The formation of Fe colloids and layered double hydroxides as sequestration agents in the natural remediation of mine drainage

Highlights: • Fe colloids are formed from ferrous Fe in a circumneutral mine drainage. • Core-shell ferrihydrite aggregates incorporate As thereby aiding in As removal. • Seasonal colloid transportation is evaluated based on the aggregation rates. • Zn-Fe layered double hydroxides are sequestration media for Zn from the drainage. • Natural remediation is attained by Fe colloids and layered double hydroxides. The increasing need to treat wastewater from mine effluents has drawn attention to passive treatment systems. Colloids are common in mine waters and are highly reactive, so their formation, characteristics, behavior, and the critical factors that affect them need to be understood for designing efficient treatment systems. An investigation was conducted at the abandoned Ainai mine drainage, Japan, where aeration is utilized to remove Fe, As, and Zn from circumneutral wastewater drainage, during rainy and dry seasons of 2016 and 2018 respectively, based on observations of physiochemical characteristics, elemental concentrations in dissolved and colloidal fractions, transmission electron microscopy, and synthetic experiments. In this circumneutral Fe-rich mine drainage, Fe²⁺ is oxidized to Fe³⁺, resulting in the formation of Fe colloids that incorporate As during their formation. Colloid formation increases turbidity, and, in the rainy season, increased colloidal interaction enhances their aggregation and higher flow rates lead to greater mobilization of the colloids. Zn-bearing colloids are rare in Ainai mine drainage because the Zn concentrations are low. However, Zn-Fe layered double hydroxide (LDH) was identified and confirmed by geochemical modelling and experiments. The Zn-Fe LDH was formed by isomorphous substitution of Zn into an Fe²⁺–Fe³⁺–CO₃^2–LDH, at pH greater than 7.5, thereby achieving efficient natural remediation of Zn and As in the drainage.