The Regulating Mechanism Of The Occurrence Of Metal(Loid)s By Fe/S Bio-Oxidation Coupling Red Mud Transformation Under Acidic Conditions

The bio-oxidation of sulfide tailings of huge amounts generated by human mining activities,can form a large amount of acid mine drainage（AMD）containing high concentrations of metal（loid）s,which can cause great potential harm to the ecological environment.Red mud（RM）,a high salt/alkaline industrial solid waste generated during alumina production,has important application potential in the remediation of acidic metal（loid）s contaminated water.There involves RM dissolution,Fe/S-bearing secondary products formation and transformation,and the transition of the occurrence of metal（loid）s during the remediation of acidic water with RM.Currently,relevant research is scarce.The aim of this paper is to unravel the regulation mechanism of the occurrence of metal（loid）s（i.e.,As/Sb/Pb）.Based on the understanding of the effects of RM on the bio-oxidation processes of arsenopyrite and the transformation of As occurrence,and in view of the key factors in the iron and sulfur transformation process,in-depth research has been conducted on the Fe³⁺catalysis,Fe（Ⅱ）bio-oxidation,and AMD mediated dissolution and transformation behavior of RM,and the transformation rules of As/Sb/Pb occurrences.The intrinsic mechanism of Fe/S speciation transformation coupling to RM transformation and their relationship with the occurrence of metal（loid）s,and so their regulatory mechanisms have been revealed and clarified.The results of this paper have important theoretical guiding significance and application value for the development of cost-effective remediation technology based on red mud.The main research contents and conclusions are as follows:（1）The effect of RM on the bio-oxidation process of arsenopyrite by Sulfobacillus thermosulfidooxidans and As speciation occurrence were studied.The complex regulatory mechanisms were investigated with focues on solution chemistry,bacterial Fe/S oxidation activity,mineral and bacterial surface microstructure composition characteristics,and As/Fe/S/Al/Si speciation transformation.The results show that an addition of≤4 g/L of RM substantially promoted arsenopyrite bio-oxidation with formation of Si O₂@（As/Fe/Al/Si）spherical nanoparticles（composed mainly of Fe₂O₃,jarosites,amorphous ferric arsenate,scorodite,and muscovite）that can enhance the stability of the immobilized arsenic.When the addition amount over 6 g/L,the encapsulation of arsenopyrite by red mud blocks the micro interface interaction between bacteria and mineral,the biological oxidation of Fe²⁺,and the effective catalysis of Fe³⁺on the mineral surface,ultimately inhibits the dissolution of arsenopyrite and the release of As.（2）The differences in RM dissolution and composition and structure of the proudcts with different concentrations of Fe³⁺[Fe³⁺]and their correlation with As fate were comparatively studied.The results indicate that Fe³⁺is the key catalyst for driving RM conversion in acidic environments.The ion exchange between Fe³⁺and Al/Si/Ca/K/Na in RM is the main reaction that drives the transformation of red mud into spherical nanoparticles.The higher the[Fe³⁺],the greater the dissolution of RM,the more regular the spherical particles,and the greater the adsorption capacity for As（V）and As（Ⅲ）were observed.The results also reflect the corelationship between the degree of bio-oxidation of arsenopyrite and the transformation of RM.（3）Based on the iron oxidation function of acidophilic leaching bacteria and their mediation to the secondary products formation and transformation characteristics,a Fe（Ⅱ）bio-oxidation coupled with RM transformation system was constructed to form low-cost Fe（Ⅲ）/Al（hydr）oxide adsorbent.The initial Fe（Ⅱ）concentration([Fe（Ⅱ）_ini])can significantly affects the removal efficiency of As by affecting bacterial growth,Fe/Al speciation,and the surface structure of adsorbent.The aluminosilicates in RM provided numerous sites for Fe（Ⅲ）adsorption,inhibiting the transformation of amorphous Fe（Ⅲ）（hydr）oxides to crystalline Fe（Ⅲ）minerals.The newly formed Fe（OH）₃ and schwertmannite played a dominant role in As（V）immobilization.The massive dissolution of RM,together with the accumulation of crystalline Fe-minerals,i.e.,jarosites（MFe₃（SO₄）₂（OH）₆）and goethite（α-Fe OOH）,resulting in a significant reduced As（V）adsorption capacity of the adsorbent.The Fe（Ⅲ）/Al（hydr）oxide adsorbent bio-synthesized under[Fe（Ⅱ）]_ini of 1 g/L showed the highest As（V）adsorption efffciency（89.9mg/g;at p H 2.0）.（4）For the first time,an actual AMD sample collected from Dabaoshan mine area was used as a natural catalyst to mediate the transformation of red mud into Fe/S/Ca/Al/Si complexes.Based on the detailed characterization of the coupling link of transformations,the correlation between the structural composition of the composite itself,the variation of Sb/As/Pb occurrence,and their immobilization efficiency were analyzed.The RA2,RA4,and RA10 adsorption materials synthesized under solid（RM）to liquid（AMD）(S_RM/L_AMD)ratios of 2:1,4:1,and 10:1,exhibites the best removal capacity for Sb,As,and Pb,respectively.The maximum removal capacities for Sb（V）,Sb（Ⅲ）,As（V）,As（Ⅲ）,and Pb（Ⅱ）reach 1,637.8,80.2,109.2,16.4,and 518.7 mg/g,respectively.In the As immobilization process,Fe（Ⅲ）（hydr）oxides（e.g.,α-Fe OOH and Fe（OH）₃）formed in RA4 played a dominant role.The Ca²⁺from the Ca-bearing compounds determined in RA2 significantly contributed to Sb（V）immobilization via formation of Ca Sb₂O₅（OH）₂ precipitate.Convertible sulfates（e.g.,Ca SO₄ and Al₂（SO₄）₃）in RA10 ensured a high removal capacity for Pb（Ⅱ）across a wide p H range（2 to 7）via Pb SO₄ precipitation.In contrast,the formation of Pb CO₃ and Pb₃（CO₃）₂（OH）₂ was the main mechanism for Pb（Ⅱ）removal by unamended RM at p H>3.0.The utilization of RM and AMD for metal（loid）removal from contaminated waters in this study presents a novel and sustainable strategy in resource utilization and waste management.