Reduction of perchlorate using zero-valent titanium (ZVT) anode: kinetic models.

The kinetics of perchlorate reduction by zero-valent titanium (ZVT) undergoing electrical pitting corrosion was described by interactions of two domains (pit and solution). Two kinetic models were developed based on two possible inhibition mechanisms. A competitive adsorption model was developed based on surface coverage of perchlorate and chloride on bare ZVT, and a Ti(II) consumption model was developed based on Ti(II) oxidation by electrochemically developed chlorine. Both models well predicted perchlorate concentration changes in the solution. The competitive adsorption model showed that chloride has a higher adsorption affinity on both sites where oxidative dissolution of ZVT occurs and where chloride oxidation occurs. Also, the rates of perchlorate removal and chloride oxidation were directly proportional to current applied. For the Ti(II) consumption model, the rate constant of Ti(II) production was dependent on current. The rate of chloride oxidation is also believed to be proportional to current, but this conclusion cannot be made with confidence. Both kinetic models described changes in perchlorate concentration well. However, the Ti(II) consumption model was limited in its ability to predict chloride concentration. This limitation was probably caused by a lack of available information like electrochemical oxidation of chloride on bare ZVT and Ti(II) oxidation by chlorine.

Perchlorate reduction during electrochemically induced pitting corrosion of zero-valent titanium (ZVT).

Zero-valent metals and ionic metal species are a popular reagent for the abatement of contaminants in drinking water and groundwater and perchlorate is a contaminant of increasing concern. However, perchlorate degradation using commonly used reductants such as zero-valent metals and soluble reduced metal species is kinetically limited. Titanium in the zero-valent and soluble states has a high thermodynamic potential to reduce perchlorate. Here we show that perchlorate is effectively reduced to chloride by soluble titanium species in a system where the surface oxide film is removed from ZVT and ZVT is oxidized during electrochemically induced pitting corrosion to produce reactive soluble species. The pitting potential of ZVT was measured as 12.77±0.04 V (SHE) for a 100 mM solution of perchlorate. The rate of perchlorate reduction was independent of the imposed potential as long as the potential was maintained above the pitting potential, but it was proportional to the applied current. Solution pH and surface area of ZVT electrodes showed negligible effects on rates of perchlorate reduction. Although perchlorate is effectively reduced during electrochemically induced corrosion of ZVT, this process may not be immediately applicable to perchlorate treatment due to the high potentials needed to produce active reductants, the amount of titanium consumed, the inhibition of perchlorate removal by chloride, and oxidation of chloride to chlorine.

Arsenic stabilization on water treatment residuals by calcium addition.

A common method of removing arsenic from contaminated water is the co-precipitation or sorption of arsenic onto oxy-hydroxides formed by the addition of metal salts. Arsenic co-precipitation produces solids containing high concentrations of arsenic. The elevated arsenic content poses leaching problems requiring expensive disposal in certified hazardous impoundments. The objective of this research is to determine the effect of calcium addition as a stabilization agent, on arsenic desorption from ferric water treatment residuals. Due to the treatment residual's buffer capacity, desorption experiments in this study did not follow the standard Toxicity Characteristic Leaching procedure (TCLP) test. Arsenate desorption was induced in two ways: controlling solution pH in de-ionized water, and controlling solution pH in a 1.33 mM phosphate solution where phosphate is a competing anion. Desorption from laboratory treatment residuals did not generate any arsenic when calcium was present in solution, especially when excess calcium that did not join the surface of the treatment residual was present. Similarly, arsenic leaching decreased when field treatment residuals were treated with lime as stabilizing agent. Ordinary Portland cement (OPC) was also tested as a stabilizing agent in conjunction with lime since long term lime stabilization can be slowly consumed when directly exposed to atmospheric CO(2). The solidification and stabilization (S/S) technique with lime and OPC was shown to be successfully applied to the immobilization of arsenic tainted water treatment residuals.

An evaluation of the colloidal stability of metal working fluid.

The effect of calcium on the stability of a commercial MWF is characterized through the experimental determination of the stability ratio, W. Three experimental methods of stability ratio evaluation are investigated. (1) The initial slope of the absorbance versus time curve is used to estimate the rate of coagulation. (2) Absorbance measurements are used to estimate N(0)/N with time. The stability ratio is determined from the slope of N(0)/N versus time. (3) Photon correlation spectroscopy (PCS) measurements of the volume distribution with time are used to estimate N(0)/N with time. Electrophoretic mobility was also measured and used to determine the fast coagulation concentration of the MWF. The accuracy of the experimentally determined stability ratios is evaluated using a population balance coagulation model. The model predicts the population distribution of a coagulating dispersion with time based on an initial particle size distribution and stability ratio. The model results were compared with the PCS-measured distributions to determine which stability ratio evaluation method best describes the stability of the MWF emulsion studied. Using the initial slope of the absorbance versus time curve to determine the fast coagulation concentration correlates well with electrophoretic mobility measurements. However, using absorbance measurements to determine the rate of coagulation underestimates the stability ratio of the MWF studied by orders of magnitude. N(0)/N values calculated from absorbance measurements provide a reasonable estimate of the stability ratio but inconsistencies in the method decrease its reliability. The stability ratio derived from PCS measurements appears to provide the most accurate, reliable description of MWF stability.

