Experimental Study Of Environmental Stability Of As-containing Schwertmannites In Mine Drainage

Arsenic?As?is one of the most problematic contaminants in mine drainage mainly existing as the oxyanions of As?III?and As?V?.As?V?is the predominant As form in well-oxidized waters,while As?III?exists predominantly in reduced environments.Aqueous arsenic mainly comes from the natural processes that include weathering of arsenic-bearing sulfide minerals,geothermal sources,as well as anthropogenic activities in mining,agriculture and metallurgy.The occasional exposure to arsenic cause a series of health problems such as cardiovascular diseases,disturbance in nervous systems and skin cancer.Recently,the secondary iron minerals such as ferrihydrite,goethite and schwertmannite precipitated in mining drainage has attracted much attention for their great abundance and strong binding affinity to toxic As species.Arsenic retention ways may be achieved by either co-precipitation or adsorption to the surface of Fe minerals.Among all the Fe minerals,schwertmannite has been considered as an important host mineral for arsenic.As a metastable mineral,schwertmannite will transform to goethite or to other more crystalline minerals mainly influenced by factors such as pH,temperature and the presence of dissolved organic matter,etc.Changes in the composition of aqueous phase,mainly in total Fe,SO₄^2-,As,have taken place during transformation of schwertmannite.In this study,we synthesized pure schwertmannite[Sch],schwertmannite with structurally-incorporated As?V?[CO-Sch]and schwertmannite with adsorbed As?V?[AD-Sch]through chemical methods;and we also quantitatively analyzed the effect of natural environmental factors,including pH,high temperature or typical heavy metal ionson the aqueous ions released from CO-Sch and AD-Sch;and we also qualitatively analyzed the effect of pH,high temperature or typical heavy metal ions on the dissolution and transformation of CO-Sch and AD-Sch by using SEM/EDS,XRD,FTIR and other characterization methods.The following conclusions may be obtained:?1?CO-Sch and AD-Sch samples contained 38.6 and 19.4 mg As?V?g^-1 Sch,respectively.The Sch and AD-Sch are both mainly composed of spheroids around 700nm in diameter,while in CO-Sch,the particles exist in aggregates and the particle size decreases obviously to less than 100 nm in diameter.The incorporation of As?V?caused a slight degeneration in the structure of schwertmannite.The sulfate in Sch,AD-Sch and CO-Sch mainly exist as outer-sphere complexes,and the inner-sphere complexes of sulfate in AD-Sch is more than that in CO-Sch.The thermal stability of AD-Sch or CO-Sch is stronger than that of Sch.?2?The concentrations of arsenate,iron and sulfate released from AD-Sch and CO-Sch under typical pH conditions?pH 3-9?remained relatively constant within 7 d.With the increase of pH value,the release of Fe³⁺from schwertmannites gradually decreased,while the release of SO₄^2-and As?V?increased.At acidic pH,As?V?on the CO-Sch is more stable,while at neutral pH or alkaline pH,As?V?adsorbed by AD-Sch is more difficult to be released.The chemical stability of AD-Sch under typical pH conditions is stronger than that of CO-Sch.?3?The concentrations of arsenate,iron and sulfate released from AD-Sch and CO-Sch under extreme pH conditions?pH 2 and 10?reached a constant value within 30-d.At pH 2,the iron or sulfate released from the CO-Sch was more than that from the AD-Sch,but less arsenate was liberated by CO-Sch.At pH 10,the CO-Sch may release more sulfate and arsenate at the end of incubation.The environmetal stability of AD-Sch under extreme pH conditions is stronger than that of CO-Sch.?4?The concentrations of arsenate,iron and sulfate released from AD-Sch and CO-Sch under high temperature conditions remained relatively constant within 7 d.With the increase of temperature,the liberation of Fe³⁺,SO₄^2-and As?V?from both AD-Sch and CO-Sch increased.Under high temperature conditions,the stability of adsorbed arsenateon AD-Sch is stronger than that on CO-Sch,and CO-Sch released less iron and sulfate.When the temperature increased up to 75°C,AD-Sch partially dissolved and then amorphous products was generated within 7 d,CO-Sch transformed to the needle-like goethite phase within 7 d,and the amount of this new mineral phase increased with the increase of temperature.The environmetal stability of AD-Sch under high temperature conditions is stronger than that of CO-Sch.?5?The concentrations of arsenate,iron and sulfate released from AD-Sch and CO-Sch in the presence of Pb²⁺and Zn²⁺achieved relatively constant within 7 d.The introduction of Pb²⁺and Zn²⁺led to more Fe³⁺,SO₄^2-and As?V?released from both AD-Sch and CO-Sch to aqueous phase.AD-Sch exhibited stronger adsorption stability for As?V?.There was no goethite or other new mineral phases appeared on AD-Sch or CO-Sch within 7 d,and the crystalline structure and surface morphology of the schwertmannites remained relatively stable.The environmetal stability of AD-Sch in the presence of Pb²⁺and Zn²⁺is stronger than that of CO-Sch.