Co-transport Behaviors Of Biochar Nanoparticles And Contaminants In Soils

Land application of biochar generated from the pyrolysis of biomass has been increasingly recommended as a powerful strategy for nutrient retention,carbon sequestration and soil remediation.However,the biochar particles,especially those in the nanoscale range,may migrate or carry the inherent contaminants along the soil profile.Furthermore,the biochar nanoparticles?NPs?rich in pore structures,abundant functional groups,and clay minerals exhibit high sorption affinities for a wide array of environmental contaminants.Thus,they are likely to expedite the transport and wide dissemination of sorbed contaminants via‘colloid-facilitated transport',causing the secondary pollution.Therefore,the objective of this study is to elucidate the transport and retention of biochar NPs in natural soils at environmental factors,and then to investigate the effect of biochar NPs on the transport and retention behaviors of phosphorus and heavy metals.The results obtained from this study will provide scientific basis for the safety evaluation of biochar application in the soil.The main results are as following:The first part of this study investigated the transport and retention of wood chip-derived biochar nanoparticles?NPs?at pyrolysis temperature of 500°C in water-saturated paddy soil columns.The environmentally-relevant soil solution chemistry including ionic strength,electrolyte type?NaCl and CaCl₂?,and natural organic matter were tested to elucidate their effects on the biochar NPs transport.Lower mobility of biochar NPs was observed in the soil at higher ionic strengths,because of weeker electrostatic repulsions between biochar NPs and soils based on DLVO interaction energy calculations.Moreover,biochar NPs formed large aggregates even clusters at higher ionic strengths,which exhibited a lower mobility in soil.CaCl₂ electrolyte was more effective than NaCl in decreasing biochar NPs transport.The divalent Ca²⁺was found to have a more marked influence on the transport of biochar NPs than that of monovalent Na⁺,due to charge screening effect of biochar NPs by divalent Ca²⁺.Humic acid increased the mobility of biochar NPs,likely due to enhanced electrostatic and steric repulsions between biochar NPs and soils.The transport behaviors of biochar NPs could be well interpreted by a two-site kinetic retention model that assumes reversible retention for one site,and irreversible retention for the other site,which could potentially expedite the assessment of NPs mobility in natural soils.The second part of this study investigated the transport and retention of phosphorus?P?in two acidic and two alkaline soils as affected by wood chip-derived biochar NPs at pyrolysis temperature of 500°C.For acidic soils,biochar NPs facilitated the retention of P in paddy?56.3%?and red?90.9%?soils,increasing by about 24%and 16%,respectively,compared to the biochar absence?45.5%and 78.9%for paddy and red soils?.It is because biochar NPs stabilized soil Fe/Al oxides and dissolved organic carbon?DOC?,thereby reducing the release of Fe/Al oxides-P and DOC-P.In contrast,in alkaline huangmian and chao soils,retention of P was reduced in the presence of biochar NPs,decreasing by about 23%and 18%,respectively.It was mainly due to the increased transport of Fe/Al oxides-associated P in effluents.Moreover,biochar NPs could also act as a P carrier,mediating the retention of P.The diffusive gradients in thin films provided in-suit measurement of labile P in soil profiles,showing much lower labile P from retained P in acidic soils than that from alkaline soils though the labile P with biochar NPs presence was increased in all soils.The third part of this study investigated the effect of wheat straw-derived biochar NPs at pyrolysis temperature of 350°C and 500°C on the transport of cadmium?Cd?in in water-saturated red soil columns under different ionic strengths in the absence and presence of humic acid.The transport of Cd in red soil was enhanced at higher ionic strength.Because the amount of cation increased with increasing solution ionic strength,which could compete with Cd for the