Interaction of divalent metals with struvite: sorption, reversibility, and implications for mineral recovery from wastes

The Simultaneous Efficient Recovery of Ammonia Nitrogen and Phosphate Resources in the Form of Struvite: Optimization and Potential Applications for the Electrochemical Reduction of NO3. Molecules 2024;29:2185. [PMID: 38792046 PMCID: PMC11123745 DOI: 10.3390/molecules29102185] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

In response to the need for improvement in the utilization of ammonium-rich solutions after the electrochemical reduction of nitrate (NO3--RR), this study combined phosphorus-containing wastewater and adopted the electrochemical precipitation method for the preparation of struvite (MAP) to simultaneously recover nitrogen and phosphorus resources. At a current density of 5 mA·cm-2 and an initial solution pH of 7.0, the recovery efficiencies for nitrogen and phosphorus can reach 47.15% and 88.66%, respectively. Under various experimental conditions, the generated struvite (MgNH4PO4·6H2O) exhibits a typical long prismatic structure. In solutions containing nitrate and nitrite, the coexisting ions have no significant effect on the final product, struvite. Finally, the characterization of the precipitate product by X-ray diffraction (XRD) revealed that its main component is struvite, with a high purity reaching 93.24%. Overall, this system can effectively recover ammonium nitrogen from the NO3--RR solution system after nitrate reduction, with certain application prospects for the recovery of ammonium nitrogen and phosphate.

Electrochemical route outperforms chemical struvite precipitation in mitigating heavy metal contamination

Electrochemically mediated struvite precipitation (EMSP) offers a robust, chemical-free process towards phosphate and ammonium reclamation from nutrients-rich wastewater, i.e., swine wastewater. However, given the coexistence of heavy metal, struvite recovered from wastewater may suffer from heavy metal contamination. Here, we systematically investigated the fate of Cu2+, as a representative heavy metal, in the EMSP process and compared it with the chemical struvite precipitation (CSP) system. The results showed that Cu2+ was 100% transferred from solution to solid phase as a mixture of copper and struvite under pHi 9.5 with 2-20 mg/L Cu2+ in the CSP system, and varying pH would affect struvite production. In the EMSP system, the formation of struvite was not affected by bulk pH, and struvite was much less polluted by co-removed Cu2+ (24.4%) at pHi 7.5, which means we recovered a cleaner and safer product. Specifically, struvite mainly accumulates on the front side of the cathode. In contrast, the fascinating thing is that Cu2+ is ultimately deposited primarily to the back side of the cathode in the form of copper (hydro)oxides due to the distinct thickness of the local high pH layer on the two sides of the cathode. In turn, struvite and Cu (hydro)oxides can be harvested separately from the front and back sides of the cathode, respectively, facilitating the subsequent recycling of heavy metals and struvite. The contrasting fate of Cu2+ in the two systems highlights the merits of EMSP over conventional CSP in mitigating heavy metal pollution on recovered products, promoting the development of EMSP technology towards a cleaner recovery of struvite from waste streams.

Co-precipitation of Cd with struvite during phosphorus recovery

During the struvite recovery process, Cd, a hazardous metal commonly found in waste streams, can be sequestered by struvite. This study investigated the influence of Cd2+ on the precipitation of struvite. Quantitative X-ray diffraction (QXRD) results showed that the purity of struvite decreased from 99.1% to 73.6% as Cd concentration increased from 1 to 500 μM. Scanning electron microscopy (SEM) revealed a roughened surface of struvite, and X-ray photoelectron spectroscopy (XPS) analysis indicated that the peak area ratio of Cd-OH increased from 19.4% to 51.3%, while the area ratio of Cd-PO4 decreased from 86.6% to 48.7% as Cd concentrations increased from 10 to 500 μM. The findings suggested that Cd2+ disrupted the crystal growth of struvite, and mainly combined with -OH and -PO4 to form amorphous Cd-bearing compounds co-precipitated with struvite. Additionally, Mg-containing amorphous phases were formed by incorporating Mg2+ with -OH and -PO4 during struvite formation.

Competitive adsorption of heavy metals between Ca-P and Mg-P products from wastewater during struvite crystallization

Wastewater usually contains high concentration of calcium (Ca), posing a competitive reaction with magnesium (Mg) on phosphorus (P) recovery during the struvite crystallization. The differences in the adsorption of heavy metals by Ca-P and Mg-P (struvite) generated are still unclear. Herein, we analyzed the residues of four kinds of common heavy metals (Cu, Zn, Cd, Pb) in Ca-P and Mg-P (struvite) under varying conditions (solution pH, N/P ratio, Mg/Ca ratio) in the swine wastewater and explored their possible competitive adsorption mechanisms. The experiments using synthetic wastewater and real wastewater have similar experimental patterns. However, under the same conditions, the metal (Pb) content of struvite recovered from the synthetic wastewater (16.58 mg/g) was higher than that of the real wastewater (11.02 mg/g), as predicted by the Box-Behnken Design of Response Surface Methodology (BBD-RSM). The results demonstrated that Cu was the least abundant in the precipitates compared to Zn, Cd, and Pb of almost all experimental groups with an N/P ratio greater than or equal to 10. The fact might be mainly attributed to the its stronger binding capacity of Cu ion with NH₃ and other ligands. Compared with struvite, the Ca-P product had a higher adsorption capacity for heavy metals and a lower P recovery rate. In addition, the higher solution pH and N/P ratio were favorable to obtain qualified struvite with lower heavy metal content. It can be applied to reduce the incorporation of heavy metals by modulating pH and N/P ratio through RSM, which is suitable for different Mg/Ca ratios. It is anticipated that the results obtained would offer support for the safe utility of struvite from wastewater containing Ca and heavy metals.