Biochar-Induced Oxidation Of As(?) Or Reduction Of Cr(?) And Their Subsequent Immobilizations In Water:Effects Of Environmental Factors

In several decades,biochar has received increasing attention on environmental remediations.In this dissertation,we summarized biochar sources,compositions,structural properties,and their structure-application relationships.Subsequently,we reviewed the advances in As???oxidation and Cr???reduction,and their immobilizations by biochar,especially potential effects of environmental factors on As???oxidation and Cr???reduction and their subsequent immobilizations.To date,however,the mechanistic effects of environmental factors involving pH,dissolved O₂,environmentally coexisting substances?i.e.,extrinsic Ca???and Fe?II??,and/or intrinsic iron oxides on As???oxidation or Cr???reduction and their immobilizations still remain poorly understood.Herein we systematically investigated the mechanistic effects of the environmental factors on As???oxidation or Cr???reduction and their subsequent immobilizations by rice-husk-derived biochar,the main contents and results of the dissertation were summarized as follows:?1?Here we investigated effects of pH?3.0,7.0,and 9.5?and dissolved O₂?anoxic and oxic conditions?on As???oxidation by biochar.The results showed that the promoted oxidation of As???by biochar is strongly pH and O₂ dependent.Under anoxic conditions,no As???oxidation by biochar occurred at pH 3.0 and 7.0,while As???was thoroughly oxidized by biochar within 12 h(2.161×10^–1 h^–1)at pH 9.5.More interestingly,the promoted As???oxidation by biochar was observed at all the pH values under oxic conditions(with1.402×10^–1 h^–1at pH 3.0,0.513×10^–1 h^–1 at pH 7.0,and 3.817×10^–1 h^–1 at pH 9.5),while only O₂ slightly oxidized As???at pH 9.5.Under acidic and neutral pH conditions,only the dominant oxidation of As???by·OH produced from activation of O₂ by phenolic–OH and semiquinone-type PFRs occurred.In contrast,the oxidation by both semiquinone-type PFRs and quinoid C=O and H₂O₂?if O₂ was introduced?appeared under alkaline conditions..This pH-dependent oxidation mode was mainly attributed to the varying redox activities of redox-active moieties?RAMs,i.e.,phenolic–OH and semiquinone-type PFRs,and quinoid C=O?,as confirmed by multiple characterization and validation experiments using biochar with tuned RAMs compositions,as well as thermodynamics.?2?Oxidation and immobilization of As???by undissolved?rUBC?and dissolved?rDBC?fractions of young biochar were investigated in the presence of Ca???under alkaline and anoxic conditions.The results demonstrated that the co-presence of Ca???and rUBC synergistically oxidized and immobilized As???.rUBC completely oxidized As???within8 h,but failed to immobilize As???and newly produced As?V?,while Ca???neither oxidized nor immobilized As???.Compared with rUBC,rDBC just led partly to As???oxidation,but could not immobilize As???and As?V?.The presence of Ca???significantly enhanced As???immobilization by rDBC,but had little effect on As???oxidation.Multiple characterization and chemical precipitation model were applied to decipher the interesting phenomena.In the rUBC+Ca???+As???system,As???was oxidized into As?V?by oxidized moieties?i.e.,PFRs and quinoid C=O?on rUBC,and followed by co-precipitation with Ca???.In the rDBC+Ca???+As???system,however,As???was directly immobilized by forming rDBC–Ca???–As???ternary complex.In addition,low pH?10.0–12.5?and Ca???concentration?0–100 mM?were both not conducive to oxidation and immobilization of As???by rUBC and rDBC.?3?Fate and oxidation of As???was explored in the presence of biochar and Fe?II?under circumneutral pH and oxic conditions.The experimental results showed that As???oxidation was partly oxidized by biochar,but no As???and As?V?immobilization occurred over the whole experimental period of 21 d.At the oxidation-precipitation stage?1 d?,Fe?II?oxidative precipitation quickly induced that aqueous As???concentration decreased from100 to 19.8?M within 3 h,and to 0.1?M,while the newly produced and aqueous As?V?concentration increased from 0 to 5.2?M,but subsequently decreased to 0.The presence of biochar facilitated oxidative precipitation of Fe?II?and As????i.e.,aqueous As???concentration decreased from 100 to 3.1?M within 3 h,and to 0.5?M,while the newly produced and aqueous As?V?concentration elevated from 0 to 11.0?M,but subsequently decreased to 0.8?M?,but contributed to As???oxidation.At the aging stage?1–21 d?,aqueous As?tot??mostly As????in the Fe?II?+As???system increased from 0.1 to 4.6?M due to the transformation of arsenic-containing lepidocrocite and amorphous iron mineral to goethite.In contrast,aqueous As?tot??mostly As????in the BC+Fe?II?+As???system elevated significantly from 1.3 to 12.0?M because of the decomposition of biochar,especially silicate release,rather than transformation of these iron minerals.The presence of biochar dramatically inhibited the transformation of arsenic-containing lepidocrocite and amorphous ferric iron mineral to goethite,facilitating As???retention on the iron minerals.?4?In this work,magnetic biochar?MBC?synthesized by microwave co-pyrolysis of solid-state FeSO₄ and rice husk was employed to remove Cr???from water.In comparison to the rice husk biochar?BC?,the MBC exhibits the 3.2-and 11.7-fold higher adsorption and reduction efficiency of Cr???.Multiple characterization results revealed that the high Cr???removal performance of MBC was mainly attributed to the presence of active Fe₃O₄and carbon-centered PFRs in the porous and graphitic MBC.The Fe₃O₄ not only provided active chemisorption/reduction sites for Cr???,but also facilitated the generation of more active electron donating carbon-centered PFRs than carbon-centered PFRs with an oxygen atom in the graphitic structure to reduce Cr???.The presence of Fe₃O₄ also elevated BET-surface area and pore volume of MBC,promoting the Cr???removal.The Fe₃O₄ and carbon-centered PFRs contributed to?81.8%and?18.2%of total Cr?III?generation,respectively.In addition,the initial solution pH was responsible for determining the relative significance of Cr???adsorption and reduction by MBC.