Co-engineering biochar and artificial humic substances: advancing photoreduction performance through structure design

Abstract

Engineered biochar with enhanced photochemical properties holds great potential for environmental remediation. However, natural humic substances, crucial players in environmental redox processes, are structurally complex and slow-forming, hindering mechanistic insights and practical applications. Here, we propose a co-engineering strategy that combines biochar with artificial humic substances synthesized from pine sawdust via controlled hydrothermal humification (180–340 °C). Modulating the hydrothermal temperature can yield artificial humic substances with diverse degradation degrees of lignin, yielding tailored phenolic architectures and electron-donating capacities (EDC). Using Ag⁺ photoreduction as a model reaction, we demonstrate that artificial humic substances produced at 340 °C exhibit optimal phenol content and the strongest reducing capacity (19.2-fold greater than that of substances synthesized at 180 °C). Notably, higher molecular weight fractions (> 5 kDa) of artificial humic substances were found to dominate Ag⁺ photoreduction due to their enriched phenolic content and superior EDC. Mechanistic investigations reveal that photo-excited phenolic groups generate superoxide radical (O₂^•−), initiating Ag⁺ reduction via a ligand-to-metal charge transfer (LMCT) pathway. Moreover, we discovered a previously overlooked phenomenon: hydrochar undergoes photo-induced dissolution, further enhancing photoreduction. This work provides new insights into the temperature-dependent lignin transformation into redox-active artificial humic substances and highlights the dynamic photochemical behavior of engineered biochar (hydrochar) under solar irradiation.

Keywords

Engineered biochar / Hydrothermal humification / Artificial humic substance / Photoreduction / Electron-donating capacities / Dynamic photochemical behavior

References

[1] Adegboyega NF, Sharma VK, Siskova K, Zboril R, Sohn M, Schultz BJ, Banerjee S. Interactions of aqueous Ag⁺ with fulvic acids: mechanisms of silver nanoparticle formation and investigation of stability. Environ Sci Technol, 2013, 47: 757-764

[2] Aeschbacher M, Sander M, Schwarzenbach RP. Novel electrochemical approach to assess the redox properties of humic substances. Environ Sci Technol, 2010, 44: 87-93

[3] Akaighe N, Maccuspie RI, Navarro DA, Aga DS, Banerjee S, Sohn M, Sharma VK. Humic acid-induced silver nanoparticle formation under environmentally relevant conditions. Environ Sci Technol, 2011, 45: 3895-3901

[4] Chen J, Fan X, Zhang L, Chen X, Sun S, Sun RC. Research progress in lignin-based slow/controlled release fertilizer. Chemsuschem, 2020, 13: 4356-4366

[5] Dong B, Liu G, Zhou J, Wang J, Jin R. Transformation of silver ions to silver nanoparticles mediated by humic acid under dark conditions at ambient temperature. J Hazard Mater, 2019, 383 121190

[6] Dong B, Liu G, Zhou J, Wang J, Jin R, Zhang Y. Effects of reduced graphene oxide on humic acid-mediated transformation and environmental risks of silver ions. J Hazard Mater, 2020

[7] Guo X, Yin Y, Tan Z, Liu J. Environmentally relevant freeze-thaw cycles enhance the redox-mediated morphological changes of silver nanoparticles. Environ Sci Technol, 2018, 52: 6928-6935

[8] Hao S, Zhu X, Liu Y, Qian F, Fang Z, Shi Q, Zhang S, Chen J, Ren ZJ. Production temperature effects on the structure of hydrochar-derived dissolved organic matter and associated toxicity. Environ Sci Technol, 2018, 52: 7486-7495

[9] Hou WC, Stuart B, Howes R, Zepp RG. Sunlight-driven reduction of silver ions by natural organic matter: formation and transformation of silver nanoparticles. Environ Sci Technol, 2013, 47: 7713-7721

[10] Huang Y-N, Qian T-T, Dang F, Yin Y-G, Li M, Zhou D-M. Significant contribution of metastable particulate organic matter to natural formation of silver nanoparticles in soils. Nat Commun, 2019, 10: 3775

[11] Jain S, Mehata MS. Medicinal plant leaf extract and pure flavonoid mediated green synthesis of silver nanoparticles and their enhanced antibacterial property. Sci Rep, 2017, 7: 15867

[12] Khan Z, Yang X-J, Fu Y, Joseph S, Khan MN, Khan MA, Alam I, Shen H. Engineered biochar improves nitrogen use efficiency via stabilizing soil water-stable macroaggregates and enhancing nitrogen transformation. Biochar, 2023, 5: 52

[13] Liu H, Ge Q, Xu F, Qu X, Fu H, Sun J. Dissolved black carbon induces fast photo-reduction of silver ions under simulated sunlight. Sci Total Environ, 2021, 775: 145897

[14] Li L, Wang X, Fu H, Qu X, Chen J, Tao S, Zhu D. Dissolved black carbon facilitates photoreduction of Hg(II) to Hg(0) and reduces mercury uptake by lettuce (Lactuca sativa L.). Environ Sci Technol, 2020, 54: 11137-11145

[15] Li M, Dang F, Fu Q-L, Zhou D-M, Yin B. Effects of molecular weight-fractionated natural organic matter on the phytoavailability of silver nanoparticles. Environ Sci Nano, 2018, 5: 969-979

