (2019) pointed out that the partitioning of atrazine to hydrochar may be the primary adsorption mechanism due to the rich C–OR bonds in hydrochar. Rattanachueskul et al. (2017) investigated the adsorption of tetracycline by magnetized hydrochar with iron loading and found that the adsorption was mainly based on hydrogen bonding interactions between the OFGs on the modified hydrochar and the –OH, C=O, and –NH2 groups on the tetracycline molecule; in addition, the aromatic structure on hydrochar induced π-π electron donor-receptor (EDA) interactions, or cation-π bonding, and could be conjugated to the ring of the TC molecule. Hydrochar has a large specific surface area and a good microporous structure, which can also increase the hydrochar adsorption capacity for tetracycline (Chen et al. 2017b). Leng et al. (2015) found that the adsorption capacity of rice husk hydrochar prepared with ethanol as solution for the cationic dye malachite green was significantly increased compared to the conventional method (water as solution) because the richer OFGs in hydrochar prepared with ethanol as solvent induce the stronger electrostatic interaction and hydrogen bonding between the negatively charged carboxylic acid anion and the positively charged malachite green. Tran et al. (2017b) investigated the adsorption of the cationic dye methylene green on hydrochar; hydrochar formed a hydrogen bonding and n-π interactions with the N and O atoms of methylene green, and acrylic-modified hydrochar improved the surface OFGs of hydrochar and reduced the adsorption capacity; therefore, it was speculated that π-π interactions between the aromatic structure of hydrochar and the benzene ring of methylene green and pore filling dominated the adsorption. Li et al. (2018) investigated the anionic dye methyl orange adsorption on hydrochar modified with protonated amines; electrostatic interactions were the adsorption mechanism for methyl orange because protonated amines-modified hydrochar was positively charged. Surface hydrophobicity and the carbonyl groups in hydrochar are conducive to removing aromatic pollutants such as 2-naphthol through hydrophobic interactions and hydrogen bonding (Li et al. 2021b). In addition, the high aromacity, hydrophobicity, porosity and low surface OFGs of hydrochar result in higher adsorption capacities for 1-butanol (Han et al. 2017).

Fig. 8
figure 8

A schematic diagram of possible adsorption and degradation mechanisms of organic pollutants by hydrochar

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Researchers have studied technology combinations to enhance the organic pollutant degradation capabilities of hydrochar as shown in Fig. 8. It is mainly combined with advanced oxidation processes such as photocatalytic technology, Fenton-like technology, electrochemical catalytic technology, sonocatalytic technology, and persulfate (PS)/peroxymonosulfate (PMS) oxidation technology to increase the catalytic degradation capacity for organic pollutants of hydrochar.

The surface OFGs (O–C=O, C–O) of hydrochar can be used as a photosensitizer. Molecular oxygen can be activated by hydrochar under sunlight irradiation to generate a large number of reactive oxygen species (ROS), including superoxide anions (·O2−) and hydroxyl radicals (·OH), to enhance the degradation of pollutants (Chen et al. 2017a). Metal oxides (Ti, Zn, and Ag) modified hydrochar owns the higher light response, faster electron transfer, photogenerated carriers, and electron-hole pairs, thus enhancing the degradation of organic pollutants by generating ROS (holes, ·OH, and ·O2−) (Leichtweis et al. 2021; Zhou et al. 2019). Ye et al. (2020b) prepared a novel multiphase photocatalytic composite by combining hydrochar and FeAl layered double hydroxide (FeAl-LDH). The composites as electron-hole pairs, Fe in the LDH layer, and surface OFGs of hydrochar were found to facilitate the electron transfer process and generate more ·O2−, hydrogen peroxide (H2O2), and ·OH. On the other hand, surface OFGs of hydrochar and hydrochar-derived DOM may undergo energy transfer to produce singlet oxygen (1O2) and form electron-transferred ·OH.

Fe oxide-modified hydrochar can improve the performance of Fenton-like oxidation technology. Liang et al. (2017) synthesized oxidized Fe-Zn hydrochar as a nonhomogeneous photo-Fenton catalyst. They found that oxidized Fe-Zn hydrochar can generate electrons and holes under visible light irradiation. H2O2 traps electrons to produce ·OH; at the same time, Fe(II) and H2O2 can also generate ·OH. As a result of forming iron (hydr)oxides on the surface of hydrochar (HC@FeOOH), Liang et al. (2018) found that H2O2 predominantly reacted with photo-generated holes in HC@FeOOH rather than Fe(II)/(III) and photo-generated electrons to produce ·O2−.

