Previous studies show mercury (Hg) can be effectively removed from solution by biochar, but limited attention was paid on the complexation between Hg and components released from biochars, e.g. dissolved organic matter (DOM). Here, aqueous data from batch-style experiments were modeled using PHREEQC, incorporating thermodynamic constants between Hg and DOM, which was assumed to be composed of thiol, carboxylic, and phenolic functional groups. Modelling results suggest that > 99% Hg complexed with thiol groups in DOM. The modelled concentrations of Hg–DOM complexes from low-T (low-temperature, 300°C) biochars were greater than from high-T (600°C) biochars. The concentrations of Hg–DOM complexes were lower in wood-based than in agricultural residue- and manure-based biochars. Hg–DOM complexes may affect Hg speciation, bioavailability, transport, and methylation processes. This research describes a method to evaluate Hg–DOM interactions, and the results indicate extra caution regarding Hg–DOM complex formation is required in the selection of biochar for Hg remediation.
Biochar has been proposed as a reactive material for removing mercury (Hg) from aqueous solutions and as an additive to stabilize Hg in soils and sediments by co-blending (Bussan et al. 2016; Gomez-Eyles et al. 2013; Liu et al. 2017, 2016; Shu et al. 2016; Zhang et al. 2018). A number of studies have documented moderate to high rates of Hg uptake by biochars (Boutsika et al. 2014; Gomez-Eyles et al. 2013; Tang et al. 2015). However, primary concerns related to the application of biochar for Hg removal are the facilitated Hg transport and the conversion of inorganic Hg to more toxic methyl Hg (MeHg).
The process of producing biochars leads to the stabilization of carbon and other elements and also to the release of soluble constituents, including dissolved organic matter (DOM), anions, major cations, major nutrients, and trace elements (Jin et al. 2016; Liu et al. 2015, 2018; Xie et al. 2016; Yargicoglu et al. 2015; Zornoza et al. 2016). These components released from biochar may affect Hg speciation, distribution, transport, and bioavailability. Further understanding of the complexation between Hg and released components is required to assess the potential impacts of the use of biochar as a reactive material for removal of Hg and other contaminants.
Previous studies show DOC comprises < 0.1%–3% of the total C in biochar (Liu et al. 2015). These carbon forms can be utilized as electron donors by potential Hg methylators to convert Hg to MeHg (Desrochers et al. 2015; Kerin et al. 2006; Paulson et al. 2016). DOM is also widely known to form strong complexes with Hg2+ ions, a process that can affect Hg speciation (Benoit et al. 2001). DOM can potentially enhance the transport of Hg in flood plain soil, fresh water, and sediment pore water either by limiting adsorption onto solid phases or enhancing the solubility of Hg precipitates (Haitzer et al. 2002; Ravichandran 2004; Wallschläger et al. 1996). Therefore, improved understanding of the formation of Hg–DOM complexes can assist in understanding Hg complexation and bioavailability in aqueous solution.
Liu et al. (2015, 2016) evaluated a series of biochars produced from a variety of feedstocks at different pyrolysis temperatures. These studies focused on the effectiveness and mechanisms of Hg uptake by different groups of biochars, the forms of S and other functional groups in the biochars, and the release of soluble carbon and sulfur constituents from the biochars. The present study complements this previous work by modelling the potential formation of Hg–DOM and other complexes. These new findings are integrated with the results of Liu et al. (2015, 2016) with the overall goal of identifying biochar types that optimize Hg uptake but have minimal impacts on aqueous Hg speciation.
