Sludge-based biochar adsorbent: pore tuning mechanisms, challenges, and role in carbon sequestration

Sludge biochar, a carbonized product of raw sludge, contains porous architectures that can act as epicenters for adsorbing external molecules through physical or chemical bonding. Sludge biochar also immobilizes innate micropollutants, which is advantageous over conventional sludge disposal methods. To date, numerous strategies have been discovered to improve sludge biochar morphology, but the influential factors, pore tuning mechanisms, and process feasibility remain imprecise. This knowledge gap limits our ability to design a robust sludge-based biochar. Herein, we present state-of-the-art sludge biochar synthesis methods with insight into structural and chemical transformation mechanisms. Roadblocks and novel concepts for improving sludge biochar porous architecture are highlighted. For the first time, sludge biochar properties, adsorption performances, and techno-economic perspectives were compared with commercial activated carbon (AC) to reveal the precise challenges in sludge biochar application. More importantly, sludge biochar role in carbon sequestration is detailed to demonstrate the environmental significance of this technology. Eventually, the review concludes with an overview of prospects and an outlook for developing sludge biochar-based research. Utilizing sludge's unique feature can unlock many new and feasible biochar modulation strategies. Sludge carbonization and activation mechanisms reveal the crucial factors for micropore decorating. Sludge biochar is compared with activated carbon to reveal requirements for practicality. Sludge-based biochar application supports carbon sequestration and economic sustainability. Utilizing sludge's unique feature can unlock many new and feasible biochar modulation strategies. Sludge carbonization and activation mechanisms reveal the crucial factors for micropore decorating. Sludge biochar is compared with activated carbon to reveal requirements for practicality. Sludge-based biochar application supports carbon sequestration and economic sustainability.


Graphical Abstract 1 Introduction
Sludge biochar has been a subject of research for more than 50 years (Beeckmans and Ng 1971).The worldwide growing concern over the pollutants emission risk of conventional sludge disposal approaches [e.g., fertilizing (Pradas del Real et al. 2016;Zhang et al. 2020b), landfilling (Feng et al. 2015b), or incinerating (Zhang et al. 2013)] galvanizes researchers' and policymakers' interest in a carbonization-based sludge valorization approach (Peccia and Westerhoff 2015;Faragò et al. 2022;UKWIR 2023).What mirrors this act is the rapid growth of sludge biochar-based research in the last decade (Fig. 1).Carbonization not only improves sludge stability but also produces valuable byproducts, such as biochar, syngas, and bio-oil (Liu et al. 2015).Numerous micropollutants can also be efficiently immobilized into the sludge biochar (Liu et al. 2018).Designing sludge biochar with numerous micropores and mesopores reduces apparent density and improves interfacial mass transfer, which is highly anticipated in adsorption-based pollutant removal processes (Wang et al. 2020a;Mian et al. 2022).Besides, highly porous sludge-based biochar also found application in the catalysis field, where the material acts as an adsorbent and catalyst (Liu et al. 2018;Diao et al. 2020Diao et al. , 2021)).Unfortunately, tailoring highly microporous architecture on sludge biochar, because of its features, is not as straightforward as creating on other biocarbon (Yu 2020).The complex composition plays a crucial role in the carbonization process and the morphology of the resulting carbon (Januševičius et al. 2022).Consequently, the underlying mechanisms of chemical and structural changes in sludge biochar differ from common biomass pyrolysis or activation processes, but the literature lacks insight in this regard (Mian et al. 2022).
Over the past years, numerous modulation approaches have been adopted to upgrade sludge biochar morphology.Of those, activation strategies were reported as efficient in creating microporous architecture (Feng et al. 2015a;Li et al. 2020c;Wu et al. 2022), while simple pyrolysis, deashing, and conditioning improve mesopores.Interestingly, sludge polymeric properties have been studied recently (Guellil et al. 2001;Sarvajith and Nancharaiah 2023) and utilized to tailor sludge porosity through template methods (Li et al. 2020b;Zhang et al. 2020a).Likewise, many other promising sludge biochar synthesis strategies are documented in the literature (Feng et al. 2015a;Du et al. 2022).However, most successful micropore tuning approaches sacrifice a high dose of chemicals and energy, which is cost-prohibitive.Therefore, despite numerous efforts on sludge-based biochar research, progress towards practicability is slow and there has been no significant breakthrough so far.In prior studies, less attention has been paid to demonstrating the practicability of sludge biochar synthesis methods and exploring viable paths to develop sludge biochar morphology.
Furthermore, sludge biochar-based adsorbent has a significant potential to sequester carbon upon land disposal because of its stable carbon content.Sequestering carbon through biochar technology is currently a globally acknowledged approach to mitigating climate change (Lehmann et al. 2021).Nevertheless, sludge biochar's contribution to carbon sequestration has not been reviewed yet (Zhang et al. 2013(Zhang et al. , 2020b;;Mohamed et al. 2023).Considering that fact, it is high time to present sludge biochar-based adsorbent's potential in carbon sequestration to display the extent of this technology's importance in the field.
Therefore, this review covers three crucial aspects of sludge biochar-based adsorbent.Firstly, we present the progress in sludge biochar synthesis methods with insight into pore-forming mechanisms; secondly, we reveal the challenges in sludge biochar application and the gap between research progress and commercialization by comparing sludge biochar with commercial AC; thirdly, we demonstrate the crucial role of sludge biochar in carbon sequestration; and finally, our perceived prospects for the improvement of this technology are summarized.

Mechanistic understanding of sludge biochar synthesis methods
Understanding biomass thermochemical conversion mechanisms is crucial for optimizing carbon porosity and surface area.Sludge conversion mechanisms thus need to be demonstrated beyond the theoretical biomass conversion processes because their ash influences carbonization and activation processes.Considering the pore tuning mechanisms and the feasibility, we classify sludge biochar modification strategies into three main categories: (1) pyrolysis and activation methods, (2) template removal methods, and (3) conditioning methods.Each approach has its unique role in morphology and functionality with individual strengths and weaknesses.The underlying mechanisms of these approaches, with appropriate examples, are detailed in the following sections.

