Effects Of Extracellular Polymers On Hydroxyl Radical Generation And Arsenic Migration And Transformation In AMD In Karst Areas

Acid mine drainage(AMD)produced in the process of mining in karst areas has the characteristics of high Fe(?)concentration,low p H and amout of various toxic elements(As/Sb,etc.),which will damage the ecosystem around the mining area.Our previous study found that the increase of p H induced by carbonate rocks in karst area would promote the oxidation of Fe(?)by O₂ to produce hydroxyl radical(·OH).·OH is the most reactive which can unselectively oxidize most organics and redox-sensitive elements.In addition,under the natural conditions,a large number of microorganisms in AMD can secrete extracellular polymers substance(EPS)that can complexate Fe(?),which may affect the chemical oxidation of Fe(?)to produce·OH and affect the transformation and transfer of heavy metals such As.However,there is still a lack of understanding of the above mechanisms.Therefore,this research firstly have done a field study of AMD in Maoshitou Reservoir,Xingren County of Guizhou Province.The AMD of maoshitou reservoir has low p H(3.00?3.52),high concentrations of Fe(?)(259.03 mg/L),the toxic heavy metal of As(?)(585.3?g/L),and SO₄^2-(1453.55 mg/L).In the process of AMD flow,total As,total Fe(?),SO₄^2-and other trace ions content decreased with increasing distance of migration.The cumulative·OH concentration increased with the increase of AMD distance,and the maximum concentration was 24.6?mol/L.Fe(?)size distribution showed that the particles and colloids of Fe(?)ratio decrease,and truly dissolved Fe(?)ratio gradually increased.The same sampling point glue posture Fe(?)accounts for a percentage is greater than the particle state.The results of microbial community at the dew point of AMD showed that the main microbial group was Beta-Proteobacteria.On the basis of the above research,this part further studied the mechanism of different concentrations of EPS,p H and Fe³⁺on the production of·OH in the process of chemical oxidation of Fe(II)by O₂ promoted by the reaction of extracellular polymer with simulated AMD.Results of this study showed that the cumulative·OH increased from 56.75 to 158.70?mol/L within 24 h at p H 3 with the increase in EPS concentration from 0 to 12 mg/L.An appropriate p H(about 6)and EPS(6 mg/L)concentration were required for the moderate rate of Fe(?)oxidation,corresponding to the maximum production of·OH.The presence of EPS enhanced the·OH production from Fe(II)oxidation in simulated AMD under acid conditions.In the presence of EPS,·OH production is attributed mainly the complexation of Fe(?)with EPS,of which is rich of carboxyl and hydroxyl groups.Besides,the yield of·OH increased remarkably with the addition of Fe(?).It is most likely that EPS can contribute to reduce Fe(?)to Fe(II),which is beneficial to the production of·OH.The findings reveal from this study supplement the fundamental of·OH production from Fe(II)oxidation by microorganisms in natural AMD.Based on these findings,the effect of extracellular polymers on the transfer and transformation of arsenic(As)was further explored.In order to detect the acidic conditions in AMD extracellular polymer affect As(?)migration,0,6,12,24 mg/L EPS were added into the 500?g/L As(?)the containing 5 mg/L Fe(?)in simulated AMD,and then added 0.32 g/L Ca CO₃ start the oxidation reaction of 4 h.Results showed that the presence of EPS enhanced the·OH production from Fe(II)oxidation in simulated AMD under acid conditions,thus promoted the As(?)oxidation of the pollutants.As(?)oxidation rate as same as Fe(?)chemical oxidation rate.At the same time,the high concentration of EPS could promote the formation of collidal As in the As(?)oxidation process,and low concentration in the reaction system of EPS As a large number of adsorption in Fe(?)(hydr)oxides and remove oxide precipitation.SEM analysis of the products with or without EPS showed that the solid phase products with EPS had a large number of cluster structures and porous surface,and had strong adsorption and solidification ability,which effectively affected the migration and transformation behavior of arsenic.This study confirmed that·OH production in nature AMD,and base on this finding,we explained that EPS of microbial secretion will promote·OH production and EPS affect the oxidation and deposition of As(?)in AMD.