Removal of dissolved organic phosphorus from synthetic agro-industrial wastewater using ferric chloride

Abstract

The available phosphorus removal/recovery technologies can achieve low concentrations of orthophosphate phosphorus (P-OP). Furthermore, P-OP can be recovered for reuse as fertiliser. However, in some agro-industrial wastewater, the dissolved organic phosphorus (DOP) is a relevant fraction that remains after conventional phosphorus removal processes and is not recovered [1]. Therefore, improvements and new technologies to recover DOP are necessary to advance the circular phosphorus economy [1,2]. Ferric chloride is frequently used to control P-OP effluent concentrations [3]. When FeCl3.6H2O is added, hydrous ferric oxides (HFO) are formed [4]. The dominant P-OP removal mechanism is the H2PO4− adsorption on HFO [3,4]. However, there are no reports about the mechanisms of DOP removal into HFO. Operational conditions can affect DOP adsorption/removal mechanisms. In this work, batch experiments were conducted using FeCl3.6H2O and synthetic wastewater (2 mg-P-OP/L as KH2PO4 + 18 mg-DOP/L as myo-inositol-1,2,3,4,5,6-hexakisphosphate) to quantify the effects of alkalinity (Alk=50-100 mg-CaCO3/L), Fe/total phosphorus molar relationship (r=0.6-1.5) and pH (pH=4.0-6.0) on DOP removal efficiency (RE_DOP). With Alk=75 mg-CaCO3/L, r=1.05 and pH=5.31, DOP was not detected. For low r, the RE_DOP was adversely affected by increasing alkalinity and decreasing pH. The results suggested that competition for the active sites depends on the decreasing preference: H+>HCO3->DOP>H2PO4−.