Phosphorus recovery from wastewater using extracellular polymeric substances (EPS)-like hydrogels

Abstract

Phosphorus (P) recovery from wastewater is meaningful in wastewater treatment and resource recycling. Natural extracellular polymeric substances (EPS) of microbes show impressive P uptake ability. However, natural EPS is difficult to assist in P recovery due to the high harvesting cost in extraction. Thus, EPS-like hydrogels constructed by sodium alginate (SA)-Fe(III) and humic acids(HA) were designed. The results showed that the optimal formation conditions of EPS-like hydrogel were 0.1 M Fe(III) and m(HA):m(SA) = 1. Fe(III) ions were the key crosslinkers in constructing the 3-dimensional sodium alginate (SA)-Fe(III) hydrogels and in the embedding of humic acid (HA) for HA@SA-Fe(III) hydrogels. HA@SA-Fe(III) hydrogels showed bigger pore sizes, lesser Fe leaching, and higher phosphate selectivity in SO42− compared to SA-Fe(III) hydrogels. The highest P adsorption amounts were 56 and 134.33 mg-P/g for the HA@SA-Fe(III) and SA-Fe(III), respectively. The adsorption isotherm results exhibited that when the initial P concentration was in a range of 0–35 mg/L, the adsorption mainly existed on the surface of the adsorbent. While in the pore-filling stage (i.e., the initial P concentration was higher than 35 mg/L), internal diffusion controlled the adsorption process based on the kinetics. The X-ray photoelectron spectroscopy (XPS) analysis detected an appearance of Fe-O-P and O-P bonds and a disappearance of chloride ions in the hydrogels after adsorption, which confirmed the major driving forces on adsorption (ligand exchange and ion exchange). Moreover, HA@SA-Fe(III) hydrogel saturated by P adsorption passed a test as sustainable-release P fertilizers. Overall, the EPS-like hydrogels developed here provide new absorbents for P recovery.