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Open Access Publications from the University of California

Scholarly Works (6 results)

  • Article
  • Peer Reviewed
UC Berkeley Previously Published Works (2024)
To oxidize trace concentrations of organic contaminants under conditions relevant to surface- and groundwater, air-diffusion cathodes were coupled to stainless-steel cathodes that convert atmospheric O2 into hydrogen peroxide (H2O2), which then was activated to produce hydroxyl radicals (·OH). By separating H2O2 generation from its activation and employing a flow-through electrode consisting of stainless-steel fibers, the two processes could be operated efficiently in a manner that overcame mass-transfer limitations for O2, H2O2, and trace organic contaminants. The flexibility resulting from separate control of the two processes made it possible to avoid both the accumulation of excess H2O2 and the energy losses that take place after H2O2 has been depleted. The decrease in treatment efficacy occurring in the presence of natural organic matter was substantially lower than that typically observed in homogeneous advanced oxidation processes. Experiments conducted with ionized and neutral compounds indicated that electrostatic repulsion prevented negatively charged ·OH scavengers from interfering with the oxidation of neutral contaminants. Energy consumption by the dual-cathode system was lower than values reported for other technologies intended for small-scale drinking water treatment systems. The coordinated operation of these two cathodes has the potential to provide a practical, inexpensive way for point-of-use drinking water treatment.
    Cover page: Electrochemical Hydrogen Peroxide Generation and Activation Using a Dual-Cathode Flow-Through Treatment System: Enhanced Selectivity for Contaminant Removal by Electrostatic Repulsion.
    • Thesis
    • Peer Reviewed
    UC Berkeley Electronic Theses and Dissertations (2022)

    Distributed water treatment has emerged as an alternative to centralized water systems as a means of providing drinking water in locations where population density is too low to support the distribution networks needed to move treated water to places where it will be used. By exploitation of non-traditional water sources, distributed water treatment also has the potential to address the problem of water scarcity. Electrochemical advanced oxidation processes are promising techniques for distributed water treatment because of their small footprint, flexible operation and ability to remove contaminants without any chemical input. However, their adaptation has been hampered by challenges including the high cost of electrodes, slow mass transport of contaminants, high energy consumption, and difficulties in scaling up treatment processes. This research attempts to develop new electrochemical advanced oxidation processes for distributed water treatment by providing both a mechanistic understanding of the chemical reactions taking place and a better understanding of practical engineering considerations needed to deploy the technologies.Urban stormwater is one of the non-traditional water sources that is attractive to water-stressed cities because it is available in large quantities and does not require large investments in conveyance systems. To provide a means of removing chemical contaminants in distributed water systems, an electrochemical advanced oxidation process that is compatible with high-capacity stormwater recharge systems (e.g., drywells) was developed. The treatment system consisted of an electrochemical module for H2O2 production and an ultraviolent (UV) reactor for converting hydrogen peroxide (H2O2) into hydroxyl radical (●OH). To minimize the energy consumption and system footprint, production of a concentrated H2O2 stock solution prior to the storm event was evaluated. The H2O2 generation was optimized in terms of its energy efficiency and oxidant production rate. The solution pH was identified as the main factor affecting the H2O2 stability, with basic conditions leading to greater loss through dismutation. Using this information, the stability was enhanced by mixing the basic H2O2-containing catholyte with the acidic anolyte. The proposed advanced oxidation process demonstrated effective removal of trace organic contaminants in full-scale stormwater treatment system. However, the energy efficiency was limited by the inefficient UV light utilization, due to light reflection and backscattering at the water-air interface in the UV reactor as well as competition for UV light absorption with H2O2. In response to the inefficient light utilization by the UV/H2O2 process, an alternative approach for converting H2O2 to ●OH was developed. In Chapter 3, the reaction mechanism and reactive oxidant yields produced by activation of H2O2 on an inexpensive stainless-steel electrode was investigated. The stainless-steel electrode was investigated at different pH values and applied potentials using probe compounds to differentiate the production of different reactive oxidants (●OH versus Fe[IV]) from H2O2 activation. The stainless-steel electrode demonstrated high yields for converting H2O2 into ●OH at circumneutral pH conditions. In the presence of H2O2 concentrations comparable to those that