Electrochemical Precipitation of Struvite

Abstract

Pure struvite ͑magnesium ammonium phosphate hexahydrate͒ layers were deposited by an electrochemical method, from an aqueous solution containing phosphate, ammonium, and magnesium ions. According to interfacial pH measurements, it was shown that the process is governed by a strong pH increase in the vicinity of the cathodic plate, due to the electrochemical reduction of oxygen and/or water molecules. The composition, the structure, and the morphology of the struvite layers were characterized by EDX analysis, X-ray diffraction, and Raman spectroscopy. Struvite electrochemical precipitation appears as an alternative method to recover P, N, and Mg from waste waters. Magnesiumammonium phosphate hexahydrate, MgNH 4 PO 4 ·6H 2 O, referred to by its mineralogical name struvite is a poorly soluble mineral, which causes severe and costly problems by forming tenacious crystalline scale, especially in the animal wastewater treatment plants. [1][2][3] Animal sludge, in particular swine manures, which are particularly rich in nitrogen and phosphorous, 4 are currently treated by an anaerobic digestion process which produces ammonia by degradation of protein, and dissolved magnesium and phosphate by cell lysis. In order to prevent scaling problems, and to remove a large part of these pollutants from the supernatant, an efficient solution consists in forcing the precipitation of struvite. 2,5,6 The solution pH is increased either by addition of a chemical such as NaOH or Mg͑OH͒ 2 , 7 or by extracting the dissolved CO 2 gas by air bubbling or by a vigorous turbulent agitation. 8,9 This process presents an additional economical benefit: magnesium ammonium phosphate hexahydrate is an excellent fertilizer, and the solid precipitate can be advantageously recovered and valorized in agriculture. 10-12 Struvite chemical precipitation has been studied in full detail by several authors, and various analytical methods are available to calculate the crystallization conditions. 13,14 Crystallization can occur when the ionic product of magnesium, ammonium, and phosphate ions becomes larger than the solubility product K s 0 . According to Babic-Ivancic et al. 15 and Ohlinger et al., 2 K s 0 is very small ͑pK s 0 = 13.2 to 13.36 at 25°C͒. It was observed that the homogeneous precipitation becomes effective past a threshold of relative supersaturation value of 2.0. 16 In the presence of foreign ions, the solubility is expressed by the conditional solubility product K so , which decreases with increasing pH. 17 Some authors found that a ratio of ͓NH 4 ͔/͓Mg͔ in the range 0.6 to 1 and a pH larger than 9 were required for struvite precipitation. 18 Practically, it was observed that the rates of nucleation and growth of this mineral become significant when the pH was larger than about 8. It was recently shown that the presence of calcium in the media has a significant impact on struvite crystallization. 19 On the other hand, electrochemical deposition is an alternative attractive method for the synthesis of mineral layers at ordinary temperature. It...

Influence of tempereature on Struvite precipitation by CO2‐deagassing method

“…The precipitate is composed of dispersed individual crystals. The crystals have needle-like shape, which has been observed previously [14,15] using electrochemical and degassing by air bubbling precipitation methods. No amorphous phase was observed in the SEM photos.…”

Phosphorus recovery as struvite from farm, municipal and industrial waste: Feedstock suitability, methods and pre-treatments

“…2). During the process, electrochemical reduction of water or oxygen takes place at the cathode forming hydroxide ions (O 2 +2H 2 O+4e ), while hydrogen gas (H 2 ) is released (H 2 O+e 2 H 2 +OH ) (Moussa et al, 2006;Wang et al, 2010). This reaction elevates the pH in the vicinity of the cathode into the alkaline range and results in rapid precipitation of struvite (Wang et al, 2010).…”

Formation of pure struvite at neutral pH by electrochemical deposition

“…Electrochemical deposition was earlier reported as an efficient method to trap calcium carbonate (CaCO 3 ) for softening hard water [22]. Clearly, Moussa et al [23] also used this method to deposit pure struvite on electrode surface from an aqueous solution containing phosphate, ammonium and magnesium ions, in which the experimental cell was a three-electrode system consisting of gold grid and nickel rod as the working electrodes respectively and the saturated Hg/HgSO 4 reference electrode (SSE). Inspired by this successful performance, we decided to investigate whether electrochemical deposition could be applied to trap and recover struvite with a high purity near the neutral pH from synthetic wastewater and/or real wastewater with cheaper electrodes.…”