Removal and stabilization of arsenic from anode slime by forming crystal scorodite

A process was proposed for removing and stabilizing arsenic(As) from anode slime. The anode slime with high arsenic concentration was pretreated by circular alkaline leaching process. Then, the arsenic in the leaching solution can be further precipitated as a form of scorodite crystalline(FeAsO4·2H2O). In the precipitating arsenic reaction, in which ferrous ions were oxidized by air gas, the effects of acidity(p H), reaction temperature, air flow rate, initial concentration of arsenic and initial molar ratio of Fe(II) to As(V) on arsenic precipitation were investigated. The results showed that sufficiently stable crystal scorodite could be achieved under the condition of initial arsenic concentration of 10 g/L, pH 3.0-4.0, Fe/As molar ratio of 1.5, the temperature of 80-95 °C, and the air flow rate higher than 120 L/h. Under the optimal condition, more than 78% of arsenic could be precipitated as a form of scorodite crystalline. The As leaching concentration of the precipitates was less than 2.0 mg/L and the precipitates may be considered to be safe for disposal.

Synthesis, in-situ coating and characterization of scorodite with high leaching stability

To improve stability of scorodite,a method of simultaneous synthesis and in-situ coating of scorodite was proposed.Scorodite particles with polyhedral and raspberry-like morphologies were synthesized in an Fe(Ⅱ).As(Ⅴ).H2O system at 90℃and pH 1.5 by blowing oxygen gas into the system.When the initial Fe/As molar ratio exceeded 1:1,a coating of sulfate-containing iron(hydr)oxides formed on the surfaces of scorodite particles during synthesis.To evaluate the leaching stability of synthesized scorodite samples,toxicity characteristic leaching procedure(TCLP)tests were conducted at pH 4.93 for 60 h,and long-term leaching tests were conducted for 30.40 d within a pH range of 5.40.10.88.The leaching results indicated that the release of arsenic from scorodite was noticeably postponed by the coating,and the average arsenic concentrations in the leaching solutions were as low as 0.12 mg/L in the TCLP tests and lower than 0.5 mg/L in the long-term leaching tests.

Arsenic release from microbial reduction of scorodite in the presence of electron shuttle in flooded soil

Scorodite (FeAsO_(4)·H_(2)O) is a common arsenic-bearing (As-bearing) iron mineral in nearsurface environments that could immobilize or store As in a bound state.In flooded soils,microbe induced Fe(Ⅲ) or As(Ⅴ) reduction can increase the mobility and bioavailability of As.Additionally,humic substances can act as electron shuttles to promote this process.The dynamics of As release and diversity of putative As(Ⅴ)-reducing bacteria during scorodite reduction have yet to be investigated in detail in flooded soils.Here,the microbial reductive dissolution of scorodite was conducted in an flooded soil in the presence of anthraquinone-2,6-disulfonate (AQDS).Anaeromyxobacter,Dechloromonas,Geothrix,Geobacter,Ideonella,and Zoogloea were found to be the dominant indigenous bacteria during Fe(Ⅲ) and As(Ⅴ) reduction.AQDS increased the relative abundance of dominant species,but did not change the diversity and microbial community of the systems with scorodite.Among these bacteria,Geobacter exhibited the greatest increase and was the dominant Fe(Ⅲ)-and As(Ⅴ)-reducing bacteria during the incubation with AQDS and scorodite.AQDS promoted both Fe(Ⅲ) and As(Ⅴ) reduction,and over 80%of released As(Ⅴ) was microbially transformed to As(Ⅲ).The increases in the abundance of arrA gene and putative arrA sequences of Geobacter were higher with AQDS than without AQDS.As a result,the addition of AQDS promoted microbial Fe(Ⅲ) and As(Ⅴ) release and reduction from As-bearing iron minerals into the environment.These results contribute to exploration of the transformation of As from As-bearing iron minerals under anaerobic conditions,thus providing insights into the bioremediation of As-contaminated soil.