Hydrochar can be combined with electrochemical oxidation to improve the ability to degrade 2,4-dichlorophenol. Electrochemical anodes can produce ·O2− and ·OH, and the surface OFGs of hydrochar can form persistent free radicals (PFRs), acting as electron donors, to produce ROS to remove pollutants (Cao et al. 2020). Khataee et al. (2021) pointed out that the prominent role of Cu oxide-modified hydrochar is to produce electron-hole pairs to facilitate the sonocatalytic dyes process, and oxidation of adsorbed water molecules or surface hydroxyl groups can produce ·OH and ·O2−. The increase in the specific surface area of Cu oxide-modified hydrochar can indirectly affect the sonocatalytic degradation capacity.

Hydrochar can activate PS/PMS and H2O2, generating ·SO4− and ·OH, improving the degradability of pollutants. The PFRs of Hydrochar are primarily responsible for the activation of PS/PMS, and its defective structure and graphite structure also promote ROS production (Wei et al. 2020). Hydrochar not only plays as a carrier for the dispersion of Co3O4 nanoparticles but also acts as an electron shuttle between Co3O4 and PMS/sulfamethazine molecules (Tian et al. 2020). Moreover, hydrochar can generate 1O2 through non-radical PMS activation because of its highly graphitized C domain with low polarity and enriched ketonic (C=O) functionality (Liu et al. 2021a). Graphite-N in N-doped hydrochar can enhance the PFRs of hydrochar, as it not only promotes PMS activation in nonradiative pathways by showing strong electron transfer but it also promotes the formation of oxygen-centered PFRs by structural defects, increasing the ability of the product to degrade contaminants through free radical oxidation (Yu et al. 2020).

4.4 Effects of hydrochar on heavy metal adsorption and metal bioavailability

Heavy metal pollution has been the focus of attention in multiple areas, including environmental applications. Research on the applications of hydrochar for heavy metal immobilization increased dramatically between 2011 and 2021. Early articles focused mainly on the adsorption of heavy metals (Pb, Cu, Cd, Sb) by hydrochar or modified hydrochar in soil and water (Chen et al. 2015; Elaigwu et al. 2014; Sun et al. 2015; Xue et al. 2012). The heavy metal adsorption mechanisms by hydrochar can be broadly classified into the following four categories as shown in Fig. 9: (1) electrostatic interactions between heavy metal cations and negatively charged minerals on hydrochar; (2) cation exchange between metal ions and mineral ions (e.g., K+, Na+, Ca2+, and Mg2+) on hydrochar; (3) complexation of metals with functional groups (e.g., –OH, –COOH, –CHO, –C–) on the surface of hydrochar; and (4) complexation of heavy metals and mineral ions (e.g., SiO32−, PO43−, and CO32−) on hydrochar to induce precipitation.

Fig. 9
figure 9

A schematic diagram of possible adsorption mechanisms of heavy metal by hydrochar

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Several studies have aimed to increase the heavy metal removal capability of hydrochar by enhancing interactions between heavy metals and hydrochar through hybridization, acid and alkali treatments, or other modifications. For example, Tang et al. (2016) used Ni/Fe hybridized hydrochar to remove Pb(II), and adsorptive precipitation dominated Pb(II) removal by Ni/Fe hybridized hydrochar. Researchers have found that Pb adsorption to phosphoric acid-modified hydrochar is a synergistic process involving surface complexation, cation exchange, and partial precipitation because phosphoric acid-modified hydrochar is rich in surface acidic OFGs (–COOH) that can complex with Pb(II), while hydrochar also contains mineral ions that can be ion exchanged (K+, Na+, Ca2+, and Mg2+) and can precipitate (CO32−) with Pb(II) (Zhou et al. 2017). The adsorption capacity of Cd on nitric acid-modified hydrochar increases 1.9–9.9 times relative to that on unmodified hydrochar, and the adsorption process involves electrostatic interactions, oxygen-containing complexation, ion exchange, and π-π interactions (Zheng et al. 2021). KOH-modified hydrochar increases Pb removal by a factor of 5 compared with unmodified hydrochar, demonstrating that the adsorption mechanism is mainly ion exchange followed by surface complexation (Petrovic et al. 2016). As a result of modifications with anaerobic fermentation, the adsorption capacity of Cd is enhanced by approximately 3.1–3.4 times, and the adsorption mechanisms include electrostatic attraction, ion exchange, and functional group complexation (Fu et al. 2021). A cyclodextrin-functionalized hydrochar results in 47.28 mg g−1 of Cd adsorption, with surface complexation and electrostatic interactions being the main mechanisms (Qu et al. 2021). In a study by Shi et al. (2018) the heavy metals Cr and Ni were removed from water using polyethyleneimine-modified hydrochar; the results showed that N-containing groups were the main adsorption sites.