might be generated by electrochemical reduction of O2, the stainless-steel electrode removed trace organic contaminants from an authentic water sample. However, loss of ●OH on or near the electrode surface decreased the utilization efficiency of ●OH. Finally, a reagent-free dual-cathode treatment process was evaluated. The system coupled the air-diffusion electrode with the stainless-steel electrode. The performance of the system was optimized separately for H2O2 generation and H2O2 activation. The optimal potential for H2O2 production on the air-diffusion electrode was identified as -0.04 V and the optimal potential for trace organic contaminant transformation on the stainless-steel electrode was identified as +0.02 V. The different optimal potential for each process suggests that the treatment could overcome the imbalance between H2O2 generation and H2O2 activation encountered in previously described composite electrodes. In tests of the combined electrochemical treatment system, the dual-cathode treatment process oxidized uncharged compounds (e.g., atrazine) but was unable to oxidize negatively charged benzoate. The efficiency of the system was also much less susceptible to competition for oxidants from negatively charged macromolecules (e.g., humic substances). These findings suggest that electrostatic repulsion minimizes the scavenging effect of negatively charged oxidant scavengers. The dual-cathode treatment system consumed about 10 kWh/m3, which is comparable to that of anodic oxidation and point-of-use UV/H2O2 process. Collectively, results of the experiments described in this dissertation indicate that electrochemical advanced oxidation processes hold promise as distributed drinking water treatment systems. The air-diffusion electrode can produce concentrated H2O2 stock solutions in an off-line reactor or lower levels of H2O2 can be produced in situ. Two H2O2 activation techniques, UV light and a stainless-steel electrode, converted H2O2 into reactive oxidant species that are capable of removing trace organic contaminants. Additional research is needed to scale up the lab scale systems, understand long-term performance of the electrodes, and develop strategies for treatment automation.

      Cover page: Electrochemical Advanced Oxidation Processes for Distributed Water Treatment
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2023)
      Low-cost stainless-steel electrodes can activate hydrogen peroxide (H2O2) by converting it into a hydroxyl radical (•OH) and other reactive oxidants. At an applied potential of +0.020 V, the stainless-steel electrode produced •OH with a yield that was over an order of magnitude higher than that reported for other systems that employ iron oxides as catalysts under circumneutral pH conditions. Decreasing the applied potential at pH 8 and 9 enhanced the rate of H2O2 loss by shifting the process to a reaction mechanism that resulted in the formation of an Fe(IV) species. Significant metal leaching was only observed under acidic pH conditions (i.e., at pH <6), with the release of dissolved Fe and Cr occurring as the thickness of the passivation layer decreased. Despite the relatively high yield of •OH production under circumneutral pH conditions, most of the oxidants were scavenged by the electrode surface when contaminant concentrations comparable to those expected in drinking water sources were tested. The stainless-steel electrode efficiently removed trace organic contaminants from an authentic surface water sample without contaminating the water with Fe and Cr. With further development, stainless-steel electrodes could provide a cost-effective alternative to other H2O2 activation processes, such as those by ultraviolet light.
        Cover page: Surface Processes Control the Fate of Reactive Oxidants Generated by Electrochemical Activation of Hydrogen Peroxide on Stainless-Steel Electrodes
        • Article
        • Peer Reviewed
        UC Berkeley Previously Published Works (2019)
        Under the conditions employed when in situ chemical oxidation is used for contaminant remediation, high concentrations of H2O2 (e.g., up to ~10 M) are typically present. Using 13C NMR, we show that in carbonate-rich systems, these high concentrations of H2O2 result in a reaction with HCO3 - to produce peroxymonocarbonate (HCO4 -). After formation, HCO4 - reacts with phenol to produce di- and tri-hydroxyl phenols. HCO4 - reacts with substituted phenols in a manner consistent with its electrophilic character. Exchanging an electron-donating substituent in the para position of a phenolic compound with an electron-withdrawing group decreased the reaction rate. Results of this study indicate that HCO4 - is a potentially important but previously unrecognized oxidative species generated during H2O2 in situ Chemical Oxidation (ISCO) that selectively reacts with electron-rich organic compounds. Under conditions in which HO· formation is inefficient (e.g., relatively high concentration of HCO3 -, low total Fe and Mn concentrations), the fraction of the phenolic compounds that are transformed by HCO4 - could be similar to or greater than the fraction transformed by HO·. It may be possible to adjust treatment conditions to enhance the formation of HCO4 - as a means of accelerating rates of contaminant removal.
