Chelating agent-assisted inhibition and dissolution of struvite for subsequent recovery: experimental and modeling analysis

This study investigates the prevention of uncontrolled precipitation and dissolution of the mineral struvite to recover it in a controlled environment. A typical mechanism has been proposed for inhibiting struvite precipitation or dissolving already precipitated struvite from unwanted parts (e.g. pipes, pumps, heat exchangers etc.) of a wastewater treatment plant (e.k.a municipal resources recovery facilities) and get struvite precipitated and recovered in controlled systems. Maximum 92.7% magnesium removal within 24 hours with the greatest drop in magnesium (81%) and phosphorus (66%) concentrations within 2 hours showed the possibility of quicker removal and recovery of struvite (confirmed by XRD) in a recovery system. The inhibition experiments showed no decrease of 5 mM Mg2+ and PO43−-P at 25 mM concentrations of EDTA and HEDTA chelating agents. Similar inhibition results are expected of biodegradable chelating agents. Extensive dissolution experiments were conducted for wide-range of degradation resistant and biodegradable chelating agents. Dissolution experiments with both degradable resistant (EDTA, NTMP, HEDTA, DTPA EDTA, DTPA, NTMP, DTPMP) and biodegradable (GLDA, MGDA, NTA, acetic acid, citric acid, oxalic acid, IDA) chelating agents were conducted at same the pH 9.5 maintained by 100 mM CHES buffer. The dissolution results showed the complete dissolution of 5 mM of struvite in 100 mM CHES buffer with degradation resistant chelating agents such as 10 mM EDTA and 10 mM HEDTA within 24 hrs by producing 5 mM dissolved concentration of Mg2+ & PO43−-P. Furthermore, biodegradable chelating agents (e.g. GLDA and NTA) completely dissolved 5 mM struvite at 15 mM concentrations. Additionally, dissolution experiments investigating the synergistic effect of different chelating agents (EDTA, DTPA, NTMP, and GLDA) in the presence of synergists like oxalic acid and citric acid were conducted. No synergy was observed rather the dissolution ability of 7.5 mM DTPA and GLDA was suppressed due to the presence of 2.5 mM of oxalic acid. Successive effects of different types of chelating agents on biological systems are being evaluated. Degradation potential of the biodegradable chelating agents is being evaluated as well. Successful completion of those two types of experiments will provide optimum conditions and chelate types with concentrations required for struvite re-precipitation in a controlled system. This work can lead to resources recovery of other types of minerals such as vivianite, brushite, calcite etc as well.