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Hypophosphite (P(I)) is ubiquitous in some waste streams. It may contribute to the eutrophication of water bodies, yet conventional methods are ineffective in treating P(I)-laden wastewater. Here, we propose a novel CaO2/ultraviolet (UV) system that can simultaneously remove P(I) and recover Ca-phosphate. This system lies in the simultaneous release of OH-, Ca2+, and H2O2, with the last being activated and producing reactive oxygen species (ROS) under UV irradiation. Subsequently, the generated ROS converts P(I) to phosphite and phosphate, forming Ca-phosphate with the released Ca2+ at pH 11.0. Beyond activating H2O2, UV light also promotes the dissolution of CaO2 powders. The system removes 97.0% of 1.0 mM P(I) in 6 h with dosing 2.0 mM CaO2. The presence of 5.0 mM HCO3- drops P(I) removal from 97.0 to 14.1% by competing with P(V) toward Ca2+. Nitrate and humic acid show similar effects by adsorbing UV light or quenching ROS. However, we may overcome the adverse effects with an enhanced CaO2 dose. For example, with 10 mM CaO2, the CaO2/UV system removed more than 78% of non-ortho P from actual electroless rinse water. These results confirmed that CaO2/UV is an exciting all-in-one system for treating P(I)-laden wastewater and recovering valuable P products.

Phosphorus (P) removal and recovery from waste streams is essential for a sustainable world. Here, we updated a previously developed abiotic electrochemical P recovery system to a bioelectrochemical system. The anode was inoculated with electroactive bacteria (electricigens) which are capable of oxidizing soluble organic substrates and releasing electrons. These electrons are then used for the reduction of water at the cathode, resulting in an increase of pH close to the cathode. Hence, phosphate can be removed with coexisting calcium ions as calcium phosphate at the surface of the cathode with a much lower energy input. Depending on the available substrate (sodium acetate) concentration, an average current density from 1.1 ± 0.1 to 6.6 ± 0.4 A/m2 was achieved. This resulted in a P removal of 20.1 ± 1.5% to 73.9 ± 3.7%, a Ca removal of 10.5 ± 0.6% to 44.3 ± 1.7% and a Mg removal of 2.7 ± 1.9% to 16.3 ± 3.0%. The specific energy consumption and the purity of the solids were limited by the relative low P concentration (0.23 mM) in the domestic wastewater. The relative abundance of calcium phosphate in the recovered product increased from 23% to 66% and the energy consumption for recovery was decreased from 224 ± 7 kWh/kg P to just 56 ± 6 kWh/kg P when treating wastewater with higher P concentration (0.76 mM). An even lower energy demand of 21 ± 2 kWh/kg P was obtained with a platinized cathode. This highlights the promising potential of bioelectrochemical P recovery from P-rich waste streams.

Phosphorus (P) is an essential nutrient for living organisms and cannot be replaced or substituted. In this paper, we present a simple yet efficient membrane free electrochemical system for P removal and recovery as calcium phosphate (CaP). This method relies on in situ formation of hydroxide ions by electro mediated water reduction at a titanium cathode surface. The in situ raised pH at the cathode provides a local environment where CaP will become highly supersaturated. Therefore, homogeneous and heterogeneous nucleation of CaP occurs near and at the cathode surface. Because of the local high pH, the P removal behavior is not sensitive to bulk solution pH and therefore, efficient P removal was observed in three studied bulk solutions with pH of 4.0 (56.1%), 8.2 (57.4%), and 10.0 (48.4%) after 24 h of reaction time. While P removal efficiencies are not generally affected by bulk solution pH, the chemical-physical properties of CaP solids collected on the cathode are still related to bulk solution pH, as confirmed by structure characterizations. High initial solution pH promotes the formation of more crystalline products with relatively high Ca/P molar ratio. The Ca/P molar ratio increases from 1.30 (pH 4.0) to 1.38 (pH 8.2) and further increases to 1.55 (pH 10.0). The formation of CaP precipitates was a typical crystallization process, with an amorphous phase formed at the initial stage which then transforms to the most stable crystal phase, hydroxyapatite, which is inferred from the increased Ca/P molar ratio from 1.38 (day 1) to the theoretical 1.76 (day 11) and by the formation of needlelike crystals. Finally, we demonstrated the efficiency of this system for real wastewater. This, together with the fact that the electrochemical method can work at low bulk pH, without dosing chemicals and a need for a separation process, highlights the potential application of the electrochemical method for P removal and recovery.

Gold is commonly liberated from sulfide minerals by chemical and biological oxidation. Although these technologies are successful, they are costly and produce acidic waste streams. Removal of mineral-sulfur to overcome the mineralogical barrier could also be done by bioreduction, producing hydrogen sulfide (H(2)S). To make the sulfur within these minerals available for bioreduction, the use of partial bio-oxidation as a pretreatment to oxidize the sulfides to elemental sulfur was investigated in gas lift loop reactor experiments. Experiments at 35 °C using a refractory concentrate showed that at pH 2 arsenopyrite is preferentially partially oxidized over pyrite and that elemental sulfur can be subsequently converted into H(2)S at pH 5 via bioreduction using H(2) gas. A single partial bio-oxidation/bioreduction treatment increased the gold recovery of the concentrate from 6% to 39%. As elemental sulfur seems to inhibit further oxidation by covering the mineral surface, several treatments may be required to reach a gold recovery90%. Depending on the number of treatments this method could be an interesting alternative to bio-oxidation.

Scorodite is an arsenic mineral with the chemical formula FeAsO(4)*2H(2)O. It is the most common natural arsenate associated with arsenic-bearing ore deposits. In the present study we show that the thermoacidophilic iron-oxidizing archaeon Acidianus sulfidivorans is able to precipitate scorodite in the absence of any primary minerals or seed crystals, when grown on 0.7 g L(-1) ferrous iron (Fe(2+)) at 80 degrees C and pH 1 in the presence of 1.9 g L(-1) arsenate (H(3)AsO(4)). The simultaneous biologically induced crystallization of ferric iron (Fe(3+)) and arsenic to scorodite prevented accumulation of ferric iron. As a result, crystal growth was favored over primary nucleation which resulted in the formation of highly crystalline biogenic scorodite very similar to the mineral scorodite. Because mineral scorodite has a low water solubility and high chemical stability, scorodite crystallization may form the basis for a novel method for immobilization of arsenic from contaminated waters with high arsenic concentrations.