surface bonding sites of biochar NPs and soils.Biochar NPs can act as a Cd carrier and mediate the transport of Cd in red soil.At low ionic strength?1.0 mM?,more than 99%of total Cd was present as biochar NPs-Cd in effluents.Higher mobility of biochar NPs and biochar NPs-Cd occurred at low ionic strength.500°C biochar NPs were more effective for Cd enhancement than 350°C biochar NPs due to higher adsorption of Cd onto 500°C biochar NPs.Humic acid further strengthened the enhancement for Cd transport by biochar NPs,which was mainly to increase the mobility of biocahr NPs and the adsorption capacity of Cd onto biochar NPs.In contrast,low transport of biochar NPs inhibited the transport of Cd in red soil at high ionic strength?10 mM?.Because soluble Cd?77.0%-95.2%?was the major migration species of Cd.Increasing pyrolysis temperature of biochar NPs decreased the biochar NPs-Cd transport due to lower transportability of 500°C biochar NPs.Humic acid increased the adsorption capacity of Cd onto biochar NPs,which was in turn responsible for the retention of biochar NPs-Cd in red soil.The fourth part of this study investigated the effect of wood chip and wheat straw-derived biochar NPs at pyrolysis temperature of 350°C and 500°C on the transport,retention and transformation of hexavalent chromium[Cr?VI?]in water-saturated red soil.Biochar NPs facilitated the transport of Cr?VI?in soils.Meanwhile,biochar NPs induced reduction of Cr?VI?to Cr?III?,and then the resultant Cr?III?as well as Cr?VI?probably sorbed on the biochar NPs surface.Thus,biochar NPs could act as a carrier to facilitate the transport of Cr?III?in soils.Greater enhancement effect on transport of Cr?VI?and associated Cr?III?was observed by biochar NPs derived from wheat straw than wood chip.500°C biochar NPs were more effective for Cr?VI?enhancement due to a stronger competition with Cr?VI?for the adsorption sites on the surface of soils.In addition,when the pyrolysis temperature of biochar NPs increased from 350°C to 500°C,the electron donating capacities of biochar NPs derived from wood chip and wheat straw decreased from 0.75 mmol e^-/?g biochar?and 0.66 mmol e^-/?g biochar?to 0.28 mmol e^-/?g biochar?and 0.24 mmol e^-/?g biochar?,respectively.Meanwhile,the transportability of biochar NPs derived from wood chip and wheat straw decreased from 48.4%and 77.7%to 28.9%and 37.0%,respectively,with the increase of pyrolysis temperature from 350°C to 500°C.Thus,more adverse transport of Cr?III?was observed for 500°C biochar NPs than 350°C biochar NPs.The fifth part of this study investigated the co-transport behaviors of Cd with biochar-Fe₃O₄ nanocomposites?BFNCs?derived from wood chip and wheat straw at pyrolysis temperature of 500°C?and biochar-alone for comparison?in water-saturated natural soil?paddy soil and red soil?packed columns.The BFNCs greatly increased?27.1-95.5 times?Cd transport in soils mainly through BFNC-Cd complexes,compared to the negligible transport of Cd in soils without presence of BFNCs.Notably,compared to the biochar-alone?Cd mass recovery=1.28-4.07%?,the BFNCs promoted the transport of Cd?Cd mass recovery=2.71-10.5%?by 2.5-times in natural soils.Greater interplays via electrostatic attraction,complexation with hydroxyls,and?-?interaction with the aromatic complexes altogether contributed to the higher adsorption capacity and transport potential toward Cd by the BFNCs?vs.biochar-alone?.Higher mobility of BFNCs and BFNC-Cd complex occurred in the red soil than in the paddy soil due to the lower contents of Fe/Al oxides in the red soil.Greater enhancement effect on Cd was observed by BFNCs derived from wheat straw than wood chip,due to the stronger adsorption ability of wheat straw biochar towards Cd,likely stemming from more mineral composition such as CaCO₃ and KCl.In conclusion,biochar NPs could migrate and act as a carrier mediate the transport of soil contaminants along the soil profiles,which could affected by feedstocks and pyrolysis temperature of biochar NPs production as well as different environmental factors and soil types.Therefore,more attention should be paid to the potential risks of biochars for land applications.