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

Abstract

Utilisation of sodium (Na +) containing alkali in the neutralisation of acidic industrial process waters rich in sulphate (SO4 2-) produces effluents high in sodium sulphate (Na2SO4) 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 SO4 2from 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 SO4 2retention capacity and purification efficiency of the NFC gels. All NFC gels efficiently and coincidentally removed Na + and SO4 2from the mining water. We concluded that Na + ions electrostatically adsorbed onto the deprotonated carboxyl groups of the anionic NFC and attracted SO4 2ions 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 SO4 2from industrial effluents.

Resource-efficient purification of acidic multi-metal process water by means of anionic nanofibrillated cellulose

“…This finding supports the conclusion that, in addition to electrostatic adsorption, other reaction mechanisms also contributed to the removal of metals in the NFC treatment. Moreover, as our previous paper suggests (Venäläinen & Hartikainen, 2017b), the conductometric titration method used in the determination of the number of -COOH groups in the undispersed cellulose pulp may underestimate the charge density in the dispersed material. Gelation of the anionic pulp in water decreases the inter-and intra-molecular interactions (e.g.…”

“…The potential of the anionic NFCs (NFC1.8, NFC1.4, and NFC1.1) to decrease the metal and SO4 2concentrations of MW was investigated by means of adsorption tests (Venäläinen & Hartikainen, 2017b). The experiments were carried out in three sequential batches (B1, B2, and B3) as follows:…”