Single-step pyrolysis
Producing sludge biochar through single-step pyrolysis is a straightforward method.During pyrolysis, atoms in sludge move to a shorter distance for stability and form stable bonds with other carbon or heteroatoms.Meanwhile, the release of low molecular weight tarry materials creates distortions and vacantness in the solid carbon structure, called pores (Marsh and Rodríguez-Reinoso 2006a).However, sludge activation temperatures and pore-forming mechanisms differ from other lignocellulosic biomass for having a complex composition (Weber and Quicker 2018).Generally, sludge (biological or chemical) contains a blend of organic and inorganic ash fractions.The organic fraction encompasses lipids, protein, carbohydrates, and humic substances (Liu et al. 2021a), and the inorganic fraction contains metalloids, earth metals, transitional metals (TMs), and heavy metals (HMs) (Mian et al. 2019a).The organic macromolecules in sludge decompose at a relatively low temperature (Yuan et al. 2022).For example, The decomposition temperatures of lipids, proteins, and carbohydrates range from 200-635, 209-309, and 164-497°C, respectively, while the decomposition temperature of lignin, the primary element of the woody biomass skeleton, is 400-900°C (Chen et al. 2018).In addition, the inorganic metals in sludge catalyze the decomposition process and could possess faster carbonization.
The organic decomposition starts with the decarboxylation and dehydration of protein and polysaccharides at low temperatures.As temperature rises, reactions become more dynamic, such as breaking glycosidic linkage in protein, releasing amine groups, Maillard reactions, retroaldol condensation between amino acids and sugar, and mass transfer diffusion (Ren et al. 2011;Liu et al. 2017;Yuan et al. 2022).The charring process matures after 450 °C, while the rearrangement, radical formation, secondary cracking of humic acid intermediates, and catalytic reaction of metals occur.Therefore, sludge biochar should be carbonized above 450 °C for better stability.Similar to other biomass, sludge biochar surface porosity also depends on pyrolysis temperature.Nevertheless, the optimization temperature range is lower than that of common biomass because of its early activation and volatilization or organics.In earlier studies, sludge biochar with optimum porous structures was found at a temperature range of 600-800 °C (Jellali et al. 2021;Mian et al. 2022).The excess heat after optimum volatile release and generation of pores leads to pore collapse and reduced surface area.The humic acid in sludge usually decomposes at a comparatively higher temperature.Thus, humic acid derivatives and ash components primarily act as sludge biochar skeleton structures (Yuan et al. 2022).
The ash content in sludge is mainly silicon, followed by aluminum, iron, and other metals.As the pyrolysis severity enhances, ash in sludge biochar increases (Januševičius et al. 2022), which essentially plays no role in porosity development.Instead, the reactive metal groups and the reactive nonmetal heteroatoms undergo polymerization during carbonization, which results in the compaction of the bulk biochar structure (Zielińska et al. 2015).The ash in sludge biochar thus can be considered a roadblock to developing morphology.Consequently, creating a highly porous surface on sludge biochar through only pyrolysis is challenging.The ash and organic contents in sludge depend on the treatment process in the wastewater treatment plant, more precisely on the source of sludge, such as industrial, municipal, or domestic wastewater.It can vary from industry to industry, or the sludge collected from different treatment zones or times of the same industry (Grobelak et al. 2019).For Example, municipal wastewater sludge collected from four locations in Poland exhibited ash content from 55.8% to 79.1% and C contents from 18.1% to 27.8%.Consequently, their biochar surface area and porosity varied significantly (Zielińska et al. 2015).Sludge composition unpredictability creates ambiguity among the stakeholders about their reproducibility and application.Thus, for standardization, sludge and its ash/organic content and detailed properties should be considered during preparation and reporting.Ideally, without additional treatment, typical pyrolysis can enhance sludge biochar surface area by approximately 2 to 80 m 2 g −1 , depending on its composition (Mian et al. 2022).Modulating heating operating conditions, such as temperature, residence time, and heating rate, can optimize porosity and surface area.Nevertheless, it cannot satisfy the requirements if the parent material is enriched with ash.

Chemical activation
Chemical activation is a solution to overcoming the limited surface porosity.Despite sludge's high ash, chemical activation tunes a highly microporous framework through eroding reactive C atoms from the surface (Marsh and Rodríguez-Reinoso 2006b).It can be treated as an assured process for improving sludge biochar porosity.The outcome of chemical activation depends on the type and dose of activators.KOH, an alkali reagent, is efficient in creating micropores of a size ranging from 1 to 4 nm (Wickramaratne et al. 2014;Feng et al. 2015a).The process is accomplished in two steps, producing reactive intermediates and eroding C atoms.At the outset, KOH reacts with the surface functional groups, creating vacancies and K 2 CO 3 byproducts (R1).The vacancies are then quickly tuned by − OH of KOH and surface oxygen groups are excessively increased.Another theory of producing K 2 CO 3 is the reaction of KOH and biomass pyrolyzed intermediate CO 2 (R2) (Liu et al. 2015).Below 700 °C, these are the primary chemical transformation processes (Chen et al. 2020).Sludge biochar produced in this condition obtains high O atoms compared to other biocarbons because of the contribution of oxygen in bulk metals along with the surfacetuned oxygen complexes (Mian and Liu 2019b).The pore tuning ideally starts after reaching a temperature above 700 °C (Fig. 2a).KOH intermediates, such as K 2 CO 3 and K 2 O, can be reduced by reactive C atoms.Concurrently, the reaction erodes C atoms from the surface (R2-R5).This C erosion is the primary micropore tuning process (Marsh and Rodríguez-Reinoso 2006b;Pinij et al. 2021).The byproducts of the reaction, K atoms, stay intercalated in the carbon matrix.Removing K atoms by acid washing also creates new pores.A high dose of activators may be required if an optimum porous framework is to be obtained because surface C erosion ascribes to the availability of KOH-derived reactive intermediates.For example, The surface area and porosity of sludge biochar increased from 853 and 0.34 to 1686 m 2 g −1 and 0.64 m 2 g −1 , respectively, as the KOH dose increased from 1 to 3 magnitude (Ros et al. 2006).
Sludge can be activated with H 3 PO 4 or ZnCl 2 to avoid toxic alkali use.These reagents are known as dehydrating reagents, while the dehydration ability of ZnCl 2 is far greater than that of H 3 PO 4 (Ros et al. 2006;Kapatel et al. 2022).Upon heating, these chemicals transform into liquid and are intercalated in the sludge structure (Marsh and Rodríguez-Reinoso 2006b).At preferential temperatures, the evaporation of activators creates cavities.Nevertheless, H 3 PO 4 has been reported as ineffectual in tuning sludge porosity, while it creates pores in many biocarbons (Ros et al. 2006;Heidarinejad et al. 2020;Kapatel et al. 2022).H 3 PO 4 lacks a stable structure and evaporates at a relatively low temperature (Li et al. 2015).After evaporation, the ash and other macromolecules in sludge could polymerize and diminish as-created pores.In contrast, ZnCl 2 has a stable structure and is reported as efficient in supporting sludge macromolecules during activation.Initially, ZnCl 2 hydrolysis forms Zn intermediates (R6), which intercalate into the sludge matrix and maintain stability till a high temperature (e.g., 850 °C) (Grabda et al. 2011).Meanwhile, the SiO 2 in sludge could polymerize with Zn intermediates and form new reactive species Zn 2 SiO 4 (R7).As the heat increases, Zn intermediates reduce and create micropores by eroding C atoms (1) et al. 2019).Zn atom has a boiling point of 907 °C.Heating at this temperature causes molecular Zn atoms to evaporate from biochar producing more micropores.At low temperatures, Zn can remain intercalated in the biochar matrix (Fig. 2b).The char yield of ZnCl 2 activation can be higher than that of the alkali activation due to the polymerization of ZnCl 2 with sludge macromolecules, formation of aromatic compounds, and selective reaction with carbon atoms (Heidarinejad et al. 2020).Besides, Sludge biochar through ZnCl 2 activation has more controllable pore size than other chemical activators (Marsh and Rodríguez-Reinoso 2006b;Shi et al. 2014;Wu et al. 2022).
Unlike carbon-rich biomasses, the suitability of chemical activation on sludge biochar production is highly related to their composition.For instance, Yao et al. (2012) applied a simple ZnCl 2 activation (without pre or post-treatment) on sludge collected from different sources and found that the surface area and porosity difference in activated biochar ranged from 23.5 to 297.5 m 2 g −1 and 0.07 to 0.2 cm 3 g −1 , respectively, which was due to the parent sludge composition.Municipal sludge typically contains higher organic fractions than industrial sewage due to bacterial treatment and organic-rich wastewater input.The organic, particularly the carbon content, has a significant opposite correlation with the ash content (Wang et al. 2023).Thus, organic-rich municipal sludge can be a suitable candidate for chemical activation since chemical activation is the process of creating pores on the surface by etching reactive carbon.On the other hand, high ash sludge processes a chance of removing excess carbon from the surface, resulting in high ash residues, which eventually lead to poor surface area and porosity. (6)

