Electrochemical crystallization for recovery of phosphorus and potassium from urine as K-struvite with a sacrificial magnesium anode

Abstract

Declining earth resources, rising ore cost and pollution call for better recycling of wastewater in the context of the circular economy. In particular, urine is a potential huge source of phosphorus (P) and potassium (K) agricultural nutrients, yet the e ciency of actual methods for P and K recovery are limited.Here we designed a electrochemical crystallization system using sacri cial magnesium anodes to recover P and K in the form of K-struvite (MgKPO 4 •6H 2 O) from simulated urine at low (P/K=0.25) and high (P/K=0.6) phosphate levels, respectively. Results show optimal recoveries of 88.5% for P and 35.4% for K in the form of rod-shaped K-struvite at 3.5 mA/cm 2 , yet higher current density reduced recovery due to side reactions and pH increase. Adding phosphate to urine increased K recovery to 35.4% versus 15.0% without phosphate. Adding prefabricated struvite crystals at 1.6 g/L into urine enhanced the recovery of K by 14.7% and of P by 23.7% compared to the control group. Overall, our ndings show that electrochemical crystallization is promising for the recovery of K-struvite fertilizers.

Is K-Struvite Precipitation a Plausible Nutrient Recovery Method from Potassium-Containing Wastes?—A Review

“…SEM analysis of the precipitate from the electrochemical treatment of simulated urine at 1.0 and 5.0 mA•cm −2 using a sacrificial Mg anode showed some difference in response to the change in current density [13]. The precipitate recovered at 1.0 mA•cm −2 had finer rod-like crystals with few impurities on the surface.…”

“…On the contrary, the rod-like product at 5.0 mA•cm −2 was longer and had more amorphous impurities attached to the crystals. EDS analysis revealed the presence of small amounts of MgNaPO4·6H2O and Ca3(PO4)2 that increased with the current density [13].…”

“…The chemicals also play a determining role in the process efficiency, purity of MPP crystals, and effluent quality [3]. Although comprehensive reviews have focused on these chemicals, magnesium sources for MAP precipitation in particular were already available in the literature [1,3,[68][69][70], and until now, very limited data have been published for MPP precipitation [13,32,71].…”

“…In effect, the definition of a virtuous cycle of nutrients to reduce the use of natural resources while also avoiding pollution phenomena is currently a key challenge. In this context, magnesium ammonium phosphate (MgNH 4 PO 4 •6H 2 O; MAP) precipitation application began some 30 years ago and research in this area is still continuing [2][3][4][5][6][7][8][9][10][11][12][13]. The impetus behind these works was that the precipitate obtained from the treatment of waste is a valuable product containing two essential elements (nitrogen and phosphorus) that can be used as a slow-release fertilizer in agronomic practices [3,14].…”

“…The studies in the field also covered bench scale experiments [16], baffle reactors [29], fluidized bed reactors [30], and even electrochemical applications [13].…”