Binding mechanisms of Pb(II) adsorption by biochar-derived dissolved organic matter: unraveling site heterogeneity and kinetics through advanced spectral analysis

Abstract

Biochar-derived dissolved organic matter (DOM) is a highly active component that plays a critical and complex role in the immobilization of heavy metals. This study systematically investigated the impact of DOM on Pb(II) adsorption by comparing the adsorption capacities of biochar before and after DOM removal, thereby unveiling the underlying mechanisms through advanced spectroscopic techniques. Adsorption experiments demonstrated that water-washed biochar (WBC) exhibited a markedly reduced adsorption capacity (35.0 mg g⁻¹) compared to untreated biochar (BC) (96.2 mg g⁻¹), highlighting the essential role of DOM in enhancing Pb(II) adsorption. Kinetic and isothermal analyses revealed that the adsorption process was predominantly chemical in nature, as evidenced by the excellent fit of experimental data to the pseudo-second-order, Freundlich, and Temkin models. FTIR and XPS analyses confirmed that oxygen-containing functional groups, including hydroxyl, carboxyl, carbonyl, and ether groups, actively participated in Pb(II) complexation in BC, WBC, and DOM. Spectral shifts and changes in the relative abundance of C–O and C = O bonds further supported this conclusion. The Pb 4f spectra indicated that Pb(II) was primarily retained as Pb₃(OH)₂(CO₃)₂, with complexation identified as the dominant mechanism, followed by co-precipitation. UV differential log-transformed absorption spectra derived from titration experiments, revealed the heterogeneity of Pb(II) binding sites within DOM. Furthermore, excitation-emission matrix fluorescence spectroscopy coupled with parallel factor analysis (EEM-PARAFAC) identified three humic-like components. Among these, component C3 (humic-like and tyrosine substance) exhibited the strongest binding affinity for Pb(II). Hetero-2DCOS analysis, combined with additional spectroscopic techniques, demonstrated that carboxyl groups in humic-like substances were the most reactive sites for Pb(II) binding. These findings provide molecular-level insights into the structural and functional characteristics of biochar-derived DOM-Pb(II) complexes, offering a scientific basis for optimizing biochar-based strategies for heavy metal pollution remediation.

Keywords

Biochar-derived DOM / Lead (Pb) / Adsorption mechanism / Differential absorption spectra / EEM-PARAFAC / Two-dimensional correlation spectroscopy (2DCOS)

Highlight

•	Hetero-2DCOS/PARAFAC/UV-differential synergy decodes DOM-Pb(II) binding mechanisms.
•	The role of biochar-derived DOM in enhancing Pb(II) adsorption was investigated.
•	The UV humic-like substance (C1) reacts preferentially in humic-like components.
•	Carboxyl groups in humic substances play a crucial role in DOM-Pb(II) complexation.

References

[1] Ahmad M, Rajapaksha AU, Lim JE, Zhang M, Bolan N, Mohan D, Vithanage M, Lee SS, Ok YS. Biochar as a sorbent for contaminant management in soil and water: a review. Chemosphere, 2014, 99: 19-33.

[2] Aiken GR, Hsu-Kim H, Ryan JN. Influence of dissolved organic matter on the environmental fate of metals, nanoparticles, and colloids. Environ Sci Technol, 2011, 45: 3196-3201.

[3] Bai YC, Wu FC, Liu CQ, Li W, Guo JY, Fu PQ, Xing BS, Zheng J. Ultraviolet absorbance titration for determining stability constants of humic substances with Cu(II) and Hg(II). Anal Chim Acta, 2008, 616: 115-121.

[4] Chen J, Gu B, Leboeuf EJ, Pan H, Dai S. Spectroscopic characterization of the structural and functional properties of natural organic matter fractions. Chemosphere, 2002, 48: 59-68.

[5] Chen W, Habibul N, Liu XY, Sheng GP, Yu HQ. FTIR and synchronous fluorescence heterospectral two-dimensional correlation analyses on the binding characteristics of copper onto dissolved organic matter. Environ Sci Technol, 2015, 49: 2052-2058.

