title: The precipitation of magnesium potassium phosphate hexahydrate for P and K recovery from synthetic urine.
abstract: Nutrients recovery from urine to close the nutrient loop is one of the most attractive benefits of source separation in wastewater management. The current study presents an investigation of the thermodynamic modeling of the recovery of P and K from synthetic urine via the precipitation of magnesium potassium phosphate hexahydrate (MPP). Experimental results show that maximum recovery efficiencies of P and K reached 99% and 33%, respectively, when the precipitation process was initiated only through adding dissolvable Mg compound source. pH level and molar ratio of Mg:P were key factors determining the nutrient recovery efficiencies. Precipitation equilibrium of MPP and magnesium sodium phosphate heptahydrate (MSP) was confirmed via precipitates analysis using a Scanning Electron Microscope/Energy Dispersive Spectrometer and an X-ray Diffractometer. Then, the standard solubility products of MPP and MSP in the synthetic urine were estimated to be 10(-12.2 ± 0.0.253) and 10(-11.6 ± 0.253), respectively. The thermodynamic model formulated on chemical software PHREEQC could well fit the experimental results via comparing the simulated and measured concentrations of K and P in equilibrium. Precipitation potentials of three struvite-type compounds were calculated through thermodynamic modeling. Magnesium ammonium phosphate hexahydrate (MAP) has a much higher tendency to precipitate than MPP and MSP in normal urine while MSP was the main inhibitor of MPP in ammonium-removed urine. To optimize the K recovery, ammonium should be removed prior as much as possible and an alternative alkaline compound should be explored for pH adjustment rather than NaOH.
authors.affiliation: College of Environmental Science and Engineering, Beijing Forestry University, Beijing 100083, PR China; School of Environment, Tsinghua University, Beijing 100084, PR China; Beijing Key Lab for Source Control Technology of Water Pollution, Beijing 100083, PR China. Electronic address: xukangning@bjfu.edu.cn.
authors.forename: Kangning
authors.initials: K
authors.lastname: Xu
authors.affiliation: School of Environment, Tsinghua University, Beijing 100084, PR China.
authors.forename: Jiyun
authors.initials: J
authors.lastname: Li
authors.affiliation: School of Environment, Tsinghua University, Beijing 100084, PR China.
authors.forename: Min
authors.initials: M
authors.lastname: Zheng
authors.affiliation: School of Environment, Tsinghua University, Beijing 100084, PR China.
authors.forename: Chi
authors.initials: C
authors.lastname: Zhang
authors.affiliation: School of Environment, Tsinghua University, Beijing 100084, PR China.
authors.forename: Tao
authors.initials: T
authors.lastname: Xie
authors.affiliation: School of Environment, Tsinghua University, Beijing 100084, PR China.
authors.forename: Chengwen
authors.initials: C
authors.lastname: Wang
mesh_terms: D000465:Algorithms; D011232:Chemical Precipitation; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D017616:Magnesium Compounds / Q000737:chemistry*; D008855:Microscopy, Electron, Scanning; D008956:Models, Chemical; D010710:Phosphates / Q000737:chemistry* / Q000502:physiology; D010758:Phosphorus / Q000737:chemistry*; D011188:Potassium / Q000737:chemistry*; D017680:Potassium Compounds / Q000737:chemistry*; D015203:Reproducibility of Results; D012995:Solubility; D013052:Spectrometry, X-Ray Emission; D013816:Thermodynamics; D014556:Urine / Q000737:chemistry*; D014865:Waste Disposal, Fluid / Q000379:methods; D014961:X-Ray Diffraction
chemical_list: D017616:Magnesium Compounds; D010710:Phosphates; D017680:Potassium Compounds; C024788:ammonium phosphate; D010758:Phosphorus; C030781:magnesium phosphate; C013216:potassium phosphate; D011188:Potassium; C018279:sodium phosphate
keywords: Magnesium potassium phosphate hexahydrate; Nutrient recovery; Solubility product; Source-separated urine; Thermodynamic model