Physical activation
Physical activation is an economic process of creating pores on biocarbon.The process involves the diffusion of reactive gas, such as steam, CO 2 , or air, over the surface of reactants and tuning pores via eroding reactive C atoms by gas-surface C reactions (Marsh and Rodríguez-Reinoso 2006c;Yi et al. 2021).Typically, a high temperature (800-1000 °C) requires initiating the physical activation process (Zhu et al. 2017).Unfortunately, the activation condition is unfavorable for sludge for two causes: (1) low carbon content in sludge is susceptible to burn-off (Yang et al. 2021) and (2) high temperature leads to significant pore cracking (Mian et al. 2019a;Hu et al. 2022;Rangabhashiyam et al. 2022).Generally, heating above 800 °C reduces sludge biochar surface area and increases pore collapsing.In such a process, the application of reactive gases amplifies the burn-off of C atoms and produces high ash residues.For example, in an earlier study, adjusting the CO 2 gas flow from 0 to 100% has been reported as ineffective in sludge biochar morphological development compared to N 2 pyrolysis (Guo et al. 2021).In some exceptions, CO 2 or steam activation slightly improves sludge biochar morphology (Wu et al. 2022;Deng et al. 2023).Nevertheless, the overall output is inferior to other physically activated biocarbons.

Pyrolysis followed by activation
A two-step heating process, pyrolysis followed by activation, provides an opportunity to increase interfacial interactions between the activators and the carbon through deashing its surface (Feng et al. 2015a;Li et al. 2020c).It is an efficient method for designing abundant micropores on sludge-activated carbon (Shi et al. 2014;Cheng et al. 2021;Kapatel et al. 2022).Since sludge is rich in ash particles, removing ash after pyrolysis can open up new macro and mesoporous channels where the activators can infiltrate and increase the contacts between activators and the carbon surface.In addition, the activator can form strong bonds with surface oxygen groups, such as phenol and carboxylic groups, which assist the activators' homogeneous dispersion and anchoring over the surface (Illingworth et al. 2022).In a two-step process, activators only interact with the already prepared char surface, while the activators can intercalate into the bulk sludge macromolecules during mixing in a single-step process.As a result, the two-step process could require fewer activators than the single-step process since it could consume more activators by reacting with volatiles.Following the pyrolysis-deashing-activation path, sludge biochar with ultra-high surface area and porosity has been produced.For instance, pyrolysis-hydrofluoric acid (HF) deashing-KOH activation produced sludge biochar with a surface area of 2839 m 2 g −1 and porosity of 2.65 cm 3 g −1 (Feng et al. 2015a).Similarly, sludge-extracted organic pyrolysis-deashing-KOH activation constructed a surface area and porosity of 3473 m 2 g −1 and 1.77 cm 3 g −1 , respectively (Du et al. 2022).
For sludge physical activation, a two-step heating process is crucial since the reactive gas diffusion can be hindered by the outward movement of volatile organics in a single-step activation (Zhou et al. 2021).Besides, in a single step, the surface C atoms burn off leaving an ashrich surface, whereas the activated gases are unable to tune pores (Ros et al. 2006;Silva et al. 2016;Deng et al. 2023).Removing ash after pyrolysis can increase relative C contents (Feng et al. 2015a;Zhang et al. 2020a), which is the primary reactive site for gas activation, and construct new porous channels, which increase gas diffusion (Fig. 2c).The study by Alvarez et al. (2016) is an ideal example of this theory.While sludge biochar was activated with CO 2 without deashing, a rapid burn-off reduced almost all C atoms and enhanced ash contents to 89%.Deashing after pyrolysis enhanced porosity and C content.As a result, although activations possess allowed C burn-off, the resulting material obtained a high porosity and surface area.Table 1 lists some selective articles from the literature to demonstrate the role of activators and deashing on sludge biochar topography and chemical composition.

Inherent and added templates
Hard or soft template for producing highly porous carbon from polymers is a mature topic.The process involves infiltrating templates into a polymer or growing them on the template by polymerization of monomers, followed by stabilization and removing templates by an appropriate etcher to create an inverse replica of used templates (Wang et al. 2020a).Typically, this approach applies to polymer-based materials and is inapplicable for biomass due to rigidity.However, biopolymer porosity can be designed by template technique (Xing et al. 2017;Wang et al. 2020b).Municipal sludge possesses good polymeric properties because of its rich extracellular polymeric substances, such as polysaccharides and proteins.In particular, the residues of glycosaminoglycans, such as hexosamine, uronic acid/galactose, and sialic acids, can bind other polysaccharides and develop a hybrid polymeric structure with improved mechanical stability (Yu 2020).Glycoconjugates and pretentious components also possess diverse functional groups, such as amino, phenol, carboxyl, sulfhydryl, and phosphoric, which can crosslink with a wide range of micro-or nanoparticles (Guellil et al. 2001;Sarvajith and Nancharaiah 2023).Upon sludge dewatering, the organic, inorganic, and particulates in the matrix are aggregated and form a complex interconnected solid (Mowla et al. 2013).The dry sludge composite can be treated similarly to the template-impregnated polymers, in which the SiO 2 and other metals act as inherent templates.SiO 2 is typically inert to C atoms during carbonization; thus, it is widely used in template processes (Wang et al. 2020a).Inspired by this fact, recent studies have improved sludge pore architectures by removing inherent SiO 2 (Fig. 3a).As an example, a SiO 2 reduction from 48.3% to 8.7% improved sludge biochar porosity from 0.12 to 0.57 cm 3 g −1 and enhanced surface area three times more than pristine one (Zhang et al. 2020a).More similar examples are available in the literature (Feng et al. 2015a;Dai et al. 2019;Liang et al. 2020;Mian et al. 2020).Since sludge composition is uncertain, it is not possible to modulate a process of sludge biochar by inherent ash removal alone.Designing pores with a controlled dose of added templates may shed light on this process.The study by Li et al. (2020b) study on improving sludge biochar porosity by using seawater salt (e.g., NaCl) as a template is an ideal example of this perspective.Although NaCl is not thermochemically stable as SiO 2 (Jacobson et al. 2017), it can play a role in tuning porosity.The process involves intercalation-activation-crystallizationetching (Fig. 3b).The dehydration of sludge macromolecules can dissolve some NaCl and allow intercalation to as-created cavities at low temperatures.Infiltrated molten salt in pores transfers mass and additional heat, which can facilitate the volatilization of macromolecules and other minerals.Usually, NaCl melts at a temperature above 800 °C, but in the case of activating sludge, the innate minerals possess a eutectic system that reduces the melting point to 750 °C and facilitates NaCl conversion (Madsen et al. 2018).At such temperature, NaCl reduces its dimension from 2.8 to 0.76 μm and remains intercalated in the sludge matrix as crystals.After cooling and removing salts and their intermediates by acid washing many new mesopores and micropores are created.Following this cost-effective strategy, biochar surface area and porosity have been improved from 24 and 0.05 to 480 m 2 g −1 and 0.53 cm 3 g −1 , respectively.Similarly, Xin et al. (2022) attempted to introduce hydrophilic polyacrylamide as a soft template in sludge.Polyacrylamide as a template didn't play any remarkable role in sludge biochar porosity.It decomposes at a relatively low temperature (300 °C) (Xiong et al. 2018), while sludge polymerization and conversion continue at a high temperature.Therefore, low-temperature degradable templates or activators are unsuitable for sludge biochar pore modulation.Designing sludge porous structures using naturally available templates undoubtedly shows a cost-effective and facile upgradation path.However, more effort is required to usher sludge biochar acceptable properties.

