Electroless Production of Fertilizer (Struvite) and Hydrogen from Synthetic Agricultural Wastewaters

Abstract

The drive toward sustainable phosphorus (P) recovery from agricultural and municipal wastewater streams has intensified. However, combining P recovery with energy conservation is perhaps one of the greatest challenges of this century. In this study, we report for the first time the simultaneous electroless production of struvite and dihydrogen from aqueous ammonium dihydrogen phosphate (NH4H2PO4) solutions in contact with either a pure magnesium (Mg) or a Mg alloy as the anode and 316 stainless steel (SS) as the cathode placed in a bench-scale electrochemical reactor. During the electroless process (i.e., in the absence of external electrical power), the open circuit potential (OCP), the formation of struvite on the anode, and the generation of dihydrogen at the cathode were monitored. We found that struvite is formed, and that struvite crystal structure/morphology and precipitate film thickness are affected by the concentration of the H n PO4 n–3/NH4 + in solution and the composition of the anode. The pure Mg anode produced a porous 0.6–4.1 μm thick film, while the AZ31 Mg alloy produced a more compact 1.7–9.9 μm thick struvite film. Kinetic analyses revealed that Mg dissolution to Mg2+ followed mostly a zero-order kinetic rate law for both Mg anode materials, and the rate constants (k) depended upon the struvite layer morphology. Fourier-transform infrared spectrometry, X-ray diffraction, and scanning electron microscopy indicated that the synthesized struvite was of high quality. The dihydrogen and Mg2+ in solution were detected by a gas chromatography–thermal conductivity detector and ion chromatography, respectively. Furthermore, we fully demonstrate that the reactor was able to remove ∼73% of the H n PO4 n–3 present in a natural poultry wastewater as mainly struvite. This study highlights the feasibility of simultaneously producing struvite and dihydrogen from wastewater effluents with no energy input in a green and sustainable approach.

Facet-Specific Dissolution–Precipitation at Struvite–Water Interfaces

“…2 Struvite (MgNH 4 PO 4 •6H 2 O), a poorly soluble crystalline mineral (the solubility product constant (K SP ) is 4.37 × 10 −14 ), 3 is generally considered as an optimal phosphate mineral for recovery from wastewater, as it contains 51.8% of P 2 O 5 based on MgNH 4 PO 4 and could potentially be used as a slow-release fertilizer. 4,5 However, naturally abundant metal ions such as Fe 3+ and Ca 2+ in soil solutions can affect the bioavailability of orthophosphates released from dissolved struvite surfaces through the precipitation of relatively insoluble iron/calcium phosphates (Fe/Ca−P). 6,7 When struvite is added as a fertilizer to soils, dissolution via the nucleation of two-dimensional (2D) vacancy islands (etch pits) or step propagation from dislocation defects is initiated due to soil humidity, which is related to the liquid−crystal interfacial free energy.…”

“…Moreover, minable phosphate rock used as a fertilizer is a nonrenewable and dwindling resource, so a potential future agricultural risk is imminent for the vast majority of countries . Struvite (MgNH 4 PO 4 ·6H 2 O), a poorly soluble crystalline mineral (the solubility product constant ( K SP ) is 4.37 × 10 –14 ), is generally considered as an optimal phosphate mineral for recovery from wastewater, as it contains 51.8% of P 2 O 5 based on MgNH 4 PO 4 and could potentially be used as a slow-release fertilizer. , However, naturally abundant metal ions such as Fe 3+ and Ca 2+ in soil solutions can affect the bioavailability of orthophosphates released from dissolved struvite surfaces through the precipitation of relatively insoluble iron/calcium phosphates (Fe/Ca–P). , …”

Self‐Co‐Electrolysis for Co‐Production of Phosphate and Hydrogen in Neutral Phosphate Buffer Electrolyte

“…Although self‐powered systems have been considered an efficient method to produce high‐value chemicals, two major drawbacks are associated with the applications of currently available SPSs: i) strong acidic and/or alkaline electrolytes are used, thus posing a safety risk to the operators, and ii) anodes are consumed without any value‐added chemicals being produced. [ 10 ]…”

“…Although self-powered systems have been considered an efficient method to produce high-value chemicals, two major drawbacks are associated with the applications of currently available SPSs: i) strong acidic and/or alkaline electrolytes are used, thus posing a safety risk to the operators, and ii) anodes are consumed without any value-added chemicals being produced. [10] Herein, we report a general and efficient self-co-electrolysis system (SCES) for the co-production of high-value chemicals at both electrodes in a neutral phosphate buffer solution (PBS) that does not require external power. This method assimilates the favorable chemical production by co-electrocatalysis and the self-energy supply of SPSs in one system.…”

Wastewater‐recovered struvite evaluation as a fertilizer‐phosphorus source for corn in eastern Arkansas

“…Struvite has been recovered from a variety of waste products through chemical, biological, and in recent years, electrochemical precipitation techniques (De‐Bashan & Bashan, 2004; Kékedy‐Nagy, Teymouri, et al., 2020; Latifian et al., 2012). Unlike chemical precipitation of struvite from wastewater, where external chemical additions are needed, electrochemical precipitation is a newly adopted approach designed to also permit energy recovery and uses a reactor with an anode that supplies Mg ions through corrosion when an electrical current is present (Kékedy‐Nagy et al., 2019; Kékedy‐Nagy, Abolhassani, et al., 2020).…”