          Cover page: Impact of Peroxymonocarbonate on the Transformation of Organic Contaminants during Hydrogen Peroxide in Situ Chemical Oxidation
          • Article
          • Peer Reviewed
          UC Berkeley Previously Published Works (2023)
          Several billion metric tons per year of durable carbon dioxide removal (CDR) will be needed by mid-century to prevent catastrophic climate warming, and many new approaches must be rapidly scaled to ensure this target is met. Geologically permanent sequestration of carbon dioxide (CO2) in carbonate minerals-carbon mineralization-requires two moles of alkalinity and one mole of a CO2-reactive metal such as calcium or magnesium per mole of CO2 captured. Chemical weathering of geological materials can supply both ingredients, but weathering reactions must be accelerated to achieve targets for durable CDR. Here, a scalable CDR and mineralization process is reported in which water electrolysis is used to produce sulfuric acid for accelerated weathering, while a base is used to permanently sequester CO2 from air into carbonate minerals. The process can be integrated into existing extractive processes by reacting produced sulfuric acid with critical element feedstocks that neutralize acidity (e.g., rock phosphorus or ultramafic rock mine tailings), with calcium- and magnesium-bearing sulfate wastes electrolytically upcycled. The highest reported efficiency of electrolytic sulfuric acid production is achieved by maintaining catholyte feed conditions that minimize Faradaic losses by hydroxide permeation of the membrane-separated electrochemical cell. The industrial implementation of this process provides a pathway to gigaton-scale CO2 removal and sequestration during the production of critical elements needed for decarbonizing global energy infrastructure and feeding the world.
            Cover page: Electrolytic Sulfuric Acid Production with Carbon Mineralization for Permanent Carbon Dioxide Removal
            • Article
            • Peer Reviewed
            UC Berkeley Previously Published Works (2020)
            Point-of-use (POU) devices with satisfactory lead (Pb2+) removal performance are urgently needed in response to recent outbreaks of lead contamination in drinking water. This study experimentally demonstrated the excellent lead removal capability of two-dimensional (2D) MoS2 nanosheets in aqueous form and as part of a layer-stacked membrane. Among all materials ever reported in the literature, MoS2 nanosheets exhibit the highest adsorption capacity (740 mg/g), and the strongest selectivity/affinity toward Pb2+ with a distribution coefficient Kd that is orders of magnitude higher than that of other lead adsorption materials (5.2 × 107 mL/g). Density functional theory (DFT) simulation was performed to complement experimental measurements and to help understand the adsorption mechanisms. The results confirmed that the cation selectivity of MoS2 follows the order Pb2+ > Cu2+ ≫ Cd2+ > Zn2+, Ni2+ > Mg2+, K+, Ca2+. The membrane formed with layer-stacked MoS2 nanosheets exhibited a high water flux (145 L/m2/h/bar), while effectively decreasing Pb2+ concentration in drinking water from a few mg/L to less than 10 μg/L. The removal capacity of the MoS2 membrane is a few orders of magnitude higher than that of other literature-reported membrane filters. Therefore, the layer-stacked MoS2 membrane has great potential for POU removal of lead from drinking water.
            • 1 supplemental ZIP
            Cover page: Superselective Removal of Lead from Water by Two-Dimensional MoS2 Nanosheets and Layer-Stacked Membranes
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