Ash and template etching process
Removing SiO 2 from sludge biochar is a laborious process and depends on suitable etchant and etching conditions.It has been reported that Si from the solid sludge matrices can be etched by wet HF (Dai et al. 2019), KOH (Mian and Liu 2019a), NaOH (Wang and Wang 2019), and Na 2 CO 3 (Alvarez et al. 2016).Some stimuli, such as NH 2 OH, can amplify the process (Swarnalatha et al. 2020).HF is a popular etcher because of its rapid Si dissolution ability in an acidic environment (Dai et al. 2019;Liang et al. 2020;Du et al. 2022).The process involves two major reactions, such as elimination-addition and substitution reactions (Fig. 3c).Initially, the O − H of the silanol and the H − F of the H 2 F 2 or H 2 F coordinate parallelly and place the uncoordinated F atom of H 2 F close to the reaction epicenter Si atom.Then, the elimination and addition take place successively.Since − OH of silanol is a poor leaving group, external assistance, such as a closed system or heat, is required (Feng et al. 2015a).Once the Si − F forms on the SiO 2 matrix, the nucleophilic substitution reaction is quickly performed by HF 2 or H 2 F 2 to remove Si − F units from the SiO 2 matrix and produce an initial reactive silanol site (Knotter 2000).An HF and heat-induced Si-fly approach was reported as efficient in removing 100% of Si atoms from sludge biochar, consequently enhancing C contents from 11.6% to 84.7% (Feng et al. 2015a).Although HF is efficient in removing Si from sludge biochar, it discourages use for toxicity.Even a little exposure can cause devastating consequences (Cheong and Kim 2023).
Wet anisotropic etchants, such as KOH and NaOH, are an alternative to HF, while NaOH is more efficient than KOH because of its high corrosion ability towards Si and less toxicity (Joo et al. 2012;Cheng et al. 2018).In some studies, NaOH has been reported as more effective than HF in creating microspores (Wan et al. 2021;Xia et al. 2022).However, the drawback of alkali etchers is the requirement of a highdose, a long aging time, and additional energy support.
In the KOH etching process, H terminates from the Si surface because of the high electronegativity of − OH, consequently, forming silanol with two − OH dangling bonds and becoming susceptible to nucleophilic attack (Fig. 3d).The external heat provides adequate energy for executing the reaction and detaching SiOH 4 from the matrix (Pal et al. 2009;Monteiro et al. 2015).Zhao et al. (2020) reported that a sludge biochar treated with 4 M of NaOH at 25 °C for 24 h increased surface area (12.4 times) and porosity (7.08 times) compared with the untreated one by reducing Si content from 27.5% to 3.9%.More evidence of this approach is in Table 1.Research in template strategies for decorating sludge pores is still in the early stage and has adequate room for further development.

Pre-conditioning with biomass
Sludge conditioning with biomass is an efficient technique to modulate high-ash sludge (Mian et al. 2022).Municipal sludge with high organic content is conducive to conversion into biochar with improved morphology, while sludge derived from chemical treatment, such as Fenton sludge, pharmaceutical industrial sludge, and textile dying sludge, contains excessive ash and less organic fraction.This sludge endows biochar with low porosity and high metal leaching propensity and concurrently becomes inefficient for adsorption application.Conditioning high ash sludge with lignin-based waste biomass is a cost-effective valorization approach.The strategy inevitably upgrades sludge biochar physical properties, such as creating new pores, increasing stability, reducing metal leaching, and surface chemical properties, such as introducing oxygen functional groups and changing surface charge, which improves adsorption performance.However, the morphological change after carbonizing biomass-conditioned sludge is unpredictable and dependent on heating operating conditions (Dai et al. 2022b).For example, a blend of sludge and lignin carbonization reduces surface porosity due to increasing tar and surface aggregation (Dai et al. 2022c).Despite that, tuning numerous oxygen groups enhances 12.8 times adsorption performance.Heavy metals (HMs) leaching from high ash sludge is a potential risk for application (Yang et al. 2022).Lignocellulosic biomass conditioning can provide an organic framework for immobilizing HMs and TMs in the char and reduce toxicity (Li et al. 2020a).It has been reported that HMs could be stabilized up to 98% into sludge biochar by optimizing the co-conditioning and heating operating conditions (Mohamed et al. 2023).Consequently, more oxygen functional groups, due to the presence of cellulose, hemicellulose, and lignin, can be obtained.Besides, biomass conditioning also improves sludge dewatering ability.sludge particles are fine and possess jell or colloidal properties, which lead to poor water permeability (Houghton and Stephenson 2002).Additional biomass grains increase sludge permeability and compressibility by increasing pores and mechanical strength (Wu et al. 2020).

