Electrochemically mediated calcium phosphate precipitation from phosphonates: Implications on phosphorus recovery from non-orthophosphate
Lei, Yang ; Saakes, Michel ; Weijden, Renata D. van der; Buisman, Cees J.N. - \ 2020
Water Research 169 (2020). - ISSN 0043-1354
Calcium phosphate - Local high pH - Organic phosphorus - Oxidation - Precipitation
Phosphonates are an important type of phosphorus-containing compounds and have possible eutrophication potential. Therefore, the removal of phosphonates from waste streams is as important as orthophosphate. Herein, we achieved simultaneously removal and recovery of phosphorus from nitrilotris (methylene phosphonic acid) (NTMP) using an electrochemical cell. It was found that the C–N and C–P bonds of NTMP were cleaved at the anode, leading to the formation of orthophosphate and formic acid. Meanwhile, the converted orthophosphate reacted with coexisting calcium ions and precipitated on the cathode as recoverable calcium phosphate solids, due to an electrochemically induced high pH region near the cathode. Electrochemical removal of NTMP (30 mg/L) was more efficient when dosed to effluent of a wastewater treatment plant (89% in 24 h) than dosed to synthetic solutions of 1.0 mM Ca and 50 mM Na2SO4 (43% in 168 h) while applying a current density of 28 A/m2 and using a Pt anode and Ti cathode. The higher removal efficiency of NTMP in real waste water is due to the presence of chloride ions, which resulted in anodic formation of chlorine. This study establishes a one-step approach for simultaneously phosphorus removal and recovery of calcium phosphate from non-orthophosphates.
Influence of Cell Configuration and Long-Term Operation on Electrochemical Phosphorus Recovery from Domestic Wastewater
Lei, Yang ; Remmers, Jorrit Christiaan ; Saakes, Michel ; Weijden, Renata D. Van Der; Buisman, Cees J.N. - \ 2019
ACS sustainable chemistry & engineering 7 (2019)7. - ISSN 2168-0485 - p. 7362 - 7368.
Calcium phosphate - Current density - Electrode distance - Energy consumption - Local high pH
Phosphorus (P) is an important, scarce, and irreplaceable element, and therefore its recovery and recycling are essential for the sustainability of the modern world. We previously demonstrated the possibility of P recovery by electrochemically induced calcium phosphate precipitation. In this Article, we further investigated the influence of cell configuration and long-term operation on the removal of P and coremoved calcium (Ca), magnesium (Mg), and inorganic carbon. The results indicated that the relative removal of P was faster than that of Ca, Mg, and inorganic carbon initially, but later, due to decreased P concentration, the removal of Ca and Mg became dominant. A maximum P removal in 4 days is 75% at 1.4 A m -2 , 85% at 8.3 A m -2 and 92% at 27.8 A m -2 . While a higher current density improves the removal of all ions, the relative increased removal of Ca and Mg affects the product quality. While the variation of electrode distance and electrode material have no significant effects on P removal, it has implication for reducing the energy cost. A 16-day continuous-flow test proved calcium phosphate precipitation could continue for 6 days without losing efficiency even when the cathode was covered with precipitates. However, after 6 days, the precipitates need to be collected; otherwise, the removal efficiency dropped for P removal. Economic evaluation indicates that the recovery cost lies in the range of 2.3-201.4 euro/kg P, depending on P concentration in targeted wastewater and electrolysis current. We concluded that a better strategy for producing a product with high P content in an energy-efficient way is to construct the electrochemical cell with cheaper stainless steel cathode, with a shorter electrode distance, and that targets P-rich wastewater.