Anionic nanofibrillated cellulose—A sustainable agent to recover highly soluble salts from industrial effluents

Abstract

Abstract Utilisation of sodium (Na + ) containing alkali in the neutralisation of acidic industrial process waters rich in sulphate (SO 4 2 − ) produces effluents high in sodium sulphate (Na 2 SO 4 ) reluctant to precipitate. Discharge of the saline effluents leads to permanent chemical stratification of the recipient freshwater systems, which prevents their annual overturn and the subsequent oxygen supply to hypolimnion. Novel and sustainable technologies are desperately needed to prevent the hazardous environmental impacts of saline effluents. We investigated the ability of anionic nanofibrillated cellulose (NFC) gels of three different consistencies to recover solubilised Na + and SO 4 2 − from authentic circumneutral mining water onto a solid phase. The water was treated with the NFC gels in three sequential batches at three sorbent-to-solution ratios. NFC-induced changes in the ion concentrations were determined to calculate the Na + and SO 4 2 − retention capacity and purification efficiency of the NFC gels. All NFC gels efficiently and coincidentally removed Na + and SO 4 2 − from the mining water. We concluded that Na + ions electrostatically adsorbed onto the deprotonated carboxyl groups of the anionic NFC and attracted SO 4 2 − ions which also acted as bridging anions between the neighbouring nanofibrils. Decrease in the consistency of the NFC gel enhanced accessibility of the sorption sites and, consequently, promoted the ion retention. A high sorbent-to-solution ratio favoured the intermolecular interactions within the NFC gels, thus decreasing the number of available sorption sites. A high ionic strength of the effluent favoured the ion retention, indicating that anionic NFC is particularly suitable for the treatment of highly saline solutions. The best purification result was obtained at a moderate sorbent-to-solution ratio with a dilute NFC gel. This lowers the demand for the cellulose raw material and the treatment expenses. We conclude that anionic NFC, made of renewable materials, may serve as an efficient and sustainable purification agent for removal and recycling of highly soluble Na + and SO 4 2 − from industrial effluents.