Phosphate removal from wastewater using slag and ion exchange resins

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Wastewater treatment is an imperative requirement in order to meet effluent quality standards before wastewater can be discharged directly into the rivers, streams and the ocean. Together with the rapid development of industry and society, large volumes of wastewater with hundreds of toxins and impurities, if not treated properly, can severely affect humans and environmental health through different manners such as drinking water contamination and habitat degradation. In particular, the excessive presence of phosphate in water encourages the growth of algae that then leads to eutrophication. Thus, wastewater treatment systems, including phosphate removal, are considered to be a principal tool to reduce the negative effects of wastewater. The main objectives of this study were to evaluate (i) the capabilities of two commercial ion exchange resins - Purolite A860S, Dowex 21K XLT and a low cost adsorbent - steel furnace slag (SFS) on phosphate removal, and (ii) the effect of modification of these materials on phosphate removal capacities. In this study, two modified materials, Dowex 21K XLT_Cu and modified steel furnace slag (M-SFS) were developed from original Dowex 21K XLT and steel furnace slag respectively. The performance of these ion exchange resins on phosphate removal was evaluated in different experimental conditions such as varying adsorbent dose, initial adsorbate concentration, contact time and pH in a series of batch studies. The Langmuir and Freundlich models were found to fit with the experimental data of all resins and slag. The results also show that the modification of ion exchange resin and slag led to a significant improvement of phosphate removal efficiency. The maximum phosphate adsorption capacities for Dowex 21K XLT_Cu, Dowex 21 K XLT and Purolite A860S were 97.09; 65.35 and 52.63 mg/g respectively. The adsorption kinetic of these resins was also fitted well with the Pseudo-Second-order kinetic model (R² = 0.99). The breakthrough data was reasonably described by using two empirical models: Thomas and Yoon-Nelson (R² ≥ 0.93). Phosphate adsorbed by resin was effectively desorbed by using 0.5M NaOH. The adsorbent capacity of resin was found to be recovered well after three adsorption/desorption cycles. This study also investigated the phosphate adsorption capacity of low cost materials, SFS and M-SFS. The phosphate adsorption capacities of SFS and M-SFS were 22.88 and 30.49 mg/g respectively, lower than those of the resins. For SFS and M-SFS, Langmuir isotherm model gave a better fit than the Freundlich model. The results also revealed that Thomas model can be used to describe the behaviour of the phosphate adsorption on SFS and M-SFS in the filter column test. In conclusion, the results show that Dowex 21K XLT_Cu was the most effective adsorbent for Phosphate removal. It can be regenerated and reused effectively. When the treatment cost is considered, SFS and M-SFS could be the potential adsorbents.
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