[16] Lv J, Han R, Huang Z, Luo L, Cao D, Zhang S. Relationship between molecular components and reducing capacities of humic substances. ACS Earth Space Chem, 2018, 2: 330-339

[17] Nie X, Zhu K, Zhao S, Dai Y, Tian H, Sharma VK, Jia H. Interaction of Ag⁺ with soil organic matter: elucidating the formation of silver nanoparticles. Chemosphere, 2019, 243 125413

[18] Okolie JA, Nanda S, Dalai AK, Kozinski JA. Optimization studies for hydrothermal gasification of partially burnt wood from forest fires for hydrogen-rich syngas production using Taguchi experimental design. Environ Pollut, 2021, 283 117040

[19] Peng X-X, Gai S, Cheng K, Yang F. Roles of humic substances redox activity on environmental remediation. J Hazard Mater, 2022, 435 129070

[20] Peretyazhko TS, Zhang Q, Colvin VL. Size-controlled dissolution of silver nanoparticles at neutral and acidic pH conditions: kinetics and size changes. Environ Sci Technol, 2014, 48: 11954-11961

[21] Qian F, Zhu X, Liu Y, Shi Q, Wu L, Zhang S, Chen J, Ren ZJ. Influences of temperature and metal on subcritical hydrothermal liquefaction of hyperaccumulator: implications for the recycling of hazardous hyperaccumulators. Environ Sci Technol, 2018, 52: 2225-2234

[22] Rtimi S, Dionysiou DD, Pillai SC, Kiwi J. Advances in catalytic/photocatalytic bacterial inactivation by nano Ag and Cu coated surfaces and medical devices. Appl Catal B Environ, 2019, 240: 291-318

[23] Tang C, Li Y, Song J, Antonietti M, Yang F. Artificial humic substances improve microbial activity for binding CO₂. iScience, 2021, 24 102647

[24] Walpen N, Getzinger GJ, Schroth MH, Sander M. Electron-donating phenolic and electron-accepting quinone moieties in peat dissolved organic matter: quantities and redox transformations in the context of peat biogeochemistry. Environ Sci Technol, 2018, 52: 5236-5245

[25] Wang BN, Liu HF, Zheng JB, Fan MT, Cao W. Distribution of phenolic acids in different tissues of jujube and their antioxidant activity. J Agric Food Chem, 2011, 59: 1288-1292

[26] Wang L, Lan X, Peng W, Wang Z. Uncertainty and misinterpretation over identification, quantification and transformation of reactive species generated in catalytic oxidation processes: a review. J Hazard Mater, 2020, 408 124436

[27] Wang Y, Gu X, Huang Y, Ding Z, Chen Y, Hu X. Insight into biomass feedstock on formation of biochar-bound environmentally persistent free radicals under different pyrolysis temperatures. RSC Adv, 2022, 12: 19318-19326

[28] Xu S, Zhan J, Li L, Zhu Y, Liu J, Guo X. Total petroleum hydrocarbons and influencing factors in co-composting of rural sewage sludge and organic solid wastes. Environ Pollut, 2023, 319 120911

[29] Yameen MZ, Naqvi SR, Juchelková D, Khan MNA. Harnessing the power of functionalized biochar: progress, challenges, and future perspectives in energy, water treatment, and environmental sustainability. Biochar, 2024, 6: 25

[30] Yang F, Zhang S, Cheng K, Antonietti M. A hydrothermal process to turn waste biomass into artificial fulvic and humic acids for soil remediation. Sci Total Environ, 2019, 686: 1140-1151

[31] Yang F, Tang C, Antonietti M. Natural and artificial humic substances to manage minerals, ions, water, and soil microorganisms. Chem Soc Rev, 2021, 50: 6221-6239

[32] Yang P, Jiang T, Cao D, Sun T, Liu G, Guo Y, Liu Y, Yin Y, Cai Y, Jiang G. Unraveling multiple pathways of electron donation from phenolic moieties in natural organic matter. Environ Sci Technol, 2023, 57: 16895-16905

[33] Yin Y, Shen M, Zhou X, Yu S, Chao J, Liu J, Jiang G. Photoreduction and stabilization capability of molecular weight fractionated natural organic matter in transformation of silver ion to metallic nanoparticle. Environ Sci Technol, 2014, 48: 9366-9373

[34] Yu S, Liu J, Yin Y, Shen M. Interactions between engineered nanoparticles and dissolved organic matter: a review on mechanisms and environmental effects. J Environ Sci, 2018, 63: 198-217

[35] Yu F, Zhao W, Qin T, Zhao W, Chen Y, Miao X, Lin L, Shang H, Sui G, Peng D, Yang Y, Zhu Y, Zhang S, Zhu X. Biosafety of human environments can be supported by effective use of renewable biomass. Proc Natl Acad Sci U S A, 2022, 119 e2106843119

[36] Zingaretti D, Lombardi F, Baciocchi R. Soluble organic substances extracted from compost as amendments for Fenton-like oxidation of contaminated sites. Sci Total Environ, 2018, 619: 1366-1374

[37] Zou X, Shi J, Zhang H. Morphological evolution and reconstruction of silver nanoparticles in aquatic environments: the roles of natural organic matter and light irradiation. J Hazard Mater, 2015, 292: 61-69

Funding

National Natural Science Foundation of China(22276040)

RIGHTS & PERMISSIONS

The Author(s)