Materials and Methods
Thirty-six biochar samples were evaluated for Hg complexation in batch-style experiments. The feedstocks included corn cobs (CC), corn stover (CS), cocoa husk (CA), cotton seed husk (CT), wheat shaft (GR2), spent hops (GR3), switchgrass (GR4), pine mulch and bark (SW), poultry (MP) and cattle (MB) manures, and mushroom soils (MU). The feedstocks were pyrolyzed at 300°C (low-T) or 600°C (high-T). In addition, three commercial products were purchased from Wicked Good Charcoal Co. (CL1), Cowboy Charcoal Co. (CL2), and Biochar Engineering Corp. (CL5). Two batches of activated carbon (AC1 and AC2) were used as benchmark materials (Sigma-Aldrich Corp.). The experiment was conducted by mixing 2 g of biochar with 150 mL (mass ratio as 1:75) of 10 µg L−1 Hg-spiked water, representative of environmental Hg concentrations (Ranchou-Peyruse et al. 2009). The ratio is selected to differentiate the removal percentages of Hg using different biochars (Liu et al. 2016). No other metals were added and the mixture was reacted for 2 days. At the termination of the experiment, aqueous phase was analyzed for total Hg, anions, cations, DOC, nutrient, and trace elements. The ionic strength of the majority of the solutions were < 0.004 mol L−1. Details of the biochar preparation method and experiments are described elsewhere (Liu et al. 2015, 2016).
Speciation modeling was conducted to assess the extent of complexation of Hg with inorganic species and thiol, carboxylic, and phenolic functional groups of DOM. The model calculations were executed with PHREEQCi (Parkhurst and Appelo 1999) using the MINTEQA2 database (Allison et al. 1991). The database was modified by adding thermodynamic reaction constants for Hg2+ and other metals (Al3+, Mg2+, Ca2+, Cu2+, Zn2+, Fe3+, and Fe2+) with thiol, carboxylic, and phenolic ligands (Table 1). The pH, Eh, and concentrations of cations, anions, alkalinity, OAs, and DOC (Liu et al. 2015, 2016) were used as model inputs.
where [DomsH], [DomcooH], and [DomoH] respectively represent the concentrations of thiol, carboxylic, and phenolic ligands in mmol L−1; and [DOM] and [DOC] respectively represent the concentrations of DOM and DOC in mg L−1. DOM concentrations are considered to be two times DOC concentrations by mass (Dong et al. 2010). In Eq. 1, F1 is the total sulfur content percentage in DOM by mass, F2 is the percentage of reduced sulfur content (thiol ligand) in total sulfur, F3 is the percentage of reactive thiol ligand in the reduced sulfur content, and WS is the atomic weight of sulfur. In Eqs. 2 and 3, C1 and C2 are the conversion factors for carboxylic and phenolic ligands from DOM, respectively. F1, F2, and F3 are taken to be 0.86% (Benoit et al. 2001; Dong et al. 2010; Haitzer et al. 2003), 50% (Haitzer et al. 2003; Skyllberg et al. 2006), and 2% (Skyllberg et al. 2006), respectively. C1 and C2 are taken to be 9.5 and 4.1 mmol g−1 DOC (Lu and Allen 2002). For samples with concentrations below the method detection limit (MDL), a value of half MDL was entered for modelling purposes (Ettler et al. 2007).
Results and Discussion
The results of the batch experiments in terms of DOC, SO42−, total Hg (tHg) concentrations and Hg removal are presented in Liu et al. (2015, 2016). Overall, Hg removal of > 95% was observed for the majority of the biochars. Sulfate concentrations ranged from 6.0 to 1000 mg L−1 in solutions mixed with the biochars and were elevated compared with the control. Concentrations of DOC varied as a function of pyrolysis temperature and raw materials. The highest concentration of DOC (148 mg L−1) was observed in the batch mixture containing high-T corn stover (CS1) biochar. Relatively high concentrations of DOC were observed in the batch mixtures containing low-T agricultural residue- and manure-based biochars.
The modelling results indicate that most of the Hg in the solution was bound to thiol groups (Doms− or DomscooDoms2−, > 99%) of DOM (Fig. 1). The modelled concentrations of Hg–DOM complexes were greater for low-T than for high-T biochars. The concentrations of Hg–DOM complexes were lower in charcoal and activated carbon than in other biochars as a result of low concentrations of both Hg in the spiked river water samples and DOC in the charcoal and activated carbon biochars (Liu et al. 2015, 2016). The only samples with Hg(OH)2(aq) as a major species were the control (river water) and high-T poultry manure and mushroom soil biochars, likely due to low concentrations of DOC and high pH (Liu et al. 2015).
