Behavior of nickel and cadmium ions during struvite crystallization in wastewater

Abstract

The tremendous amounts of municipal solid wastes (MSW) generated contribute to the raise of nitrogen (N) in landfill leachate, which affect water bodies and cause severe damages to the environment. Struvite crystallization is a simple method that allows N recovery in the form of environmentally friendly fertilizer. Fundamentally, struvite (MgNH4PO4.6H2O) is sensitive to changes in pH and the existence of foreign ions in the precipitation media. Therefore, heavy metals, such as nickel (Ni) and cadmium (Cd), in wastewater can reduce the quality of struvite. The current research studied the interaction behavior of Ni and Cd during struvite crystallization in landfill leachate. In addition, the thermal stability and microstructural composition under the equimolar ratio of struvite components was investigated. Struvite crystallization was conducted in synthetic solution (Phase 1) and synthetic landfill leachate (Phase 2) with pH maintained at 8.0±0.02. Struvite crystallization experiments proved that Cd and Ni have insignificant effect on the efficiency of PO4 removal; however, they affect NH4-N removal dramatically. The statistical analysis (ANOVA) shows that changing the experiment media from synthetic solution to synthetic landfill leachate did not affect the removal percentage of NH4-N and PO4. Atomic Absorption Spectrometry (AAS) analysis for liquid samples showed that the removal percentage of Ni and Cd was also high (65.68% - 99.35%), which proved that the destiny of the trace heavy metals was in solid samples. To further study the fate of Ni and Cd ions in struvite crystal, X-ray diffraction (XRD), scanning electron microscopy energy dispersive X-ray (SEM-EDX), and thermogravimetric analysis (TGA) analyses were conducted. The XRD results confirmed that highly pure crystals were formed. However, some crystals contained minor impurities such as trimagnesium phosphate, nitric acid dehydrate, nickel oxide and magnesium tetra-ammonium cyclotriphosphate tetrahydrate. In addition, SEM-EDX analysis showed that all struvite crystals interacting with Cd and Ni have changed in their surface structure, shape, and size. Moreover, the EDX analysis outcomes supported the results obtained from other analyses in Phase 1 and 2. TGA analysis for struvite showed that all struvite samples containing Cd and Ni in Phase 1 and Phase 2 were thermally stable until the thermal analysis ended with 43.32% - 45.45% remaining weight. The kinetics of struvite in the presence of Cd and Ni at pH 7.5 indicates that the kinetics constant increases up to 21.456 hr-1 with increasing Cd and Ni concentrations. The reaction of struvite with Cd at pH 8.5 followed the same trend. Meanwhile, the kinetics constants of struvite-Ni interaction at pH 8.5 demonstrate that the kinetics constant reduces to 6.408 hr-1, which is lower than kinetics of pure struvite. In future, the outcomes in this study are expected to assist the struvite crystallization process in landfill leachate.