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 effi-cient 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 compos-ite 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 similar to 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.