[6] Chen B, Huang W, Ma S, Feng M, Liu C, Gu X, Chen K. Characterization of chromophoric dissolved organic matter in the littoral zones of eutrophic lakes Taihu and Hongze during the algal bloom season. Water, 2018, 10: 861.

[7] Chen H, Li W, Wang J, Xu H, Liu Y, Zhang Z, Li Y, Zhang Y. Adsorption of cadmium and lead ions by phosphoric acid-modified biochar generated from chicken feather: selective adsorption and influence of dissolved organic matter. Bioresour Technol, 2019, 292. 121948

[8] Chen YD, Duan XG, Zhang CF, Wang SB, Ren NQ, Ho SH. Graphitic biochar catalysts from anaerobic digestion sludge for nonradical degradation of micropollutants and disinfection. Chem Eng J, 2020, 384: 11.

[9] Chen H, Yang X, Liu Y, Lin X, Wang J, Zhang Z, Li N, Li Y, Zhang Y. KOH modification effectively enhances the Cd and Pb adsorption performance of N-enriched biochar derived from waste chicken feathers. Waste Manag, 2021, 130: 82-92.

[10] Chen XT, Guo Z, Liu JM, Wu F, Cheng C, Lin H, Ren W, Zhang H. Electron transfer-based peroxydisulfate activation by waste herb residue biochar: Adsorption versus surface oxidation. Chem Eng J, 2023, 451: 12.

[11] Cui HY, Zhang SB, Zhao MY, Zhao Y, Wei ZM. Parallel faction analysis combined with two-dimensional correlation spectroscopy reveal the characteristics of mercury-composting-derived dissolved organic matter interactions. J Hazard Mater, 2020, 384. 121395

[12] Cui S, Lv J, Hough R, Fu Q, Zhang Z, Dong X, Fan X, Li YF. Imidacloprid removal by modified graphitic biochar with Fe/Zn bimetallic oxides. Environ Res, 2024, 258. 119444

[13] Dryer DJ, Korshin GV, Fabbricino M. In situ examination of the protonation behavior of fulvic acids using differential absorbance spectroscopy. Environ Sci Technol, 2008, 42: 6644-6649.

[14] Fan SS, Wang Y, Wang Z, Tang J, Tang J, Li XD. Removal of methylene blue from aqueous solution by sewage sludge-derived biochar: adsorption kinetics, equilibrium, thermodynamics and mechanism. J Environ Chem Eng, 2017, 5: 601-611.

[15] Fan QY, Sun JX, Quan GX, Yan JL, Gao JH, Zou XQ, Cui LQ. Insights into the effects of long-term biochar loading on water-soluble organic matter in soil: Implications for the vertical co-migration of heavy metals. Environ Int, 2020, 136: 9.

[16] Fan XB, Peng LL, Wang XF, Han SQ, Yang LZ, Wang HL, Hao C. Efficient capture of lead ion and methylene blue by functionalized biomass carbon-based adsorbent for wastewater treatment. Ind Crops Prod, 2022.

[17] Gao ZY, Guéguen C. Distribution of thiol, humic substances and colored dissolved organic matter during the 2015 Canadian Arctic GEOTRACES cruises. Mar Chem, 2018, 203: 1-9.

[18] Guo XJ, Peng YY, Li NX, Tian YY, Dai LC, Wu Y, Huang Y. Effect of biochar-derived DOM on the interaction between Cu(II) and biochar prepared at different pyrolysis temperatures. J Hazard Mater, 2022, 421. 126739

[19] Habibul N, Chen W. Structural response of humic acid upon binding with lead: a spectroscopic insight. Sci Total Environ, 2018, 643: 479-485.

[20] Hameed R, Li G, Son Y, Fang H, Kim T, Zhu C, Feng Y, Zhang L, Abbas A, Zhao X, Wang J, Li J, Dai Z, Du D. Structural characteristics of dissolved black carbon and its interactions with organic and inorganic contaminants: a critical review. Sci Total Environ, 2023, 872. 162210

[21] He B, Yun Z, Shi J, Jiang G. Research progress of heavy metal pollution in China: sources, analytical methods, status, and toxicity. Chin Sci Bull, 2012, 58: 134-140.

