Determination of the process variables for adsorption and desorption of phosphate from wastewater by selective ion exchangers on magnetic particles

Abstract

In this study different process variables were evaluated for the adsorption and desorption of phosphate from wastewater by using selective ion exchangers with magnetic particles modi-fied with layered double hydroxides (LDH). Adsorption and desorption kinetics for pure CaZnFeZr-LDH was performed. The best phosphate adsorption efficiency was achieved with CaZnFeZr-LDH, above 90 % after 1 and 24 h, whereas for the composite particle material with 7,6 wt% CaZnFeZr –LDH the adsorption efficiencies were 85,4 % and 92,1 % after 1 and 24 h, respectively. Ten adsorption/desorption cycles were performed by using three different desorption solutions (1 M NaHCO3, 1 M NaOH+1M Na2CO3 and 3 M NaOH) where the last one showed the best performance. The pH effect was studied by using CaZnFeZr-LDH and ZnFerZr-LDH, where the first one achieved higher phosphate uptake. The effect of adsorbent dosage was investigated on filtered and unfiltered wastewater (WW), with 10 mg/L PO4-P each, wherein the filtered had the best phosphate uptake with 99,2 % after 24 h using a ZnFrZr-LDH dosage of 259 mg/L. The adsorbent dosage was studied on unfiltered WW spiked at three different concentrations of PO4-P: 10, 100 and 300 mg/L. The phosphate adsorption was also studied at different temperatures: 15, 22, 35 and 50 °C, wherein as the temperature increases the amount of adsorbed phosphate onto the LDH increases as well. Moreover, an adsorption kinetic experiment was performed by using filtered and unfiltered WW spiked with PO4-P. The first one reached equilibrium at about 4 h with a phosphate uptake of 99 %. Desorption kinetics was developed for the filtered WW and the best performance was in the case of regeneration with a solution of 1 M NaOH in distilled water. The best adsorbent dosage for the maximal removal of 431 mg/L PO4-P in the Offenburg WW (99,9 % adsorption efficiency) was 3000 mg/L ZnFeZr-LDH. After performing five adsorption/desorption cycles with the Offenburg WW, a conclusion was drawn that it is necessary to maintain pH~7 during the adsorption cycles. The total adsorption efficiency was 94,2 % and from this, 73 % was desorbed.