As(III) oxidation and immobilization by activated carbon

Production of acidic wastewaters containing highly toxic and soluble arsenite (As(III)) is a growing concern in copper refinery industries. This study aimed to investigate the utility of readily available activated carbon (AC) for oxidation and immobilization of As(III) from acidic wastewaters at a relatively moderate temperature (45°C). For comparison, effect of bio-AC (combination of AC with moderately thermophilic Fe(II)-oxidizing bacterium, Acidimicrobium ferrooxidans ICP) was also investigated. The initial AC pulp densities ranged between 1-25% for acidic solutions (pH 1.5) containing 1000 ppm of As(III) and Fe(II). When tested with an individual ion species (1000 ppm As(III) or Fe(II)) using 1% AC, it was confirmed that AC is capable of chemically catalyzing Fe(II) oxidation and As(III) oxidation. Following the oxidation, Fe(III) was found absorbed on the AC surface, while As(V) remained in the solution. When both ions were present, 78% As was immobilized (at day 14). By elevating the AC concentrations, As(III) oxidation was increasingly accelerated and 1000 ppm (III) was completely oxidized in 2 days (10, 25% AC). At 25% AC, 94% of As was removed as amorphous ferric arsenate within 2 days. Possible additional Fe(II) oxidation effect (thus facilitating further As(III) oxidation by coupling Fe(III) reduction-As(III) oxidation on the AC surface) was evaluated by preparing bio-AC. The expected effect was seen at low AC concentrations (0.5, 1%): At 1% bio-AC, oxidation of both Fe(II) and As(III) were accelerated compared to 1% AC. As a result, 57% As was immobilization at day 6 using bio-AC. In conclusions, higher AC concentrations lead to more effective As(III) oxidation and immobilization as amorphous ferric arsenate. This approach can be utilised as pre-treatment for final scorodite crystallization (at the same time recycling AC). Whilst use of bio-AC can be advantageous when treating dilute As(III) concentrations (<1000 ppm As(III)) using lower AC concentrations.