Metal(loid) removal from highly metal rich acid mine waters using natural schwertmannite

This study evaluates the potential of natural schwertmannite for treating highly acidic and metal-rich effluents (pH 2.0) containing high concentrations of Fe (6664 mg/L), Al (910 mg/L), Zn (794 mg/L), Cu (196 mg/L), As (12.5 mg/L), and Pb (0.17 mg/L) through batch and column experiments. In batch experiments, schwertmannite interaction with acidic waters led to increased dissolved concentrations of sulfate (19 %), Fe (14 %), and Al (6 %), especially at a 1:10 solid-to-liquid ratio, likely due to schwertmannite dissolution. Other elements such as Cr, Cu, Ni, Cd, Se, U, Th, and REEs followed the same trend, with Cr later showing 22 % removal and Zn ranging from 1.3 % to 5.5 %. Most notably, As and Pb were effectively removed, with efficiencies of 82-88 % and 90-93 %, respectively. The column experiment also demonstrated high As and Pb removal rates (63-99 % and 74-92 %, respectively). After stabilization, most elements showed slight concentration increases (1-8 %) at the end of the experiment, while Cr, Ga, Se, Cd, U, and Y exhibited net removal rates of 10-49 %, 7-38 %, 3-24 %, 8-11 %, 1-15 %, and 3-20 %, respectively. Fe solubility in the column experiment was controlled by jarosite precipitation and schwertmannite dissolution. The mobility of other elements was influenced by sorption and/or coprecipitation onto these minerals, depending on their speciation. Negatively charged species were preferentially removed by sorption onto the positively charged schwertmannite surface, while others coprecipitated with newly formed jarosite. Maximum sorption values reached 97-181 mg/g for As and 0.8-0.9 mg/g for Pb. The adsorption capacity of natural schwertmannite was notable compared to synthetic schwertmannite, nanostructured cerium-manganese oxide, biochars, Fe-Mn polymers, and low-cost materials like eggshells and tea waste. Given its effectiveness, schwertmannite from AMD systems could serve as a natural filter at treatment plant inlets.