Pre-conditioning with metals
Although sludge is rich in ash, additional metal can be loaded on sludge biochar to alter the surface charge, tune new active sites, and, precisely, improve adsorption performance towards HMs.The topological development due to metal doping is minor and random.Some studies reported that metal integration improves porosity by altering surface texture (Chen et al. 2021b;Wang et al. 2022), while others found it blocks pores (Cheng et al. 2022;Li et al. 2022).In fact, less attention has been given to improving porosity while sludge is conditioned with metals since their role in HMs adsorption is minor compared to active sites.For example, a KOH-activated sludge biochar with a surface area of 997.7 m 2 g −1 showed significantly lower Cr (VI) adsorption ability than the ZVI-loaded sludge biochar with a surface area of 190 m 2 g −1 (Wang et al. 2022).Besides, the traditional organic surface functional groups play a minor role in HMs sorption (Chen et al. 2015b).Metal species on sludge biochar surface improves HMs adsorption by triggering new mechanisms, such as co-precipitation (Wang et al. 2022), ion exchange interaction (Chen et al. 2015a), electrostatic interaction (Ma et al. 2021a;Chen et al. 2022), and surface complexation (Marsh and Rodríguez-Reinoso 2006d;Chen et al. 2015b).
Metal species on sludge biochar can be introduced through precipitation or direct heating methods, while the liquid-phase precipitation method is the most popular.In the precipitation method, a reduced metal species is first produced using a reducing agent, and then it is anchored onto sludge biochar through aging in a preferable aqueous solution.Zerovalent iron, a reduced iron species produced by NaBH 4 conditioning in alkaline media (R12) (Devi and Saroha 2015), precipitated sludge biochar is the most reported one (Devi and Saroha 2015;Dai et al. 2022a;Wang et al. 2022;Xue et al. 2022;Zhang et al. 2022;Zheng et al. 2022).Precipitation is a less energyconsuming method for tuning sludge biochar with metal species.On the other hand, metal doping through heating is a straightforward method that involves the heating of metal and raw sludge or sludge biochar blend (Liu et al. 2020c).The carbothermal condition and generated reactive radicle species, such as CH 4 , H 2 , and CO, play a role in changing the metal phase and increasing surface charges.For example, The iron species produced during direct heating is sequentially reduced from Fe 3+ to Fe 2+ and then Fe 0 with increasing temperature (Mian et al. 2022).While metal-conditioned sludge biochar shows better affinity towards adsorbing polar compounds and HMs, it is less efficient in removing non-polar or large organic molecules for poor surface morphology.

Challenges in sludge biochar modulation and application
In past years, sludge biochar has been studied significantly because of sludge's rapid production and adverse environmental effects.Carbonized sludge biochar, despite showing remarkable environmental benefits along with pollutant removal abilities, has failed to acquire practicability.Thus, it is essential to understand the criteria of commercial activated carbon and the prerequisites for sludge-activated carbon to be acceptable for commercialization.

High-performance biochar modulation challenges
To date, a wide variety of biomass was carbonized or activated through different modulation approaches to produce a highly microporous framework with improved stability, while only a few selective biocarbon showed practicability.High carbon contents, hardness, bulk density, surface area, microporosity, and low ash and dusting tendency are crucial standards for AC (Gabelman 2017).Commercial AC used for water treatment ensures a considerable degree of those criteria.For example, the carbon content of raw coconut shell, a benchmark biomass for AC production, is approximately 45-50%, and ash content is below 1%.Carbonization of coconut shells leads to an increase of C contents up to 95% (Prauchner and Rodríguez-Reinoso 2012;Arena et al. 2016;Kabir Ahmad et al. 2022).Other wood-based biomass or lowgrade coal also shows a similar phenomenon.In contrast, the sludge C content is poor, but the ash content is high (29-89%) (Table 1).As a result, an economical and easy modification is ineffective for sludge morphological development.Although ash can be removed by ( 12) concentrated acid or alkali/HF treatment, it produces a large amount of semi-solid waste, which is a burden (Regkouzas and Diamadopoulos 2019).In addition, the composition of sludge varies significantly from industry to industry.For commercial product manufacturing, deviation in product quality and performance is unacceptable (Marsh and Rodríguez-Reinoso 2006a).High hardness or mechanical strength of carbon leads to low attrition, ensuring stability during filter use or backwash (Jjagwe et al. 2021).Sludge biochar must have considerable mechanical strength before being suitable for a long horizontal column or pressure filter bed.It has been reported that sludge biochar mechanical strength varies depending on its composition (Smith et al. 2012).Mesophilic anaerobic digested sludge biochar processes a high hardness equivalent to the commercial Filtrasorb 400, while most other granular sludge carbon shows an inferior attrition resistance.Sludge biochar with low stability poses risks of reducing its size during the pressured process, leaching carbon, or reducing water permeability.Besides the chances of attrition, the sludge biochar poses a risk of HMs or TMs release in the water during treatment.Sludge is a sink of diverse HMs, such as Cd, Hg, Zn, Cr, Pb, As, Ni, Cu, and Se.Some HMs can evaporate during pyrolysis, while others remain in solids (Mian et al. 2022).These HMs can be stabilized into the biochar upon carbonization, reducing the leaching risk (Li and Jiang 2017).However, the activation can reduce the strength of the material by creating inner porous channels, increasing the leaching risk as AC in the industry remains in the adsorption chamber for a long time and is reused for several cycles.The HMs immobilized in biochar typically show a high leaching tendency in acidic media.The discharge water in the industry is generally neutral in pH and has less possibility of interacting with metals compared to acidic water (Wang et al. 2022).However, more studies on the leaching tendency upon long-term adoption in filters and after regeneration are required before practical application.
The bulk density of typical ACs ranges from 250-750 kg m −3 .The high bulk density reduces pulverization during processing and is beneficial for carbon storage and transportation (Capareda 2022).The bulk density of raw sludge is higher than that of other biomass and most ACs.The increasing carbonization temperature increases sludge carbon bulk density, indicating the reduction of the inner pores and a high material compaction, while wood-based biomasses show the opposite trend (Titova and Baltrėnaitė 2021).The bulk density of sludge may be suitable for producing granular carbon, but concurrently improving morphology is challenging for higher compaction and poor porosity.Earlier studies used powdered sludge biochar with particle sizes ranging from < 0.1 mm to 0.25 mm (Table 1).The powder or small granular form of carbon is suitable for high adsorption but problematic for separation from water (Summers et al. 2010;Gabelman 2017).
The most important criteria for ranking AC quality are the porosity and surface area, particularly the microporous surface area.Macropores above 50 nm are considered open surfaces, mesopores between 2 and 50 nm are pollutant diffusion passages, and micropores below 2 nm are the adsorption epicenter (Marsh and Rodríguez-Reinoso 2006a).The high C content, hardness, bulk density, and low ash of biomaterials facilitate the design of a highly microporous framework using a simple adjustment path (Dittmann et al. 2022).For example, a low-cost charring-pacification-steam activation of coconut shells can produce a surface area above 1100 m 2 g −1 with 90% microporous area (Keppetipola et al. 2021;Dittmann et al. 2022;Homewater 2022), while chemical activation enhances it up to 1500-3000 m 2 g −1 (Keppetipola et al. 2021).On the contrary, typical physical activation methods are ineffective in improving sludge carbon morphology, while chemical activation with a high reagent dose can improve sludge surface porosity.Thus, micropore tuning process on sludge biochar is a costly process.Sludge biochar surface area can be enhanced up to 1000 m 2 g −1 by using a high dose of activators (Feng et al. 2015a;Dai et al. 2019;Du et al. 2022;Mian et al. 2023).In advanced cases, applying charring-deashing-chemical activation, sludge biochar surface area can be enhanced by more than 3000 m 2 g −1 (Du et al. 2022).However, due to the blockage of micropores by nanoscale metals in sludge, the ratio of micropores in sludge biochar is lower than in other activated biocarbons (Table 1).The obtained surface area generally ascribes to the higher degree of smaller mesopores or macropores.As a result, sludge biochar can have lower adsorption performance although it has a higher surface area than other highly microporous biocarbon.