The predominance of Hg–DOM complexes is likely due to the low Hg concentrations and the presence of abundant thiol functional groups (Liu et al. 2016). The greater concentrations of Hg–DOM complexes in solutions mixed with low-T biochars vs. high-T biochars are likely due to the much greater DOC concentrations released by low-T biochars (Liu et al. 2015). The relative prevalence of Hg(OH)2(aq) species in high-T biochars may be due to the high pH values and low DOC concentrations. When biochar is used in the field for contaminated water remediation, the geochemical parameters, solid to solution ratio, and concentrations of tHg, DOC, cations, and anions are different than considered here. Therefore, the speciation of Hg in the resulting remediated water may also be different and must be modeled using the field data.
The presence of Hg–DOM complexes may affect the speciation, transformation, and bioavailability of Hg. Complexation between Hg and DOM may inhibit Hg2+ sorption onto surfaces of minerals and facilitate the transport of Hg from polluted soils and sediments to rivers, lakes, and groundwater (Haitzer et al. 2002, Hsu-Kim et al. 2013; Liang et al. 2013; Wallschläger et al. 1996). For example, a study on Hg at Matagami Lake, Québec (Canada) shows an increase of tHg content correlated well with an increase of organic matter in the sediment cores (Moingt et al. 2014). With respect to the biogeochemical role of DOM in Hg methylation, early findings indicate the bioaccumulation of Hg in food webs may be decreased by lowering the bioavailability of Hg(II) for methylation (Benoit et al. 2001), because the DOM molecules are generally too large for microbial uptake. More recent findings suggest Hg–DOM complexes promote Hg methylation by increasing the solubility of Hg compounds (Hsu-Kim et al. 2013) and providing carbon sources for Hg methylators (Chiasson-Gould et al. 2014).
Constants, including F1, F2, F3, C1, and C2, were applied in the current study to calculate the concentrations of thiol, carboxylic and phenolic functional groups using the concentrations of DOM. The elemental composition of DOM from different sources (e.g. different types of biochar) may vary (Jamieson et al. 2014; Mancinelli et al. 2017), and the constants may also vary among the different DOM types. Therefore, future studies are required to characterize DOM and provide more data to measure or calculate the concentrations of the functional groups.
In combination with the results of Hg removal and DOC release from previous batch experiments, wood- and agricultural residue-based high-T biochars are promising reactive media for Hg removal from aqueous solution. These biochars showed high Hg removal percentages, limited release of DOC, and lower Hg–DOM complexes than the other biochars evaluated in this study.
Allison JD, Brown DS, Novo-Gradac KJ (1991) MINTEQA2/PRODEFA2: a geochemical assessment model for environmental systems. Version 3.0 user’s manual. Environmental Protection Agency, Washington, DC
Benoit JM, Mason RP, Gilmour CC, Aiken GR (2001) Constants for mercury binding by dissolved organic matter isolates from the Florida Everglades. Geochim Cosmochim Acta 65:4445–4451
Berthon G (1995) Critical evaluation of the stability-constants of metal-complexes of amino-acids with polar side-chains. Pure Appl Chem 67:1117–1240
Boutsika LG, Karapanagioti HK, Manariotis ID (2014) Aqueous mercury sorption by biochar from malt spent rootlets. Water Air Soil Poll 225:1–10
Bryan SE, Tipping E, Hamilton-Taylor J (2002) Comparison of measured and modelled copper binding by natural organic matter in freshwaters. Comp Biochem Physiol C 133:37–49
