Simultaneous recovery of ammonium, potassium and magnesium from produced water by struvite precipitation

Abstract

Unconventional oil and gas industries generate huge amounts of produced water (PW) containing high concentrations of potentially hazardous organic and inorganic contaminants. This study demonstrated the feasibility of simultaneously recovering NH4+, K+ and Mg2+ from PW by struvite precipitation after calcium pretreatment with Na2CO3 addition or CO2 stripping. Without pretreatment, calcium exhibited strong competition for phosphate through the formation of Ca3(PO4)2 precipitate. The pretreatment with a Ca2+:CO32− molar ratio of 1:1.2 achieved a relatively low loss rate of Mg2+ (31.3%) and high Ca2+ removal efficiency (95.9%). The results also revealed that the Mg/N/P molar ratio and solution pH had a remarkable effect on the struvite precipitation, while the seeding dosage and Na+ slightly influenced struvite formation. The combination of pH = 9.5 & Mg/N/P molar ratio = 1.5:1:1.5 was ideal for struvite recovery from PW, resulting in NH4+, K+ and Mg2+ recovery efficiencies of 85.9%, 24.8% and 96.8%, respectively. The results of X-ray diffraction and scanning electron microscopy further confirmed that the precipitates generated at this optimal condition were orthorhombic struvite. Moreover, along with the struvite recovery, no accumulation of heavy metals and organic contaminants was observed, indicating that the struvite quality was sufficient for field application. Furthermore, struvite recovery process was able to reduce the Microtox toxicity of PW towards Vibrio fischeri by 60%. Considering the low cost and relatively simple technology, struvite precipitation process has the potential to be used for large-scale applications for produced water treatment and resource recovery.