Adsorption and regeneration challenges
High adsorption towards a wide variety of pollutants and facile regeneration ability marked the AC's potentiality in wastewater treatment.The adsorption performance of ACs towards pollutants is evaluated by their iodine content.It is the amount of iodine adsorbed per gram of carbon, particularly indicating the adsorption strength of micropores.Usually, commercial AC's iodine number ranges from 500 to 1200 mg g −1 , while most sludgebased adsorbent did not report their iodine number.The few exceptions are the studies by Li et al. (2020c) and Wu et al. (2022) contents in which sludge biochar iodine numbers were reported as 816.2 and ~ 401.4 mg g −1 , respectively.Highly microporous sludge biochar with a high surface area can exhibit good adsorption performance.For example, Liu and coworkers (2020a;2021b) improved tetracycline adsorption from 157.4 to 379.8 mg g −1 by updating sludge biochar surface area and porosity from 319.8 and 0.537 to 814.4 m 2 g −1 and 0.645 cm 3 g −1 , respectively.Typically, micropores that are 1.3 to 1.8 times larger than the target pollutants are convenient for adsorption, in which the material hydrophobicity provides additional support (Li et al. 2002).Unlike ACs, sludge biochar is mostly hydrophilic and contains a small microporous volume and surface area (Chen et al. 2014;Schumann et al. 2023).Thus, it is not timely yet to reveal the precise role of sludge porous structure on pollutant adsorption.Besides, numerous studies reported that sludge biochar with poor surface topography performs better than the highly porous carbon because of its surface functional groups and charges (Ma et al. 2020a(Ma et al. , 2020c(Ma et al. , 2021b;;Deng et al. 2023).
A side-by-side performance comparison between AC and sludge biochar is challenging because ACs are already applied in industries for adsorbing diverse organics, while sludge biochar is still in the laboratory stage and treated against some particular pollutants.Here, we present some pollutant removal evidence from the literature to give an insight into their adsorption strength.A list of prominent sludge-based adsorbents' performances is presented in Table 2. To date, sludge based-adsorbents are applied against dyes and organics (e.g., methylene blue, bisphenol A, chlorophenol), pharmaceuticals and personal care products (PPCPs) (e.g., tetracycline, sulfamethoxazole, ciprofloxacin, ofloxacin, norfloxacin, levofloxacin), and HMs (e.g., Cu 2+ , Pb 2+ , Cd 2+ , Cr 6+ , As 5+ , Ni 2+ , and U 6+ ).Based on available data, the best adsorption performance of sludge biochar towards methylene blue, tetracycline, sulfamethoxazole, ciprofloxacinremoval was reported as 95.3 (Li et al. 2020c), 379.8 (Liu et al. 2021b), 50.6 (Minaei et al. 2023), and 83.7 mg g −1 (Ma et al. 2020b), while the pollutant adsorption performance of commercial ACs is 240 (AC F300) (Stavropoulos and Zabaniotou 2005), 471 (Merck) (Ocampo-Pérez et al. 2015), 6728 (S208C) (Nielsen et al. 2014), 250 mg g −1 (NORIT ROX 0.8) (Carabineiro et al. 2011), respectively.The adsorption performance of Acs towards organic is several magnitudes higher than that of the sludge biochar.The highly microporous surface of AC endows it with a very high adsorption ability that outperforms sludge carbon (UMass 2007).However, despite poor surface morphology, metal-rich sludge carbon has the advantage over commercial AC in removing HMs (Tang et al. 2019).It manifests the applicability of different types of sludge biochar.The industrial wastewater-derived sludge biochar with high metal content can be a suitable material for removing various HMs, while the organic-rich municipal sludge-derived biochar can be modulated to produce highly porous structures to adsorb a diverse group of organic pollutants.
Adsorbent regeneration is essential for cost efficacy.Sludge biochar has shown considerable regeneration ability when applied against organic pollutants.Chemical regeneration, such as different doses of NaOH (Liu et al. 2020a(Liu et al. , 2021b)), HCl (Xin et al. 2022), EDTA (Ifthikar et al. 2017;Jiang et al. 2022), or alcohol (Ma et al. 2021c;Guo et al. 2022) eluting assisted by ultrasound, was usually used to regenerate spent sludge biochar.NaOH eluting assisted by ultrasound has been underlined as the most efficient method (Ma et al. 2022;Wu et al. 2022).However, sludge biochar exhibits poor regeneration tendency after HMs removal.HMs adsorption on the metal-doped sludge biochar is favored by solution alkalinity.In such conditions, a passivation layer is formed on sludge biochar that hinders metal leaching and endows it with maximum adsorption (Wang et al. 2022).For regeneration, acid eluting is used since the adsorption process of HMs is related to electrostatic and ion exchange.Acid treatment hinders the formation of passivation layers and results in significant metal leach (Singh and Srivastava 2020;Ma et al. 2021c;Xin et al. 2022).The regenerated sludge biochar thus loses active sites and shows poor performance in subsequent runs.Sometimes, the spent material is no longer usable for HMs adsorption (Diao et al. 2018;Liu et al. 2020b).On the other hand, the heating treatment, a benchmark process for activated carbon regeneration, has been reported as less efficient for sludge biochar regeneration since most sludge biochar is prepared at low temperatures, and many active sites, such as N groups or surface functional groups, can be altered by high-temperature heating (800-1000 °C) regeneration process (Marsh and Rodríguez-Reinoso 2006e).

