Performance of pristine biochars in manganese removal from synthetic and mine-impacted waters

Elevated concentrations of manganese (Mn) in ground and surface waters, often originating from impounded reservoirs and abandoned mine waters, pose increasing risks to environmental and human health. Conventional Mn removal methods are often expensive and require strong chemical oxidants. This study trialled a low-cost, alternative approach by evaluating the effectiveness of six pristine biochars for dissolved Mn removal from solution. Batch and continuous flow column tests were performed on synthetic Mn solutions to investigate the effect of pH, contact time, initial Mn concentration, particle size, and biochar dose. Ground Oil Seed Rape (OSR550) biochar was the best performer, achieving a Mn uptake of 2.56 mg g 1 in batch tests and 1.19 mg g 1 in column tests. Mn removal was closely associated with an increase in solution pH induced by biochar addition, enhancing Mn retention without chemical oxidants. The influence of competing ions was also assessed. Iron (Fe) up to 20 mg L 1 1 did not affect Mn removal, but calcium (Ca) significantly reduced it because of competitive interactions. Real mine-impacted water (up to 4 mg L Mn) showed lower Mn removal than synthetic solutions. Furthermore, geochemical modelling (PhreeqC, Version 3.0) suggested that high concentrations of sulphate ions may form the neutral ion pair MnSO 4 , reducing the amount of dissolved Mn available for adsorption. This study demonstrates realistic and scalable use-cases for pristine biochar, particularly OSR550, in Mn adsorption under environmentally relevant conditions highlighting the critical role of solution chemistry, ion competition, and Mn speciation in governing performance in real waters.