Concentrating nutrients and recovering water and energy from source separated urine using osmotic microbial fuel cell

This work presents the use of osmotic microbial fuel cell (OsMFC), for the first time, to concentrate nutrients and recover water and energy from source separated urine. Four sets of concentration of fresh urine as feed and NaCl as draw were examined: 10% fresh urine vs 0.25 M NaCl; 10% fresh urine vs 2 M NaCl; fresh urine vs 0.25 M NaCl; and fresh urine vs 2 M NaCl. A maximum water flux of 14.27 LMH was attained when 10% of fresh urine and 2 M of NaCl were used as feed and draw solutions, respectively. Additionally, OsMFC concentrates ~99% of TOC, TN, NH₄⁺, and 100% of PO₄^3- and NO₃^- from urine at the feed side. Polarization studies indicate that the power generation in OsMFC is related to the rate of change of conductivity and the initial conductivity of the anolyte. The maximum (0.12187 W m^-3) and minimum power densities (5.3372 × 10^-4 W m^-3) were obtained for the conditions of fresh urine vs 0.25 M NaCl and 10% fresh urine vs 0.25 M NaCl, respectively. The study shows that OsMFC is an effective pretreatment process to concentrate nutrients from urine by recovering water and energy, simultaneously.

An efficient system for electro-Fenton oxidation of pesticide by a reduced graphene oxide-aminopyrazine@3DNi foam gas diffusion electrode

A stable rGO-AmPyraz@3DNiF gas diffusion electrode was prepared via modification of 3D nickel foam (3D-NiF) with aminopyrazine functionalized reduced graphene oxide (rGO-AmPyraz) for the electro Fenton (EF) process. The generation capacity of H₂O₂ and OH radicals by this electrode was assessed relative to 3DNiF and rGO-AmPyraz@indium tin oxide (ITO) electrodes and with/without a coated Fe₃O₄ plate. The rGO-AmPyraz@3DNiF electrode showed the maximum production of these radicals at 2.2 mmol h^-1 and 410 μmol h^-1, respectively (pH 3) with the least leaching of Ni²⁺ such as < 0.5 mg L^-1 even after 5 cycles (e.g., relative to 3DNiF (24 mg L^-1). Such control on Ni ion leaching was effective all across the tested pH from 3 to 8.5. Its H₂O₂ generation capacity was far higher than that of the nanocarbon supported on commercially available ITO conductive glass. The mineralization of dichlorvos (at initial concentration: 50 mg L^-1) was confirmed with its complete degradation as the concentrations of the end products (e.g., free Cl^-1 (5.36 mg L^-1) and phosphate (12.89 mg L^-1)) were in good agreement with their stoichiometric concentration in dichlorvos. As such, the proposed system can be recommended as an effective electrode to replace nanocarbon-based product commonly employed for EF processes.

Reduced graphene oxide and biofilms as cathode catalysts to enhance energy and metal recovery in microbial fuel cell

In this study, reduced graphene oxide (rGO) was developed and employed as cathode catalyst in a membrane-less microbial fuel cell (MFC) to improve energy and metal (copper) recovery in combined with biofilms. Results showed that rGO-based cathode exhibited better characterizations in structure and electron transfer than graphene oxide (GO)-based cathode. The voltage with rGO was about 67% increased, and Cu²⁺ removal efficiency was 43% improved as compared to GO. Cu species on cathode demonstrated the favorable Cu²⁺ reduction to Cu with the catalysis of rGO. Moreover, microbial community analysis indicated that rGO-based cathode exhibited better biocompatibility for functional bacteria that related to electron transfer and Cu²⁺ resistance, such as Geobacter and Pseudomonas, demonstrating the interspecific synergism of microorganisms for efficient energy and copper recovery. It will be of important significance for the heavy metal and energy recovery from low concentrations wastewater by using microbial fuel cell.

Hexavalent chromium reduction through redox electrolytic cell with urea and cow urine as anolyte

The present study demonstrates the potential utilization of urea/cow urine as anolyte for Cr(VI) reduction via a simple three-chambered electrolytic cell. The inherent chemical energy in the dual-waste stream (Cr(VI)-urea/urine) is employed for its self-oxidation-reduction without the need for any external energy supply. Ni foam as electroactive anode and catalyst-free carbon felt as cathode, along with the appropriate positioning of ion-selective separators, indirectly improved the cell performance by impeding electrolyte crossover. A fundamental study involving five different membrane configurations was conducted herein to improve Cr(VI) reduction efficiency. The Cr(VI) reduction efficiencies were 11.84 ± 0.27%, 10.55 ± 0.17%, 77.24 ± 0.38% at 24 h, 13.57 ± 0.25% at 72 h with glass frit, cation exchange membrane (CEM), sandwiched membrane, and anion exchange membrane (AEM) as separators in a dual-chambered H-cell, respectively, with an initial Cr(VI) concentration of 100 mg/L. The fifth configuration, consisting of a middle chamber between the anode and cathode with the CEM close to the anode and the AEM close to the cathode resulted in a reduction efficiency of 79.98 ± 2.24% within 45 min for an initial Cr(VI) concentration of 400 mg/L. The first order rate constants were determined to be 0.024, 0.018, and 0.013 min^-1 for Cr(VI) concentrations of 100, 200, and 400 mg/L, respectively. Moreover, when urea was replaced with cow urine as anolyte, a reduction efficiency of 98.94 ± 1.28% was achieved at pH 2 in 45 min with 400 mg/L as initial Cr(VI) concentration. Furthermore, the XPS spectra of reduced Cr corresponding to binding energies of 579.4 eV and 589.3 eV, respectively, confirmed the presence of low-toxic Cr(III). The effect of applied load, initial Cr(VI) and urea concentration, Cr(VI) reduction under different initial H₂SO₄ concentrations were succinctly investigated to evaluate the performance of the electrolytic cell. The redox electrolytic cell can thus be an alternative to the conventional chemical or energy intensive processes for the reduction of hexavalent chromium.

Low energy liquid plasma for direct reduction and formation of rGO-aminopyridine hybrid for electrical and environmental applications

The present works demonstrate the formation of reduced graphene oxide (r-GO) and aminopyridine (r-GO-(AmPy)_n) hybrid by submerged liquid plasma process. Raman spectra shows the ID/IG ratio of GO, r-GO and r-GO-(AmPy)_n are 2.2, 1.91 and 2.0, respectively. The decrease in ID/IG ratio of r-GO clearly indicate the reduction of oxygen functional groups and restoration of sp² network in r-GO domain. The pyridinic and pyrrolic functional group present in rGO-(AmPy)_n hybrids shows a distinct redox peaks in cyclic voltameter at the scan rates of 5, 10 and 25mV/s. The r-GO-(AmPy)_n hybrid displayed an excellent electrochemical performance with a high specific capacitance of 418, 400 and 381F/g at the current densities of 0.1, 0.2 and 0.3A/g, respectively. Electro chemical generation of H₂O₂ at the rGO-(AmPy)_n surface was evaluated and found to be 0.72mmol for the fixed reaction time of 60min. Similarly, conversion of H₂O₂ to OH radical with a coated Fe₃O₄ plate catalyst was found to be 0.15mmol. A H₂O₂/OH radicals generated in the electrochemical cell was used for the degradation of methyl parathion. During the degradation (7.5mg/L), two intermediates like methylparaoxon, and p-nitrophenol were identified by the Gas Chromatography and Mass Spectrometer analysis.