Recent studies have investigated the co-adsorption of heavy metals and organic pollutants by hydrochar. For example, Guo et al. (2018) activated rapeseed shell hydrochar with KOH to remove MB and Cr from water and found that the modified hydrochar showed excellent pH tolerance. Li et al. (2019) investigated the adsorption of MB and Cu from water using polyamine carboxylated-hydrochar and found the adsorption process to have rapid kinetics (5 min) and large capacities (1239 and 141 mg g−1, respectively). Surface complexation contributes to the adsorption of Cu(II) by amino and carboxylate groups, while π-π interactions, hydrogen bonding, and electrostatic attractions dominate the adsorption of MB.

Many researchers have studied the effects of hydrochar on the bioavailability of heavy metals in contaminated soils when used as a soil amendment since heavy metals cannot be easily degraded (Han et al. 2019). The bioavailabilities of heavy metals are essential indicators for evaluating their mobility and ecological impact, i.e., whether they can cause toxic effects on living organisms or are absorbed by them. To date, several studies have examined the effects of hydrochar on heavy metal mobility and bioavailability (Wagner and Kaupenjohann 2014; Wang et al. 2016; Watson et al. 2021). For example, heavy metals adsorbed to hydrochar are more stable than those in sewage sludge or manure feedstock, which reduces the bioavailability and environmental risks of heavy metals (Lang et al. 2018; Zhang et al. 2018). However, in addition to reducing the bioavailability of heavy metals through the hydrothermal carbonization process, hydrochar can directly adsorb heavy metals after it is applied to soils, or it can indirectly affect the bioavailability of heavy metals in soil by influencing soil properties.

Hydrochar reacts with soil heavy metals through adsorption, precipitation, complexation, and ion exchange after application. Heavy metals in soils bind to soil components in different manners, such as adsorption-desorption, dissolution-precipitation, complexation-dissociation, and oxidation-reduction, resulting in spatial migration, morphological changes, and varying bioavailabilities (Khan et al. 2021; Ren et al. 2018). Teng et al. (2020) found that soils amended with iron-modified rice husk hydrochar decreased mobility and bioavailability of the heavy metals Pb and Sb due to cation exchange, complexation, and precipitation. In contrast, the mechanisms controlling Sb immobilization in soils may be complexation, reduction, and electrostatic interactions. Moreover, iron-modified hydrochar can oxidize Sb(III) to Sb(V) and reduce the overall toxicity of Sb. Alkali-modified hydrochar is more effective than pristine hydrochar at reducing heavy metal bioavailability because it increases soil pH and forms precipitate with heavy metals (Xia et al. 2019).

As mentioned earlier, hydrochar can indirectly enhance the immobilization and adsorption of heavy metals in soils through soil properties. Adding hydrochar to soils reduces heavy metal bioavailability due to changes in soil properties, as Fei et al. (2019) showed. Soil pH and cation exchange capacity (CEC) affect the adsorption and transport of heavy metals; low pH values usually indicate that heavy metals are more easily leached, and high pH values induce heavy metal precipitation in the soil, thereby reducing the migration of heavy metal ions; in soils with high CEC, heavy metals adsorb more readily on cation exchange sites (Fei et al. 2019; Xia et al. 2020). It has been reported that amino-functionalized hydrochar derived from pinewood sawdust at 200 °C increased soil CEC by 8% and soil organic matter by 59.6%, while also lowering the bioavailability of heavy metals in plants by 45.9–52.5% (Xia et al. 2020). Therefore, hydrochar can reduce soil heavy metals bioavailability and relieve soil stress.