[22] He CJ, He XW, Li JJ, Luo Y, Li JC, Pei Y, Jiang JY. The spectral characteristics of biochar-derived dissolved organic matter at different pyrolysis temperatures. J Environ Chem Eng, 2021, 9. 9

[23] Hou JW, Liu DP, Wu FC, Gao HJ, Yu HB. Applying synchronous fluorescence spectroscopy combined with Gaussian band fitting and two-dimensional correlation to characterize interactions of copper (II) with dissolved organic matter from urban river sediments. J Environ Chem Eng, 2023, 11. 110038

[24] Hou EY, Jia XY, Wang LW, Mcgrath SP, Zhu YG, Hu Q, Zhao FJ, Bank MS, O’Connor D, Nriagu J. Global soil pollution by toxic metals threatens agriculture and human health. Science, 2025.

[25] Huang M, Li ZW, Huang B, Luo NL, Zhang Q, Zhai XQ, Zeng GM. Investigating binding characteristics of cadmium and copper to DOM derived from compost and rice straw using EEM-PARAFAC combined with two-dimensional FTIR correlation analyses. J Hazard Mater, 2018, 344: 539-548.

[26] Huang M, Li Z, Chen M, Wen J, Luo N, Xu W, Ding X, Xing W. Dissolved organic matter released from rice straw and straw biochar: contrasting molecular composition and lead binding behaviors. Sci Total Environ, 2020, 739. 140378

[27] Hur J, Lee BM. Characterization of binding site heterogeneity for copper within dissolved organic matter fractions using two-dimensional correlation fluorescence spectroscopy. Chemosphere, 2011, 83: 1603-1611.

[28] Janot N, Reiller PE, Korshin GV, Benedetti MF. Using spectrophotometric titrations to characterize Humic acid reactivity at environmental concentrations. Environ Sci Technol, 2010, 44: 6782-6788.

[29] Jiang S, Dai G, Liu Z, He T, Zhong J, Ma Y, Shu Y. Field-scale fluorescence fingerprints of biochar-derived dissolved organic matter (DOM) provide an effective way to trace biochar migration and the downward co-migration of Pb, Cu and As in soil. Chemosphere, 2022, 301. 134738

[30] Ke YX, Zhang FX, Zhang ZL, Hough R, Fu Q, Li YF, Cui S. Effect of combined aging treatment on biochar adsorption and speciation distribution for Cd(II). Sci Total Environ, 2023, 867. 10

[31] Lawaetz AJ, Stedmon CA. Fluorescence intensity calibration using the Raman scatter peak of water. Appl Spectrosc, 2009, 63: 936-940.

[32] Li Z, Ma Z, van der Kuijp TJ, Yuan Z, Huang L. A review of soil heavy metal pollution from mines in China: pollution and health risk assessment. Sci Total Environ, 2014, 468: 843-853.

[33] Li M, Zhang AF, Wu HM, Liu H, Lv JL. Predicting potential release of dissolved organic matter from biochars derived from agricultural residues using fluorescence and ultraviolet absorbance. J Hazard Mater, 2017, 334: 86-92.

[34] Li C, Wang Q, Lai Z, Mu J. One-step synthesis of novel nitrogen-doped biochar from N-rich waste straw particleboard for efficient Pb(II) adsorption. Microchem J, 2024.

[35] Li HY, Wang J, Zhu XJ. Adsorption of Pb(II) and Ag(I) using iron/manganese oxides modification biochar. Ind Crops Prod, 2024, 215. 11

[36] Liu YX, Lonappan L, Brar SK, Yang SM. Impact of biochar amendment in agricultural soils on the sorption, desorption, and degradation of pesticides: a review. Sci Total Environ, 2018, 645: 60-70.

