Shirish Agarwal

A feasibility study on Pd/Mg application in historically contaminated sediments and PCB spiked substrates.

A vast majority of literature on bimetals deals with aqueous contaminants, very little being on organics strongly adsorbed on sediments and hence very challenging to remediate. Having previously reported materials, mechanistic and parametric aspects of PCB dechlorination with Pd/Mg bimetals, here we evaluate their ability to dechlorinate (i) PCBs spiked in clean clays and sediments and (ii) historically contaminated aged sediments (Waukegan Harbor, WHS). It was determined that while water, as a medium, dechlorinated low surface area clays it was ineffective with high surface area clays and sediments. Dechlorination was governed by desorption of PCBs, hence addition of 10-25% ethanol improved dechlorination performance. Triton X-100, a surfactant, effectively desorbed PCBs from WHS but their dechlorination was not observed. Investigating possible causes, we found that while (i) Pd/Mg completely dechlorinated multi-component commercial PCB mixtures (i.e. complex PCB distribution in WHS did not limit Pd/Mg effectiveness) and (ii) active components in extraction media did not inhibit dechlorination, sulfide in WHS was poisoning Pd, compromising its activity.

Assessment of the functionality of a pilot-scale reactor and its potential for electrochemical degradation of calmagite, a sulfonated azo-dye

Electrochemical degradation (ECD) is a promising technology for in situ remediation of diversely contaminated environmental matrices by application of a low level electric potential gradient. This investigation, prompted by successful bench-scale ECD of trichloroethylene, involved development, parametric characterization and evaluation of a pilot-scale electrochemical reactor for degradation of calmagite, a sulfonated azo-dye used as a model contaminant. The reactor has two chambers filled with granulated graphite for electrodes. The system has electrical potential, current, conductivity, pH, temperature, water-level and flow sensors for automated monitoring. The reactor supports outdoor and fail-safe venting, argon purging, temperature regulation and auto-shutdown for safety. Treatment involves recirculating the contaminated solution through the electrode beds at small flow velocities mimicking low fluid-flux in groundwater and submarine sediments. The first phase of the investigation involved testing of the reactor components, its parametric probes and the automated data acquisition system for performance as designed. The results showed hydraulic stability, consistent pH behavior, marginal temperature rise (<5 degrees C) and overall safe and predictable performance under diverse conditions. Near complete removal of calmagite was seen at 3-10V of applied voltage in 8-10h. The effects of voltage and strength of electrolyte on degradation kinetics have been presented. Further, it was observed from the absorption spectra that as calmagite degrades over time, new peaks appear. These peaks were associated with degradation products identified using electrospray ionization mass spectrometry. A reaction mechanism for ECD of calmagite has also been proposed.

Chapter 25 – Magnesium-Based Corrosion Nano-Cells for Reductive Transformation of Contaminants

Magnesium, with its potential to reduce a variety of aqueous contaminants, unique self-limiting corrosion behavior affording long active life times, natural abundance, low cost, and environmentally friendly nature, promises to be an effective technology. However, nanoparticles of Mg are difficult to produce by reduction of its aqueous salts owing to the low reduction potential of Mg2+. Alternately, a top-down synthesis pathway of breaking down micro-sized Mg to nanosized particles in an inert atmosphere may be used. In the synthesis of Pd/Mg using nanotechniques, depositing nanoscale Pd islands onto microsized Mg substrates produces Pd islands with high catalytic activity at low Pd content, reduced bioavailability of Pd during the application of Pd/Mg systems, and more selective and stable Pd islands. Upon corrosion during application to aqueous systems, Mg transforms to its oxides and hydroxides, which have been reported to have anti-biofouling properties. Also, nanocrystalline MgO is known to be an efficient adsorbent for acid gases, chlorocarbons, organophosphorus compounds, alcohols, and other organics. Further, the high oxidation potential of Mg can be exploited by coupling it with a number of metals (Ni, Ag, Cu, etc.) to provide an array of efficient corrosion nano-cells. In wider environmental terms, Mg-based composites such as Pd/Mg, Ni/Mg intermetallics are being explored for hydrogen storage toward sustainable solutions to the energy crisis.

Magnesium-Based Corrosion Nano-Cells for Reductive Transformation of Contaminants

Magnesium, with its potential to reduce a variety of aqueous contaminants, unique self-limiting corrosion behavior affording long active life times, natural abundance, low cost, and environmentally friendly nature, promises to be an effective technology. However, nanoparticles of Mg are difficult to produce by reduction of its aqueous salts owing to the low reduction potential of Mg2+. Alternately, a top-down synthesis pathway of breaking down micro-sized Mg to nanosized particles in an inert atmosphere may be used. In the synthesis of Pd/Mg using nanotechniques, depositing nanoscale Pd islands onto microsized Mg substrates produces Pd islands with high catalytic activity at low Pd content, reduced bioavailability of Pd during the application of Pd/Mg systems, and more selective and stable Pd islands. Upon corrosion during application to aqueous systems, Mg transforms to its oxides and hydroxides, which have been reported to have anti-biofouling properties. Also, nanocrystalline MgO is known to be an efficient adsorbent for acid gases, chlorocarbons, organophosphorus compounds, alcohols, and other organics. Further, the high oxidation potential of Mg can be exploited by coupling it with a number of metals (Ni, Ag, Cu, etc.) to provide an array of efficient corrosion nano-cells. In wider environmental terms, Mg-based composites such as Pd/Mg, Ni/Mg intermetallics are being explored for hydrogen storage toward sustainable solutions to the energy crisis.