4.5 Effect of hydrochar on anaerobic digestion

Note that the effect of hydrochar on anaerobic digestion is a popular topic and is gaining increasing attention (Fig. 7b). Anaerobic digestion is widely used and effectively adds value to high-moisture wastes, such as food, organic residues, and sewage sludge since it can convert these products into energy. However, the limitations of anaerobic digestion usually stem from recalcitrant substrates when utilizing wastes. A significant problem is the quality of produced digestion residues (digestate) since digestion residues are often used as soil conditioners (Codignole Luz et al. 2018). There has been growing interest in using hydrochar in waste anaerobic digestion since it increases the value of anaerobic digestion and improves environmental and agronomic benefits. Many studies systematically assessed the advantages of hydrochar for the anaerobic digestion of wastes, including its effect on accelerated organic matter solubilization and hydrolysis (Ren et al. 2020; Wang et al. 2017), altering microbial diversity (Usman et al. 2020, 2021), reduced N loss (Zhang et al. 2020a), and reduced phytotoxicity (Celletti et al. 2021; Usman et al. 2020; Xu et al. 2018).

The humic acid in hydrochar accelerates the solubilization and hydrolysis of organic matter during anaerobic digestion. Humic acid functions as a surfactant, facilitating the dissolution of organic matter and hydrolysis, and competes for electrons in the reaction system with methanogenic bacteria, inhibiting their activity (Wang et al. 2017). The surface OFGs of hydrochar can mediate direct interspecific electron transfer and promote anaerobic digestion (Ren et al. 2020), and alter microbial diversity and abundance (Geobacter) (Usman et al. 2021). Additionally, hydrochar can degrade aromatic and phenolic compounds, contributing to the enrichment of methanogenic bacteria in anaerobic digestion (Usman et al. 2020). As a result, hydrochar can adsorb ammonium (NH4+) during anaerobic digestion and lower the inhibitory effect of NH4+ on anaerobic digestion; also, NH3 emissions from anaerobic digestion can be reduced, thereby reducing atmospheric pollution (Zhang et al. 2020a). Appropriate acidification and saline leaching treatment of hydrochar will reduce the potential phytotoxic effects of combined alkalinity and high electrical conductivity (Celletti et al. 2021). As a result, hydrochar is an ideal biological substrate for reducing the time required for the anaerobic digestion process and for increasing its value.

4.6 Comparison of hydrochar and pyrochar in environmental applications

Biochar is a solid carbon-rich product produced from biomass using various thermochemical methods, e.g., pyrolysis, dry torrefaction, gasification, and hydrothermal carbonization (Liu et al. 2021b). Generally, pyrochar refers to pyrolytic carbonization products (300–700 °C), and hydrochar refers to hydrothermal carbonization products (180–260 °C). There are numerous applications for both types of biochar, including soil amendment and plant growth, CO2 capture and GHGs emissions, organic pollutant removal, heavy metal adsorption, and reducing heavy metal bioavailability. Pyrochar and hydrochar differ significantly in terms of physicochemical properties as a result of different preparation methods. Compared with pyrochar produced at the typical temperature range, hydrothermal carbonization has lower dehydration and ash to liquid phase process, resulting in a lower C, ash, and pH content in hydrochar (Fang et al. 2018). A higher production temperature results in a higher specific surface area and larger pore size in pyrochar (Liu et al. 2021b), whereas pyrochar is richer in heavy metals and alkaline earth metals than hydrochar, while its H/C ratio and O/C ratio are lower (Kumar et al. 2020; Shen et al. 2019). As a result, the performance of hydrochar and pyrochar in environmental applications differs.

As a soil amendment, pyrochar can be used on acidic soils to enhance pH improvement, while hydrochar can be used on alkaline soils to reduce pH (Zhang et al. 2019). Additionally, hydrochar outperformed pyrochar in improving soil aggregates since soil aggregates correlated positively with the H/C and O/C ratios (George et al. 2012). Contrary to pyrochar, hydrochar was not as effective as pyrochar in improving soil water holding capacity because hydrochar is unstable and biodegradable, and fungal fixations create hydrophobicity (Fang et al. 2018). The decomposition of char leaching into the soil can negatively affect plant growth when hydrochar or pyrochar is applied (Lang et al. 2019a). Pyrochar has a higher heavy metal content than hydrochar, so that it may be more phytotoxic than hydrochar because of its high heavy metal content (Shen et al. 2019). In contrast, pyrochar has a larger specific surface area and is more stable than hydrochar, and contains alkaline functional groups (Liu et al. 2021b); Therefore, pyrochar can better sequester carbon and capture CO2 than hydrochar in soil. In addition, hydrochar provides fertile substrates for soil microorganisms (such as methanobacteria and actinomycetes), which results in enhanced microbial activity, which may contribute to GHG emissions due to its instability and ease of decomposition (Ji et al. 2020). Consequently, washing treatment of hydrochar (Chen et al. 2021) or alkali modification (Cui et al. 2021; Jiang et al. 2020; Shi et al. 2021a) is a good way to improve the CO2 capture capacity of hydrochar and reduce GHG emissions.