[37] Liu D, Gao H, Yu H, Song Y. Applying eem-parafac combined with moving-window 2dcos and structural equation modeling to characterize binding properties of Cu (II) with DOM from different sources in an urbanized river. Water Res, 2022, 227. 119317

[38] Lou YM, Joseph S, Li LQ, Graber ER, Liu XY, Pan GX. Water extract from straw biochar used for plant growth promotion: an initial test. BioResources, 2016, 11: 249-266

[39] Lu Y, Yan M, Korshin GV. Spectroscopic study of interactions of lead (II) ions with dissolved organic matter: evidence of preferential engagement of carboxylic groups. Geochim Cosmochim Acta, 2017, 213: 308-316.

[40] Mohan D, Sarswat A, Ok YS, Pittman CU. Organic and inorganic contaminants removal from water with biochar, a renewable, low cost and sustainable adsorbent - a critical review. Bioresour Technol, 2014, 160: 191-202.

[41] Murphy KR, Stedmon CA, Wenig P, Bro R. OpenFluor– an online spectral library of auto-fluorescence by organic compounds in the environment. Anal Methods, 2014, 6: 658-661.

[42] Nazari S, Rahimi G, Nezhad AKJ. Effectiveness of native and citric acid-enriched biochar of Chickpea straw in Cd and Pb sorption in an acidic soil. J Environ Chem Eng, 2019, 7: 6.

[43] Noda I. Techniques useful in two-dimensional correlation and codistribution spectroscopy (2DCOS and 2DCDS) analyses. J Mol Struct, 2016, 1124: 29-41.

[44] Noda, I., Ozaki, Y, (2005). Two-dimensional Correlation Spectroscopy: Applications in Vibrational and Optical Spectroscopy. John Wiley & Sons.

[45] Romero CM, Engel RE, D'Andrilli J, Miller PR, Wallander R. Compositional tracking of dissolved organic matter in semiarid wheat-based cropping systems using fluorescence EEMs-PARAFAC and absorbance spectroscopy. J Arid Environ, 2019, 167: 34-42.

[46] Sebastian A, Nangia A, Prasad MNV. Cadmium and sodium adsorption properties of magnetite nanoparticles synthesized from Hevea brasiliensis Muell. Arg. bark: relevance in amelioration of metal stress in rice. J Hazard Mater, 2019, 371: 261-272.

[47] Shu Y, Kong F, He Y, Chen L, Liu H, Zan F, Lu X, Wu T, Si D, Mao J, Wu X. Machine learning-assisted source tracing in domestic-industrial wastewater: A fluorescence information-based approach. Water Res, 2024, 268. 122618

[48] Sondergaard M, Stedmon CA, Borch NH. Fate of terrigenous dissolved organic matter (DOM) in estuaries: aggregation and bioavailability. Ophelia, 2003, 57: 161-176.

[49] Tan X, Liu Y, Zeng G, Wang X, Hu X, Gu Y, Yang Z. Application of biochar for the removal of pollutants from aqueous solutions. Chemosphere, 2015, 125: 70-85.

[50] Tian XS, Chu SY, Hu YJ, Luo LZ, Lin XA, Wang H. Removal of heavy metals from single- and multi-metal solution by magnetic microalgae-derived biochar. J Water Process Eng, 2025, 69. 11

[51] Wan Z, Xu Z, Sun Y, He M, Hou D, Cao X, Tsang DCW. Critical impact of nitrogen vacancies in nonradical carbocatalysis on nitrogen-doped graphitic biochar. Environ Sci Technol, 2021, 55: 7004-7014.

[52] Wang Y, Wang L, Li ZT, Yang D, Xu JM, Liu XM. MgO-laden biochar enhances the immobilization of Cd/Pb in aqueous solution and contaminated soil. Biochar, 2021, 3: 175-188.

[53] Wang Y, Li J, Xu L, Xu Q, Wu D, Ai Y, Li D, Liu W, Qu J, Wang L, Zhang Y. The effect and spectral response mechanism of dissolved organic matter (DOM) in Pb(II) adsorption onto biochar. J Environ Chem Eng, 2023, 11. 111115

[54] Wang M, Zhu Y, Yue C, Ge H, Yan J, Yang Y, Quan G. Preparation of a novel magnetic calcium-based biochar for arsenic removal: behavior and dominant mechanism. Sep Purif Technol, 2024, 336. 126384

[55] Xing J, Xu G, Li G. Analysis of the complexation behaviors of Cu(II) with DOM from sludge-based biochars and agricultural soil: effect of pyrolysis temperature. Chemosphere, 2020, 250. 126184

[56] Xu H, Yan M, Li W, Jiang H, Guo L. Dissolved organic matter binding with Pb(II) as characterized by differential spectra and 2D UV-FTIR heterospectral correlation analysis. Water Res, 2018, 144: 435-443.

