Efficient fertilizer production from low phosphate water using in situ-formed vaterite/calcite calcium carbonate composite microspheres

Phosphate, as an important non-renewable agricultural resource, needs to be recovered from wastewater at mg/L scale. Calcite carbonate fine powder was used in P-recovery but could only work in recovering high concentration phosphate. Herein, a new strategy is explored using in situ-formed CaCO3 microspheres (CaCO3-in situ) to realize efficient and fast P-recovery by adding CaCl2 and Na2CO3 solution into low phosphate water (10 mg-P/L). We find that freshly in situ-formed CaCO3 nanoparticles can capture phosphate ions very efficiently and self-assemble into composite CaCO3 microspheres consisting of vaterite and calcite phases. Phosphate ions are possibly immobilized between CaCO3 nanoparticles which stimulate the formation of metastable vaterite CaCO3. Under optimized conditions (Ca/P molar ratio, 6/1), 98% of phosphate can be recovered with a rather low residual phosphate level of ~0.2 mg-P/L within only 30 min which is much time-saving than existing methods. Importantly, superior class P-fertilizer can be obtained with P2O5 content of 20.8 wt% using this novel CaCO3-in situ recovery strategy, which is 4 times as high as that using prepared calcite CaCO3 nanoparticles. The yield of pakchoi, a fast-growing vegetable, was increased by 58.9% (fresh weight) when using the prepared CaCO3-in situ-P fertilizer. This strategy provides a new way of recycling low concentration phosphate while producing high quality fertilizer.