Bioremediation of acid mine water utilising red mud and Desulfotomaculum nigrificans

Abstract

The efficiency of red mud as an adsorbent for the removal of metal ions and sulphate from synthetic acid mine water was assessed. Almost complete removal of Fe^–z (98%), Fe³⁺ (99%) and Cu²⁺ (97%) and a significant removal of Zn²⁺ (84%) was achieved from acid mine water at pH 2·3. However, only about 27% of sulphate reduction could be attained. A noteworthy feature was the concomitant increase in pH from 2·3 to 7·6 with the increase in red mud loading under appropriate conditions. The adsorption isotherms adhered to Freundlich and Langmuir relationships and were endothermic in nature. The free energy of the adsorption process was found to be negative attesting to the feasibility of the reaction. The adsorption kinetics followed the first order Lagergren rate equation. The filtrate obtained after treatment of red mud with synthetic acid mine water supplemented with carbon and nitrogen sources served as a successful growth medium for the sulphate reducing bacteria, namely Desulfotomaculum nigrificans (Dsm. nigrificans). Bacterial growth studies indicated that about 22, 41 and 52% of sulphate reduction could be achieved at initial pH values of 5, 6·1 and 7·2 respectively. The pretreatment of the acid mine water with red mud followed by bioremoval using Dsm. nigrificans resulted in about 27, 47 and 62% of sulphate removal at initial pH values of 5, 6·1 and 7·2 respectively, cumulatively by adsorption and bioreduction. The mechanisms of metal ion removal utilising red mud and sulphate reduction using Dsm. nigrificans are discussed.