Arsenic-Imposed Effects on Schwertmannite and Jarosite Formation in Acid Mine Drainage and Coupled Impacts on Arsenic Mobility

This study explores interactions between As and Fe(III) minerals, predominantly schwertmannite and jarosite, in acid mine drainage (AMD) via observations at a former mine site combined with mineral formation and transformation experiments. Our objectives were to examine the effect of As on Fe(III) mineralogy in strongly acidic AMD while also considering associated controls on As mobility. AMD at the former mine site was strongly acidic (pH 2.4 to 2.8), with total aqueous Fe and As decreasing down the flow-path from similar to 400 to similar to 20 mg L-1 and similar to 33,000 to similar to 150 mu g L-1, respectively. This trend was interrupted by a sharp rise in aqueous As(III) and Fe(II) caused by reductive dissolution of As-bearing Fe(III) phases in a sediment retention pond. Attenuation of Fe and As mobility occurred via formation of As(V)-rich schwertmannite, As(V)-rich jarosite, and amorphous ferric arsenate (AFA), resulting in solid-phase As concentrations spanning similar to 13 to similar to 208 g kg(-1). Schwertmannite and jarosite retained As(V) predominantly by structural incorporation involving AsO4-for-SO4 substitution at up to similar to 40 and similar to 22 mol %, respectively. Arsenic strongly influenced Fe(III) mineral formation, with high As(V) concentrations causing formation of AFA over schwertmannite. Arsenic also strongly influenced Fe(III) mineral evolution over time. In particular, increasing levels of As(V) incorporation within schwertmannite were shown, for the first time, to enhance the transformation of schwertmannite to jarosite. This significant discovery necessitates a reevaluation of the prevailing paradigm that As(V) retards schwertmannite transformation.

Automated summary

This study explores the interactions between As and Fe(III) minerals in acid mine drainage, revealing that arsenic significantly influences the formation and evolution of Fe(III) minerals. Arsenic has a significant impact on the stability and enrichment of Fe(III) minerals, triggering new mineral formation reactions.