The synthesis of calcium phosphate from municipal wastewater and its application for the removal of metals from acidic effluents

Abstract

The contamination of the environment with wastewaters emanating from different operations and industrial activities has been an issue of prime concern to national and international research communities. Grossly, acid mine drainage (AMD) and municipal wastewater (MWW) are the most common waste streams that pose major environmental threats to surface and ground waters if left untreated. Specifically, trivalent (e.g., Fe3+ and Al3+) and divalent (e.g., Mg2+, Ca2+, and Mn2+) ions including rare earth metals contained in acid mine drainage (AMD) have strong affinity to phosphate contained in municipal wastewater (MWW). Therefore, their sequential treatment could be feasible, particularly when viewed under reuse, recycle, and resource recovery paradigm in wastewater management. Due to stringent environmental regulation, contaminants embodied in AMD and MWW exceed the prescribed limits for different defined uses hence they need to be managed prior discharge into the environment. In this study, the recovery of phosphate as calcium phosphate from municipal wastewater using hydrated lime was explored. The generated sludge, known as calcium phosphate, was used for the removal of heavy metals from acidic effluents. In light of subsequent stages, it is crystally clear that this study is stratified into two-phases, of which, phase one is the synthesis if calcium phosphate from MWW using lime, and phase two is the employment of calcium phosphate for the removal of metals from acidic effluents. To fulfil the goals of this study, the one factor-at-a-time (OFAAT) parameters were explored, and they include the effect of contact time, feedstock dosage, concentration and pH. The resultant sludge was ascertained using different state of art analytical techniques and equipment such as the X-ray diffraction (XRD), Focused ion beam (FIB) and high-resolution (HR) field emission scanning electron microscopy (FE-SEM), coupled with Electron Dispersion Spectroscopy (EDS) capabilities. Thenceforth, the PH REdox EQuilibrium (PHREEQC) (in C language) was used to substantiate the experimental results. The aqueous samples were characterised using Atomic Absorption Spectrometer (AAS) and anions were ascertained using Ion chromatography (IC). All experiments were done in triplicates and the results were reported as mean values. Findings from this study revealed the following: (a) In the first part of the study, MWW was reacted with calcium hydroxide to synthesize calcium phosphate. Approximately 99% and 30% removal efficacy for phosphate and nitrate, respectively, were observed from municipal wastewater using hydrated lime. The optimum conditions were observed to be solid: liquid ratio of 0.06 g: 100 mL and contact time of 60 min, pH of 10. An increase in feedstock dosage resulted in pH increase, this could be attributed to the neutralisation process followed by nucleation and precipitation of phosphate as calcium phosphate (Ca3(PO4)2), hence decreasing phosphate concentration in the product water. The sludge obtained from this phase of the project was used for the subsequent stage which is the removal of heavy metals from acidic effluents. (b) In the second part of this study, calcium phosphate was reacted with simulated and real acidic effluents rich in heavy metals for their attenuation. The interaction of AMD with calcium phosphate led to an increase in pH and a significant reduction in concentrations of metal species. The optimum conditions were observed to be 60 min of mixing, and 1.0 g feedstock dosage and pH 9.0. The removal of metals was observed to be dependent on pH. The metals removal efficiency obeyed the sequence: Fe (99%) > Mn (95%) > Al (80%). The pH of reacted AMD was (≥ 10). The removal of metals from aqueous solution was independent of chemical species concentration. The PHREEQC geochemical model also confirm that the species existed as divalent, trivalent and oxyanions in aqueous solutions. Furthermore, metals were predicted to precipitate as metal hydroxides. Specifically, the geochemical model explicitly showed that Al was removed as Al(OH)3, Fe as Fe(OH)3. Al and Fe precipitated as iron (oxy)-hydroxides and aluminium (oxy)-hydroxides. Mn precipitated as rhodochrosite and manganite. This would explain the reduction of metal species in product water. The XRD confirmed the formation of new mineral phases although most of it were amorphous in nature hence they could not be detected under the XRD. Thenceforth, phosphate was removed to below the SANS 241-2: 2015 specifications, WHO standards, EPA guidelines and DWS water quality guidelines. In conclusion, calcium phosphate was successfully synthesized from MWW using hydrated lime. This denotes that Ca in hydrated lime scavenged the phosphate in MWW to form calcium phosphate. Furthermore, the synthesized mineral phase proved to have strong neutralisation capacity. Calcium phosphate has neutralised AMD and attenuated high levels of metal species from acidic mine effluents. This study verified that the concept of circular economy and waste beneficiation via the recovery of valuable minerals that could be valorised for secondary application such as metals removal is feasible. This will go a long way in minimizing ecological footprints associated with acid mine drainage and municipal wastewater discharge.