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.

Quantitative, morphological, and structural analysis of Ni incorporated with struvite during precipitation.

Struvite precipitation is a promising strategy for the simultaneous recovery of nitrogen and phosphorus from waste streams. However, waste streams typically contain high amounts of metal contaminants, including Ni, which can be easily sequestered by struvite, but the behavior of Ni during struvite precipitation remains unclear. Thus, this study investigates the influence of Ni concentrations on struvite precipitation. The quantitative X-ray diffraction (QXRD) results revealed that the purity of struvite decreased from 96.6 to 41.1% with the Ni concentrations increased from 0.1-100 mg·L-1. At lower Ni concentrations of 0.1-1 mg·L-1, scanning electron microscopy (SEM) showed a roughened surface of struvite crystal, and this was combined with X-ray absorption near edge structure (XANES) data that indicated a stack of Ni-OH and Ni-PO4 on struvite surface. At Ni concentrations of 10-25 mg·L-1, Ni primarily crystalized as Ni-struvite (NiNH4PO4·6H2O), as detected by QXRD. At higher Ni concentrations of 25-100 mg·L-1, the co-precipitation of amorphous Ni phosphate(s) (e.g., Ni3(PO4)2) and Ni hydroxide (e.g., Ni(OH)2) was identified by XANES. Specifically, the X-ray photoelectron spectroscopy (XPS) analysis detected the formation of amorphous Mg hydroxide(s) and phosphate(s) at Ni of 25-100 mg·L-1. The overall results revealed that Ni formed Ni-OH and Ni-PO4 on struvite surface at 0.1-1 mg·L-1, whereas Ni precipitated as separated phases (e.g. Ni-struvite, Ni hydroxide and phosphate) at 10-100 mg·L-1. The existence of Ni disturbed the crystal growth of struvite and promoted the formation of Ni-struvite, amorphous products during struvite formation.

Effect of solution chemistry and aggregation on adsorption of perfluorooctanesulphonate (PFOS) to nano-sized alumina.

The interaction of pollutants with nanomaterials has attracted attention due to the extensive application of nanomaterials. In this study, the adsorption behavior of PFOS on nano-alumina with different shapes was investigated. First, the adsorption isotherms and kinetics of PFOS on alumina nanoparticles (NPs) and nanowires (NWs) were measured to calculate thermodynamic parameters. The effects of solution chemistry (e.g., pH, ionic strength, and the presence of humic acid) on adsorption were further studied. The different aggregation behavior of alumina NPs and NWs were the critical factor for PFOS adsorption, as demonstrated through dynamic light scattering (DLS) experiments. This study is the first to investigate the aggregation effects on PFOS adsorption on nanomaterials and the results should be useful in identifying the important roles of shape and aggregation of nanomaterials on the fate of organic pollutants in the environment.

Mineralization behavior of fluorine in perfluorooctanesulfonate (PFOS) during thermal treatment of lime-conditioned sludge.

The fate and transport of the fluorine in perfluorooctanesulfonate (PFOS) during the thermal treatment of lime-conditioned sludge were observed using both qualitative and quantitative X-ray diffraction techniques. Two main fluorine mineralization mechanisms leading to the substantial formation of CaF2 and Ca5(PO4)3F phases were observed. They had a close relationship with the thermal treatment condition and the PFOS content of the sludge. At low temperatures (300-600 °C), CaF2 dominated in the product and increases in treatment time and temperature generally enhanced the fluorine transformation. However, at higher temperatures (700-900 °C), increases in treatment time and temperature had a negative effect on the overall efficiency of the fluorine crystallization. The results suggest that in the high temperature environment there were greater losses of gaseous products such as HF and SiF4 in the transformation of CaF2 to Ca5(PO4)3F, the hydrolysis of CaF2, and the reaction with SiO2. The quantitative analysis also showed that when treating sludge with low PFOS content at high temperatures, the formation of Ca5(PO4)3F may be the primary mechanism for the mineralization of the fluorine in PFOS. The overall results clearly indicate the variations in the fate and transport of fluorine in PFOS when the sludge is subject to different PFOS contents and treatment types, such as heat drying or incineration.