Bussan DD, Sessums RF, Cizdziel JV (2016) Activated carbon and biochar reduce mercury methylation potentials in aquatic sediments. Bull Environ Contam Toxicol 96:536–539
Chiasson-Gould SA, Blais JM, Poulain AJ (2014) Dissolved organic matter kinetically controls mercury bioavailability to bacteria. Environ Sci Technol 48:3153–3161
Desrochers KAN, Paulson KMA, Ptacek CJ, Blowes DW et al (2015) Effect of electron donor to sulfate ratio on mercury methylation in floodplain sediments under saturated flow conditions. Geomicrobiol J 32:924–933
Dong W, Liang L, Brooks S, Southworth G et al (2010) Roles of dissolved organic matter in the speciation of mercury and methylmercury in a contaminated ecosystem in Oak Ridge, Tennessee. Environ Chem 7:94–102
Ettler V, Mihaljevič M, Šebek O, Grygar T (2007) Assessment of single extractions for the determination of mobile forms of metals in highly polluted soils and sediments-analytical and thermodynamic approaches. Anal Chim Acta 602:131–140
Gismera MJ, Procopio JR, Sevilla MT (2007) Characterization of mercury-humic acids interaction by potentiometric titration with a modified carbon paste mercury sensor. Electroanal 19:1055–1061
Gomez-Eyles JL, Yupanqui C, Beckingham B, Riedel G et al (2013) Evaluation of biochars and activated carbons for in situ remediation of sediments impacted with organics, mercury, and methylmercury. Environ Sci Technol 47:13721–13729
Haitzer M, Aiken GR, Ryan JN (2002) Binding of mercury(II) to dissolved organic matter: the role of the mercury-to-DOM concentration ratio. Environ Sci Technol 36:3564–3570
Haitzer M, Aiken GR, Ryan JN (2003) Binding of mercury(II) to aquatic humic substances: influence of pH and source of humic substances. Environ Sci Technol 37:2436–2441
Hsu-Kim H, Kucharzyk KH, Zhang T, Deshusses MA (2013) Mechanisms regulating mercury bioavailability for methylating microorganisms in the aquatic environment: a critical review. Environ Sci Technol 47:2441–2456
Jamieson T, Sager E, Guéguen C (2014) Characterization of biochar-derived dissolved organic matter using UV-visible absorption and excitation-emission fluorescence spectroscopies. Chemosphere 103:197–204
Jin Y, Liang XQ, He MM, Liu Y et al (2016) Manure biochar influence upon soil properties, phosphorus distribution and phosphatase activities: a microcosm incubation study. Chemosphere 142:128–135
Kerin EJ, Gilmour CC, Roden E, Suzuki MT et al (2006) Mercury methylation by dissimilatory iron-reducing bacteria. Appl Environ Microbiol 72:7919–7921
Laglera LM, van den Berg CMG (2003) Copper complexation by thiol compounds in estuarine waters. Mar Chem 82:71–89
Liang P, Li YC, Zhang C, Wu SC et al (2013) Effects of salinity and humic acid on the sorption of Hg on Fe and Mn hydroxides. J Hazard Mater 244–245:322–328
Liu P, Ptacek CJ, Blowes DW, Berti WR et al (2015) Aqueous leaching of organic acids and dissolved organic carbon from various biochars prepared at different temperatures. J Environ Qual 44:684–695
Liu P, Ptacek CJ, Blowes DW, Landis RC (2016) Mechanisms of mercury removal by biochars produced from different feedstocks determined using X-ray absorption spectroscopy. J Hazard Mater 308:233–242
Liu P, Ptacek CJ, Blowes DW, Finfrock YZ et al (2017) Stabilization of mercury in sediment by using biochars under reducing conditions. J Hazard Mater 325:120–128
Liu P, Ptacek CJ, Blowes DW, Gould WD (2018) Control of mercury and methylmercury in contaminated sediments using biochars: a long-term microcosm study. Appl Geochem 92:30–44
Lu YF, Allen HE (2002) Characterization of copper complexation with natural dissolved organic matter (DOM)—link to acidic moieties of DOM and competition by Ca and Mg. Water Res 36:5083–5101