Organic pollutants and heavy metal pollutants can be significantly reduced when used as adsorbents by hydrochar and pyrochar. The functional units of action and adsorption mechanisms of hydrochar and pyrochar are interchangeable in removing organic and heavy metal pollutants from the environment (Wu et al. 2019, 2020, 2021b; Zhang et al. 2019). Despite this, the main adsorption mechanisms or their applicability to different organic and heavy metal pollutants differ due to the different content of functional units in the two types of biochar. In the case of toluene, an organic pollutant that is removed by surface adsorption and pore filling, pyrochar has a higher specific surface area, porosity, and aromatization, so it is more efficient at removing toluene than hydrochar (Liu et al. 2021b). In the case of PAH organic pollutants, pyrochar significantly removes them by pore filling and interacting with N-heterocyclic structure (Liu et al. 2021b). Hydrochar, on the other hand, has a predominant interaction on PAH organic pollutant removal via hydrophobic interaction (Li et al. 2021b). In the case of the positively charged heterocyclic organic pollutants, hydrochar has a much higher adsorption capacity than pyrochar because of its rich surface OFGs. Hydrochar not only produces electrostatic effects on organic pollutants but also contains hydrogen bonding effects (Kambo and Dutta 2015). In the case of heavy metals, pyrochar has a large specific surface area and porous structure, and is generally alkaline, which make pyrochar a good material to remove heavy metals by electrostatic interaction, ion exchange, and precipitation; hydrochar has an advantage over pyrochar when it comes to heavy metal pollutant adsorption due to its abundant OFGs, which can easily interact electrostatically and complexity with heavy metals (Liu et al. 2021b).

5 Conclusion

This study provided a comprehensive scientometric review of the trends in research on environmental applications of hydrochar from 2011 to 2021. An overview of the present state of research in hydrochar was provided, along with some significant conclusions. First, keyword clustering analysis revealed that the main research areas for the environmental applications of hydrochar were “soil quality and plant growth”, “carbon capture and greenhouse gas emissions”, “organic pollutant removal” and “heavy metal adsorption and its bioavailability” and that research on hydrochar has diversified over time. Second, the emergence of the term anaerobic digestion was noted; in addition to the major popular topics, hydrochar and anaerobic digestion have received increasing attention. The development of these research topics will facilitate the synthesis of hydrochar-based functional materials. Finally, although hydrochar is widely used in environmental applications, the increasing number of hydrochar-related publications each year indicates that hydrochar is still a major research topic that needs further development. Because mass engineering and production of hydrochar are approaching quickly, more attention should be given to the following aspects.

6 Possible further trends

  1. (1)

    The potential environmental risks of hydrochar should be concerned during its application processes. The biological toxicity and its effects on crop growth are not addressed, especially the methods involved in eliminating or mitigating the toxic effects of hydrochar by modifying and applying it with an appropriate application ratio. In addition, the increased reactivity of hydrochar may affect their risk concerns. A careful evaluation of the scope/conditions of hydrochar applications and the environmental effects of hydrochar applications on soils in different regions under various climate conditions is a fundamental requirement before their wide and environmentally friendly applications.

  2. (2)

    The effects of hydrochar on different GHG emissions require further comprehensive study. Hydrochar should also be considered in terms of its relationship with soil microorganisms to clarify the impacts of this relationship on GHG emissions. Moreover, the stability of hydrochar should be considered in GHG emissions. A better understanding of the available active sites and the properties of hydrochar is needed to enhance CO2 capture capacity of hydrochar. Hydrochar helps promote anaerobic digestion by accelerating the solubilization and hydrolysis of organic materials and reducing N loss, GHG emissions, and organic pollutants. Future research will focus on optimizing the physicochemical properties of hydrochar, enhancing its economic benefits, and coupling hydrochar with anaerobic digestion technology to treat toxic and hazardous pollutants.

  3. (3)

    The physicochemical properties of hydrochar should be determined before its application because the application performances of hydrochar are diverse due to the different raw materials, processes, and modifications. More research needs to be conducted to select the optimal conditions and to expand the application range of hydrochar to improve its performance. The establishment of a database that records in detail the environmental applications of hydrochar from different sources and under various preparation conditions would provide a valuable theoretical basis for the development and application of hydrochar in the environmental field.