Formation And Solubility Of Secondary Arsenic Minerals:Examples From Heavily Arsenic-contaminated Mining Areas And Thermodynamic Experiments

One the one hand,arsenic is a trace element necessary for the human body;on the other hand,arsenic is also a hazard contaminant element.Different arsenic species have different toxicities.Inorganic arsenic is generally more toxic than organic arsenic;trivalent arenic is generally more toxic than pentavalent arsenic.Mining activities are the primary reason for regional arsenic contamination.Therefore,characterizing different arsenic minerals and obtaining their solubilities are essential to evaluate arsenic contaminations to the environment,thus will be of great significance regarding mining management Different arsenic species and secondary arsenic minerals forming in Shizishan Copper-gold mine,Tongling,Anhui and shimen realgar(AsS)-orpiment(As2S3)mine,Hunan,were studied,through which evolvements of arsenic species in mining areas were demonstrated from the perspective of mineralogy,based on which detailed experimental studies were conducted and thermodynamic and kinetic data for the main secondary arsenic minerals dissolution were obtained.The surficial oxidation of arsenic-bearing sulfide is the main mechanism to form secondary arsenic minerals.Mineral combinations may vary in different mining areas,causing different arsenic transferability and stability properties,thus will lead to different environmental impacts.The shimen realgar-orpiment mine is dominant with dolomite-realgar-orpiment ores and the common secondary arsenic minerals are arsenolite(As2O3),claudetite(dimorph of aesenolite),pharmacolite(CaHAsO4·2H2O),picropharmacolite(Ca4Mg(AsO3OH)2(AsO4)2·11H2O),As-gypsum et al;whereas,Fe-As minearals are depleted.Short-period weathered samples in relatively reductive conditions will form As(?)secondary minerals and primary minerals combinations,whereas long-period weathered samples in oxidative conditions are dominant with As(?)minerals such as scorodite and pharmacolite.pH value not only controls the type of secondary minerals but also determines the sequence in which secondary minerals form,e.g.as pH increases,gypsum,weilite(CaHAsO4)and other arsenic minerals precipitate sequentially in shimen tailings.Primary arsenic minerals in Copper-gold mine are mainly arsenopyrite(FeAsS)and arsenian pyrite(FeAsxS2-x)and their weathering products include not only arsenic minerals,e.g.scorodite(FeAsO4·2H2O),arsenolite(As2O5)but also arenic-bearing minerals,such as arsenic is absorbed into iron hydroxide or incorporated into sulfate minerals.To evaluate the hazard of secondary arsenic minerals to the environment,the solubility of scorodite,which is known to occur as weathering products of arsenic minerals found in tailings and waste piles of mining and mineral processing activities,was measured between 5 and 90? and the data was reduced kinetically and thermodynamically to get the dissolution rate,activation energy,solubility products,and reaction enthalpies and entropies.Thermodynamic data used include new arsenate activity coefficients derived from mean activity coefficients of potassium arsenates,arsenate ionic molal conductivities and dissociation constants derived from conductivity and pH measurements,and association constants of Fe(?)-As(V)complexes derived from redox measurements in Fe(?)-Fe(?)-As(V)-HClO4-H2O and Fe(?)-Fe(?)-As(?)-HCl-H2O systems,all of which can be used at 5-90? and ionic strengths up to 3 mol kg-1.Our studies contribute to the basic thermodynamic data for arsenic that form the database for numerous geochemical codes used in the quantitative interpretation of arsenic reactions in surficial environments such as PHREEQC,WATEQ4F,TOUGHREACT,and The Geochemist's WorkbenchTM.