Microbe-driven Iron Oxide Reduction Coupled With Arsenic Fixation-release And The Regulation Of Biochar

Extensive arsenic(As)contamination in soil has been a serious environmental problem threatening human health worldwide.The bioavailability and toxicity of arsenic depend heavily on its form.The abundant iron minerals in the environment are a good arsenic adsorbent that can reduce its mobility.However,differences in the binding mode between different types of iron minerals and arsenic.Various Fe(?)(hydrogen)oxides adsorbed with large amounts of arsenic would undergo reductive dissolution driven by anaerobic iron-reducing microorganisms,resulting in the release of arsenic.At the same time,some secondary minerals such as lepidocrocite,goethite,etc.would be produced during the reduction,and the conversion process of these mineral phases may make the originally released arsenic adsorbed again.Therefore,the coupling relationship between the iron reduction and arsenic release and fixation effect in an anaerobic environment needs further study.Biochar is widely used for soil pollution remediation,which can improve the nutrient utilization rate in soil and promote plant growth.Besides,biochar has a good redox activity to change microbial extracellular electron transport,affecting the relevant metabolic processes.Biochar can therefore regulate the reduction and transformation process by affecting the extracellular electron transport process.this regulation finally affects the release and reabsorption process of arsenic on these iron minerals,thus affecting the mobility of arsenic in the environment.To study these environmental processes,this paper explores the arsenic release and fixation caused by the dissolution and transformation of Fe(?)(hydrogen)oxides based on a model bacterium MR-1.Then a series of biochars with different redox activities was prepared to regulate the iron reduction process and related arsenic release phenomenon.The main results are as follows:(1)The reduction of ferrihydrite by MR-1 leads to the release of adsorbed As(?)and promote the fixation of As(?).This effect was enhanced with the increased MR-1concentration and arsenic loading(As/Fe).Moreover,during the experiment,arsenic was bound to iron minerals with extractable adsorption state,the lattice fixed state arsenic is almost zero;besides,increasing arsenic loading leads to the inhibition of iron reduction.But As(?)inhibited the iron reduction process of MR-1 weakly than As(?),and the Fe(?)concentration corresponding to As(?)system was higher than As(?)system;With the increase of arsenic loading,a mineral phase dominated by goethite was formed in As(?)system;while a mineral phase dominated by lepidocrocite was formed in As(?)system,indicating that As(?)also inhibited mineral transformation weakly than As(?).Finally,XPS and Mossbauer showed that As(?)was released into the liquid phase by desorption and readsorption during reduction,while the released As(?)formed ferrous arsenate phase with Fe(?)and reprecipitated on the mineral surface.(2)Biochar BC prepared from palm had no regulatory effect on ferrihydrite reduction,while biochar GC prepared from cow bone could inhibit ferrihydrite reduction.Moreover,the inhibitory effect of biochar GC was significantly improved with increasing pyrolysis temperature.Using the biochar GC1000,the iron reduction can be effectively attenuated and the related As(?)release can be reduced;besides,GC1000 can affect the transformation of mineral phases during the iron reduction process.The addition of biochar GC1000 formed a mineral phase dominated by lepidocrocite;finally,fluorescence analysis was used to confirm that biochar GC1000 shows no toxicity to MR-1.By monitoring riboflavin we indicate that GC1000 could rapidly reduce riboflavin in the system,thereby competing for electron shuttles used for iron reduction and achieving inhibition of the iron reduction process.Using electrochemical experiments,it was found that GC1000 has good capacitive characteristics and redox activity,and could compete with ferrihydrite for electron shuttles from MR-1,ultimately leading to a decrease in the ferrihydrite reduction.This paper provides valuable information for understanding the effect of biochar in the arsenic-bearing ferrihydrite reduction process and is also of great significance for evaluating the biogeochemical processes of iron and arsenic in arsenic-contaminated soils and sediments.