PEER-REVIEWED ARTICLE bioresources.com Runtti et al. (2016). “Sulphate removal,” BioResources 11(2), 3136-3152. 3136 Sulphate Removal from Water by Carbon Residue from Biomass Gasification: Effect of Chemical Modification Methods on Sulphate Removal Efficiency Hanna Runtti, a Sari Tuomikoski,* ,a Teija Kangas, a Toivo Kuokkanen, a Jaakko Rämö, a and Ulla Lassi a, b Sulphate removal from mine water is a problem because traditional chemical precipitation does not remove all sulphates. In addition, it creates lime sediment as a secondary waste. Therefore, an inexpensive and environmental-friendly sulphate removal method is needed in addition to precipitation. In this study, carbon residues from a wood gasification process were repurposed as precursors to a suitable sorbent for SO4 2- ion removal. The raw material was modified using ZnCl2, BaCl2, CaCl2, FeCl3, or FeCl2. Carbon residues modified with FeCl3 were selected for further consideration because the removal efficiency toward sulphate was the highest. Batch sorption experiments were performed to evaluate the effects of the initial pH, initial SO4 2- ion concentration, and contact time on sulphate removal. The removal of SO4 2- ions using Fe-modified carbon residue was notably higher compared with unmodified carbon residue and commercially available activated carbon. The sorption data exhibited pseudo-second-order kinetics. The isotherm analysis indicated that the sorption data of Fe-modified carbon residues can be represented by the bi-Langmuir isotherm model. Keywords: Sorption; Sulphate; Chemical modification; Carbon residue; Activated carbon Contact information: a: Research Unit of Sustainable Chemistry, University of Oulu, P.O. Box 3000, FI-90014 University of Oulu, Finland; b: Kokkola University Consortium Chydenius, Unit of Applied Chemistry, University of Jyvaskyla, Talonpojankatu 2B, FI-67100, Kokkola, Finland; *Corresponding author: sari.tuomikoski@oulu.fi INTRODUCTION Sulphate (SO4 2- ) is a major pollutant that occurs in both natural waterways and industrial effluents, such as inorganic chemical industry wastewater and acid mine drainage. Sulphate is typically present in high concentrations, especially in industrial wastewaters, which may contain several thousand milligrams of SO4 2- ions per liter. The main natural sources of SO4 2- ions are chemical weathering and dissolution of sulphur- containing minerals (Rayner-Canham 1999; Cao et al. 2011; Rui et al. 2011). Excess SO4 2- ions cause an imbalance in the natural sulphur cycle and endanger human health. While SO4 2- ions are common in drinking water (Rui et al. 2011), many countries have not set guidelines because sulphate is less toxic than heavy metals (Silva et al. 2012). Sulphate ions are corrosive to reinforced steel (Rui et al. 2011). In Finland, the SO4 2- ion limit in drinking water is set at 250 mg/L, but it is recommended that the concentration not be higher than 150 mg/L to ensure that water pipes are not damaged (MSAH 2000). Sulphate concentrations higher than 600 mg/L can cause a laxative effect and affect the taste of the water (Silva et al. 2012). Typically, domestic sewage contains less than 500 mg/L of SO4 2- ions; therefore, the removal of sulphate with a large concentration range is a considerable