[57] Xu W, Jin Y, Zeng G. Introduction of heavy metals contamination in the water and soil: a review on source, toxicity and remediation methods. Green Chem Lett Rev, 2024, 17. 2404235

[58] Yamashita Y, Jaffé R. Characterizing the interactions between trace metals and dissolved organic matter using excitation-emission matrix and parallel factor analysis. Environ Sci Technol, 2008, 42: 7374-7379.

[59] Yan MQ, Wang DS, Korshin GV, Benedetti MF. Quantifying metal ions binding onto dissolved organic matter using log-transformed absorbance spectra. Water Res, 2013, 47: 2603-2611.

[60] Yan CX, Wang WY, Nie MH, Ding MJ, Wang P, Zhang H, Huang GX. Characterization of copper binding to biochar-derived dissolved organic matter: effects of pyrolysis temperature and natural wetland plants. J Hazard Mater, 2023, 442. 13

[61] Yang YK, Luo X, Zhang J, Ma XK, Sun PZ, Zhao L. Sewage sludge-coconut fiber co-pyrolysis biochar: mechanisms underlying synergistic heavy metal stabilization and ciprofloxacin adsorption. J Cleaner Prod, 2022, 375. 10

[62] Yang W, Lu C, Liang B, Yin C, Lei G, Wang B, Zhou X, Zhen J, Quan S, Jing Y. Removal of Pb(II) from aqueous solution and adsorption kinetics of corn stalk biochar. Separations, 2023, 10: 438.

[63] Ye QT, Wang P, Liang YZ, Li R, Shi ZQ. Modeling the equilibrium and kinetics of heavy metals reactions with dissolved organic matter. Appl Geochem, 2025, 181. 11

[64] Yue X, Chen X, Kang J, Xu Y, Zhang H. Insight into the efficacy and mechanism of persulfate activation using KHCO3 modified biochar: the overlooked contribution of adsorption. Chem Eng J, 2025, 516. 163781

[65] Zeng Z, Zheng P, Ding AQ, Zhang M, Abbas G, Li W. Source analysis of organic matter in swine wastewater after anaerobic digestion with EEM-PARAFAC. Environ Sci Pollut Res, 2017, 24: 6770-6778.

[66] Zhang XY, Su C, Liu XY, Liu ZG, Gu PX, Deng M, Liu Q. Periodical changes of dissolved organic matter (DOM) properties induced by biochar application and its impact on downward migration of heavy metals under flood conditions. J Cleaner Prod, 2020, 275. 123787

[67] Zhang XL, Cai XW, Wang ZW, Yang X, Li S, Liang GW, Xie XY. Insight into metal binding properties of biochar-derived DOM using EEM-PARAFAC and differential absorption spectra combined with two-dimensional correlation spectroscopy. Environ Sci Pollut Res, 2021, 28: 13375-13393.

[68] Zhang JL, Peng YT, Liu TZ, Wang ZH. Na related nanowhisker-containing biochar obtained from self-catalyzation and functionalization of papermaking black liquor lignin with superior heavy metal immobilization capability. Bioresour Technol, 2025, 417. 13

[69] Zhou X, Zhao C, Sun J, Cao Y, Yao K, Xu M. A deep learning method for predicting lead content in oilseed rape leaves using fluorescence hyperspectral imaging. Food Chem, 2023, 409. 135251

Funding

National Key R&D Program of China(2024YFD1501700)

Distinguished Youth Science Foundation of Heilongjiang Province, China(JQ2023E001)

Young Leading Talents Project of Northeast Agricultural University, China(NEAU2023QNLJ-013)

RIGHTS & PERMISSIONS

The Author(s)