Mancinelli E, Baltrėnaitė E, Baltrėnas P, Marčiulaitienė E et al (2017) Dissolved organic carbon content and leachability of biomass waste biochar for trace metal (Cd, Cu and Pb) speciation modelling. J Environ Eng Landsc Manag 25:354–366
Moingt M, Lucotte M, Paquet S, Ghaleb B (2014) Deciphering the impact of land-uses on terrestrial organic matter and mercury inputs to large boreal lakes of central Québec using lignin biomarkers. Appl Geochem 41:34–48
Parkhurst DL, Appelo C (1999) User’s guide to PHREEQC (Version 2): a computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations. US Geological Survey, Denver
Paulson KMA, Ptacek CJ, Blowes DW, Gould WD et al (2016) Role of organic carbon sources and sulfate in controlling net methylmercury production in riverbank sediments of the South River, VA (USA). Geomicrobiol J 35:1–14
Ranchou-Peyruse M, Monperrus M, Bridou R, Duran R et al (2009) Overview of mercury methylation capacities among anaerobic bacteria including representatives of the sulphate-reducers: implications for environmental studies. Geomicrobiol J 26:1–8
Ravichandran M (2004) Interactions between mercury and dissolved organic matter—a review. Chemosphere 55:319–331
Shu R, Wang Y, Zhong H (2016) Biochar amendment reduced methylmercury accumulation in rice plants. J Hazard Mater 313:1–8
Skyllberg U, Bloom PR, Qian J, Lin CM et al (2006) Complexation of mercury(II) in soil organic matter: EXAFS evidence for linear two-coordination with reduced sulfur groups. Environ Sci Technol 40:4174–4180
Smith RM, Martell AE, Motekaitis RJ (2004) Critically selected stability constants of metal complexes, Version 8.0. Washington DC
Tang J, Lv H, Gong Y, Huang Y (2015) Preparation and characterization of a novel graphene/biochar composite for aqueous phenanthrene and mercury removal. Bioresour Technol 196:355–363
Temminghoff EJM, Plette ACC, Van Eck R, Van Riemsdijk WH (2000) Determination of the chemical speciation of trace metals in aqueous systems by the Wageningen Donnan Membrane Technique. Anal Chim Acta 417:149–157
Wallschläger D, Desai MVM, Wilken RD (1996) The role of humic substances in the aqueous mobilization of mercury from contaminated floodplain soils. Water Air Soil Poll 90:507–520
Xie T, Sadasivam BY, Reddy KR, Wang C et al (2016) Review of the effects of biochar amendment on soil properties and carbon sequestration. J Hazard Toxic Radioact Waste 20:04015013, 04015011, 04015014
Yargicoglu EN, Sadasivam BY, Reddy KR, Spokas K (2015) Physical and chemical characterization of waste wood derived biochars. Waste Manag 36:256–268
Zhang Y, Liu Y-R, Lei P, Wang Y-J et al (2018) Biochar and nitrate reduce risk of methylmercury in soils under straw amendment. Sci Total Environ 619–620:384–390
Zornoza R, Moreno-Barriga F, Acosta JA, Muñoz MA et al (2016) Stability, nutrient availability and hydrophobicity of biochars derived from manure, crop residues, and municipal solid waste for their use as soil amendments. Chemosphere 144:122–130
This research was funded by the Natural Sciences and Engineering Research Council of Canada (NSERC) and E. I. du Pont de Nemours and Company. We are grateful for the advice and assistance from R. Landis, N. Grosso, E. Mack, J. Dyer, and the South River Science Team. We also thank A.O. Wang, K. Paulson, Y.Y. Liu, J. Ma, L. Groza, and J. Hu for analytical assistance and advice on the experimental set-up and data analysis.
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Liu, P., Ptacek, C.J. & Blowes, D.W. Mercury Complexation with Dissolved Organic Matter Released from Thirty-Six Types of Biochar. Bull Environ Contam Toxicol 103, 175–180 (2019). https://doi.org/10.1007/s00128-018-2397-2
- Dissolved organic matter