Economic feasibility challenges
Economic profitability is the primary consideration for product commercialization.A sludge-based carbon can be introduced in manufacturing or reused onsite in industries to produce sludge-based carbon once it ensures profitability over the existing AC application.At the time of writing, the approximate price of AC in the USA is 2.7 US$ kg −1 , while it is 1.7 US$ kg −1 and 1.5 US$ kg −1 in India and China, respectively (Björklund and Li 2017;Index box 2023).AC can offer such prices due to its simple preparation process and cheaper raw materials.A techno-economic analysis based on Canadian standards estimated the cost of drying, carbonization, and CO 2 activation, with 18.4% carbon yield, could be 0.15 CA$ kg −1 , excluding maintenance, insurance, labor, and overhead costs (Mukherjee et al. 2022).Physical activation, particularly steam activation, is very popular with manufacturers because of its low processing cost (Heidarinejad et al. 2020).Unfortunately, sludge biochar cannot utilize the advantage of steam activation because of its low carbon contents.Physical activation is notorious for reducing carbon yield due to uncontrolled gasification from the outer surface and microdomains (Stavropoulos and Zabaniotou 2009;Yi et al. 2021).In case of high severity, carbon yield after steam activation can be as low as 2%, which eventually increases the production cost per unit of AC (Prauchner and Rodríguez-Reinoso 2012).
The price of AC is ascribed to its iodine content.The iodine content of sludge biochar can be enhanced by chemical activation.It is also more advantageous than physical activation processes in terms of high carbon yield (Prauchner and Rodríguez-Reinoso 2012).However, the activators are cost-intensive.For example, in the current Chinese market, the approximate prices per ton of NaOH, KOH, and ZnCl 2 were 380, 920, and 1690 US$, respectively.As listed in Table 1, most reported sludge biochar applied high doses of activators to achieve optimum morphology and adsorption performance.
The high activator dose plays a crucial role in biochar yield reduction.Thus, after different modulation steps and cleaning, the actual activator dose per biochar unit is enriched by several magnitudes, which eventually increases the production cost (Fig. 4a).Ultra-high surface area, similar to high-performance commercial AC, can be produced from sludge carbon through carbonizationdeashing-activation.However, the processes consume a high amount of chemicals and generate a large amount of byproducts, which is undesirable (Feng et al. 2015a;Du et al. 2022;Mian et al. 2023).
Considering the performance and production cost, ACs are more advantageous than sludge carbon.However, careful modulation, such as controlling chemical input, carbon yield, byproducts, and adsorption and regeneration performance, can reduce sludge carbon production costs.Minaei et al. (2023) reduced sludge biochar production cost to 1.2-1.3US$ kg −1 (excluding labor, tax, and overhead costs) by activating sludge pyrochar with 0.34 doses of ZnCl2.The cost can be further reduced by eliminating transportation costs and adopting advanced modulation techniques (He et al. 2022).Besides, altering sludge carbon to AC reduces the sludge disposal cost and other indirect costs, such as leachate management, greenhouse gas emission control, and building infrastructure, which are often disregarded (Li et al. 2019).Thus, concentrating efforts to innovate a feasible sludge carbon is worthwhile.Although no breakthrough so far, we believe that systematic research can shed light on this direction.An overview of different sludge biochar synthesis approaches' feasibility and their role in porosity is illustrated in Fig. 4b.

Role of sludge biochar-based adsorbents in carbon sequestration
Biochar disposal on land is one of the most efficient strategies for greenhouse gases (GHGs) (e.g., CO 2 , CH 4 , and N 2 O) reduction since its stable carbons are highly reluctant to decompose (e.g., a lifetime over 100 years) (Baskar et al. 2022).It is estimated that about 48-54% of net GHG reduction can be achieved by using biochar technology (Lehmann et al. 2021).In this context, producing sludge biochar through pyrolysis, utilizing it as an adsorbent, and landfilling spent biochar can be a win-win technology.The following sections detail the carbon sequestration potentials of sludge biochar and the necessity of its adsorption application for economic sustainability.

Sludge biochar carbon sequestration potential
Sludge is a great source of organic carbon whose volume is growing monotonously (Lishan et al. 2018;Bagheri et al. 2023).Through current disposal strategies, this waste contributes a high carbon emission to the atmosphere (Chen and Kuo 2016;Piippo et al. 2018).In 2019, Chinese sludge facilities emitted 108.18 × 10 8 kg CO 2eq (equivalent) , in which incineration, landfill, agricultural use, and building material contributed 45.11%, 23.04%, 17.64%, and 14.21%, respectively (Wei et al. 2020).Likewise, worldwide carbon emission from sludge is increasing as the conventional disposal strategy is practiced in most countries (Lishan et al. 2018;Faragò et al. 2022).
Carbon emission from sludge can be remarkably reduced by pyrolysis and capturing its byproducts.An industrial-scale sludge carbon footprint analysis by Sun et al. (2022a) reported that about 71% of carbon in the sludge biochar can be sequestered for over 100 years.Similarly,  2021) found that the CO 2eq emission per ton of dry sludge can be reduced to 1.607 tons (emitted from N-organic compounds, sludge drying, dewatering, and pyrolysis) from 2.432 tons (emitted from direct sludge disposal) by adopting the pyrolysis process.In addition to carbon sequestration, sludge pyrochar disposal on land doesn't create eutrophication or ecotoxicity.Therefore, pyrolytic conversion of sludge and the resulting materials' suitable application could be a green approach for carbon sequestration and safe sludge disposal.A concept of sludge processing and disposal methods for carbon sequestration and sustainability is illustrated in Fig. 5.The spent sludge-based adsorbent can be disposed of in landfills or reused as soil fertilizer through proper decontamination (Shepherd et al. 2016;Liu et al. 2019;Baskar et al. 2022).Usually, the spent AC, both hazardous and non-hazardous, ends its life cycle through regenerating, landfilling, or incinerating, while regeneration and landfilling are the most commonly used processes as incineration costs more (National Research Council 2009).Upon sludge biochar disposal, the fixed carbon (FC) can be retained in the land, while some labile carbon (LC) can re-enter the atmosphere through decomposition (Lehmann et al. 2021).A high ratio of FC in sludge biochar is thus essential.The FC contents in sludge-based adsorbent can be enhanced in three ways: (1) activating with alkali or minerals, (2) heating at a high temperature, and (3) amending with other biomass.Minerals, during carbonization, provide physical protection to carbon (e.g., encapsulating carbon or increasing aromaticity via catalyzing reaction) or crosslinks with them via chelating hydroxyl and ether groups, which increase the stable carbon ratio (Fuentes et al. 2008;Guo and Chen 2014;Ren et al. 2018).Si, Fe, P, Ca, Mg, Na, and K are reported as promising for that goal, where the role of K is superior (Buss et al. 2022).Sludge has the benefit of containing these minerals (Jellali et al. 2021).For the desired outcome, biochar stability and microporosity can be improved further by additional K reagents.For example, in a study, sludge pyrolysis and deashing enhanced FC from 0.98% to 66.7%, while post-KOH activation increased it to 87.61% (Feng et al. 2015a).Ash itching increases sludge biochar FC by reducing relative ash content, while activation plays a key role in altering LC to FC.The roadblock of the process is the high cost of K reagents, such as KOH and C 2 H 3 KO 2 at 920 and 1000 US$ per ton, respectively.A proper valorization of sludge biochar is thus required.It was calculated that, despite the mineral's high cost, activation is a cost-effective C sequestration approach compared to simple pyrolysis since it significantly enhanced the amount of FC in biochar (Buss et al. 2022).
Pyrolysis at a higher temperature is another efficient path for producing highly stable carbon by increasing carbon aromaticity (Nansubuga et al. 2015).The aromatic carbon shows high oxidation resistance and thus ensures long-term persistence upon land application (Ho et al. 2017).Forecasting based on an empirical study predicted that CO 2 emission reduction in ten years can be improved from 301 to 932 tons ha −1 by increasing sludge pyrolysis temperature from 300 to 600 °C (Méndez et al. 2013).Therefore, pyrolysis combined with activation can be a greener path for improving sludge C sequestration.The FC in the sludge biochar can be measured by approximate analysis at 900 °C or by calculating the H/ C org or O/C org ratio.Achieving a H/C org value of > 0.5 can Fig. 5 A concept of sludge processing and lifecycle closing for achieving carbon sequestration and environmental-economic sustainability.The carbon emission (Chen and Kuo 2016;Piippo et al. 2018), cost (National Research Council 2009), biochar performance (Zhang et al. 2020a;Wu et al. 2022), and FC and LC bars (Feng et al. 2015a;Li et al. 2020c) were generalized following the literature indicate a 50% carbon persistency of 100 years (Lehmann et al. 2021).

Sludge biochar adsorption application for carbon sequestration and economic sustainability
The primary obstacle to the biochar-based carbon sequestration process is its production cost.Biochar produced from different biomass can cost 120 to 400 US$ for per ton CO 2 reduction, where biomass costs contribute 35-55% (Buss et al. 2022).It is the primary cost of this process, followed by heating costs.Although sludge raw material cost could be low or null, carbonization at high temperatures and safe conversion are still cost-prohibitive.
A low-heat carbonization or a high-value sludge biochar application is essential to achieve economic sustainability.Hydrothermal carbonization or liquefaction of sludge uses a low temperature and produces hydrochar and bio-oil.Nevertheless, the carbon sequestration potential of the sludge hydrochar is poor because of the high LC.For example, 33 ± 8% of carbon was reported lost after one year of soil amendment by sludge hydrochar, and it was predicted that the remaining carbon half-life could be only 19 years (Malghani et al. 2015).Therefore, producing sludge biochar at high temperatures with activation and recovering production cost are crucial.
To date, sludge biochar, produced at high temperatures or activation, has found application in soil amendment, catalysis, and adsorption (Mian et al. 2022).The limitation of such biochar soil application is that it confers less accessible nutrients (e.g., N, P, and other minerals) for the plant roots because they form stable bonds with other atoms (Song et al. 2014).Achieving the goal of soil amendment at the same time economic sustainability is thus challenging.Catalytic application of sludge biochar has been reported efficient in pollutant decontamination (Mian et al. 2019a(Mian et al. , 2019b)).Nevertheless, the sludge biochar-based catalysis suffers cost efficacy.Currently, most industries use a Fenton-reaction process for pollutant degradation (e.g., activation of H 2 O 2 by FeSO 4 ).Replacing low-cost FeSO 4 (e.g., currently 60 US$ per ton), which performs rapid reaction through homogenous catalysis, with the heterogeneous sludge biochar, which exhibits slow reactivity through heterogeneous catalysis, is not economical.Besides, sludge biochar application as an efficient persulfate activator is also limited due to the environmental concern over persulfate byproducts and process cost.Considering these facts, sludge biochar as an alternative to high-cost activated carbon (e.g., cost ranges from 1500 to 2800 US$ per ton) should be worthwhile (Björklund and Li 2017;Index box 2023).Although the performance of sludge biochar is lower than that of ACs and high-cost sludge modulation processes were practiced previously, applying an advanced modulation using minimum chemicals could make significant progress in this field.Besides, the adsorbents are easily regeneratable compared to catalysts.The processing cost is thus possible to be reduced significantly.Furthermore, through this process, the industries may be eligible for claiming carbon credit (1 carbon credit is equivalent to 1 ton of CO 2 or equivalent GHG removal from the atmosphere (Oldfield et al. 2022)) as a significant amount of carbon can be sequestered by disposing of spent adsorbent in the land.Therefore, it can be stated that sludge biochar as an adsorbent is more practical than other applications.

Conclusions and perspectives
In recent years, tremendous efforts have been made toward developing sludge biochar morphology and adsorption performance.Nevertheless, a substantial gap remains in understanding the sludge biochar pore designing mechanism.Here, we detailed the pore-tuning mechanistic insights of different sludge carbon modulation methods, including pyrolysis, activation, template removal, and conditioning.Besides, roadblocks and feasible roadmaps for designing porous architecture are presented with suitable examples.In addition, sludge biochar's application challenges and role in carbon sequestration are detailed.Based on our understanding of key challenges in scaling up sludge carbon from the research stage to commercialization, we provide the following recommendations for assisting sludge carbon-based research.
Firstly and most importantly, more effort is required to explore feasible synthesis methods.While excess chemical or energy input improves sludge carbon morphology, the process is inapplicable.Instead, the facile template removal, conditioning (e.g., remediate sludge with C-rich municipal sludge), modulated physical activation, and low-chemical dose activation methods with suitable adjusting can be explored.The biochar particle shape and size should be designed considering actual applications.Granular carbon instead of powder should be prepared, and its mechanical and chemical strength should be evaluated to assess its applicability in pressure beds or filters, which has been overlooked in prior studies.
Secondly, sludge derived from different sources should be modulated according to their composition, and the resulting biochar should be applied for particular pollutant removal.It is essential to understand the applicable modification technique for different sludge compositions.For practicality, detailed measurement of the precursor, generated by-products (amount of ash, organic water, acid water, activator, and syngas), biochar yield, and preparation cost should be considered and correlated with performance.
Thirdly, sludge biochar should be used against diversified pollutants.Earlier studies applied sludge carbon only for some PPCPs, dyes, and HMs removal.It should also be applied to the treatment of other emerging contaminants, such as per-and polyfluoroalkyl substances, disinfection byproducts, natural organic matter, and volatile organics in the air.Comparative adsorption studies for removing dynamic pollutants, such as polar, nonpolar, hydrophilic, hydrophobic, long/short chain, or ionic, can be carried out to reveal the strength and weakness of sludge.
Fourthly, most of the earlier studies did not provide the complete set of sludge biochar information, which makes the efficiency of the synthesized material dubious.Reports regarding sludge-based adsorbents should contain some basic information, such as carbon yield (e.g., from precursor to the final product), ash content, particle size, mechanical strength, iodine number, surface topology with the share of micropores and mesopores, leaching propensity, adsorption energy distribution sites, and a list of all used chemical with doses.
Finally, collaboration the deployment of sludge biochar in industries requires collaboration between researchers and industry personnel.Besides, sludge biochar's carbon sequestration potential and economic value should be highlighted.Research on evaluating modified sludge biochar performance and cost, including all direct and indirect costs, can be performed to provide valuable knowledge to the stakeholders and sludge research community about the feasibility of a sludge carbon-based wastewater treatment process.

Fig. 1
Fig. 1 Annual growth of published research articles regarding sludge biochar-based adsorbents.(Source: Web of Science, accessed on 6 September 2023)

Fig. 2 a
Fig. 2 a Pore-forming mechanisms in KOH or NaOH activation, b ZnCl 2 and H 3 PO 4 activation, and c single-step and two steps physical activation of sludge biochar

Fig. 3 a
Fig. 3 a Yielding mesoporous channels and lamellar structure by removing SiO 2 and minerals for activation.Reproduced with permission from ref (Zhang et al. 2020a).Copyright 2020, Elsevier.b Melted NaCl acted as templates for sludge biochar mesopores development.Reproduced with permission from ref (Li et al. 2020b).Copyright 2020, Springer Nature.c Mechanism of SiO 2 dissolution in aqueous HF.Reproduced with permission from ref (Knotter 2000).Copyright 2000, American Chemical Society.d SiO 2 dissolution process in aqueous KOH.Reproduced with permission from ref (Pal et al. 2009) Copyright 2009, IEEE

Fig. 4
Fig. 4 Schematic illustration of feasibility and performance of sludge biochar synthesis approaches

Table 1
Sludge biochar modulation approaches and properties

Table 2
Pollutant removal by representative sludge biochar and regeneration performances