Prediction of three-dimensional fractal dimensions using the two-dimensional properties of fractal aggregates.

Fractal dimension analysis using an optical imaging analysis technique is a powerful tool in obtaining morphological information of particulate aggregates formed in coagulation processes. However, as image analysis uses two-dimensional projected images of the aggregates, it is only applicable to one and two-dimensional fractal analyses. In this study, three-dimensional fractal dimensions are estimated from image analysis by characterizing relationships between three-dimensional fractal dimensions (D(3)) and one (D(1)) and two-dimensional fractal dimensions (D(2) and D(pf)). The characterization of these fractal dimensions were achieved by creating populations of aggregates based on the pre-defined radius of gyration while varying the number of primary particles in an aggregate and three-dimensional fractal dimensions. Approximately 2000 simulated aggregates were grouped into 33 populations based on the radius of gyration of each aggregate class. Each population included from 15 to 115 aggregates and the number of primary particles in an aggregate varied from 10 to 1000. Characterization of the fractal dimensions demonstrated that the one-dimensional fractal dimensions could not be used to estimate two- and three-dimensional fractal dimensions. However, two-dimensional fractal dimensions obtained statistically, well-characterized relationships with aggregates of a three-dimensional fractal characterization. Three-dimensional fractal dimensions obtained in this study were compared with previously published experimental values where both two-dimensional fractal and three-dimensional fractal data were given. In the case of inorganic aggregates, when experimentally obtained three-dimensional fractal dimensions were 1.75, 1.86, 1.83+/-0.07, 2.24+/-0.22, and 1.72+/-0.13, computed three-dimensional fractal dimensions using two-dimensional fractal dimensions were 1.75, 1.76, 1.77+/-0.04, 2.11+/-0.09, and 1.76+/-0.03, respectively. However, when primary particles were biological colloids, experimentally obtained three-dimensional fractal dimensions were 1.99+/-0.08 and 2.14+/-0.04, and computed values were both 1.79+/-0.08. Analysis of the three-dimensional fractal dimensions with the imaging analysis technique was comparable to the conventional methods of both light scattering and electrical sensing when primary particles are inorganic colloids.

Disinfection and dewatering of wastewater solids by interstitial vapor generation.

Disinfection of wastewater solids (waste activated solids [WAS]) by interstitial vapor generation was investigated. In addition to the magnitude of disinfection, the amount of water removed and cost relative to traditional residuals disinfection processes was also examined. The process of interstitial vapor generation occurs as a result of the rapid heating of liquid in the interstices of the solid-liquid array. Intense heating causes boiling of the slurry liquid, resulting in an expanding vapor front that simultaneously dewaters the wastewater solids and contributes to the destruction of viable pathogenic microorganisms. Objectives of the study were threefold: (1) to validate disinfection of WAS using the interstitial vapor technique; (2) establish the degree of possible drying of the residuals using the techniques; and (3) establish the key operating variables for the process. Results showed a significant reduction in the most probable number of total coliforms and Escherichia coli (E. coli). Specifically, greater than four-log unit reductions were produced for both total coliform and E. coli bacteria. In addition to quantifying the reduction in bacteria, the percent solids were increased from an initial amount of 7.6% (mass basis) to a final solids content greater than 90% using optimal processing conditions. Cost comparisons were also conducted and shown to be quite favorable when compared with traditional disinfection methods such as lime addition. Because of the high level of E. coli reduction achieved, the process of interstitial vapor generation is shown to be capable of converting a class B biosolids into a class A pathogen reduced product. For example, an initial most probable number (MPN) of 1.2 x 10(6) E. coli bacteria were reduced to 19 at the extreme conditions of the process, well below the requirement of an MPN of 1000 for fecal coliform bacteria. Given its ability to disinfect and dewater wastewater solids, the interstitial vapor generation process was found to be a robust and beneficial technique to produce an environmental and publicly acceptable recyclable biosolids resource.

Adsorption and transport of arsenic(V) in experimental subsurface systems.

The adsorption and transport of As(V) in a heterogeneous, iron oxide-containing soil was investigated in batch and column laboratory experiments. The As(V) adsorbed rapidly to the soil over the first 48 h, but continued to adsorb slowly over the next several weeks, clearly indicating the potential for rate-limited transport. The equilibrium As(V) adsorption isotherm was markedly nonlinear, further indicating the potential for nonideal transport. A model developed for the adsorption of As(V) to hydrous ferric oxide (HFO) was able to predict the pH-dependent adsorption of As(V) to the soil in batch experiments within 0.116 to 0.726 root mean square error (RMSE). Arsenic(V) was significantly retarded in column transport experiments. The column transport experiments were modeled using the one-dimensional advection-dispersion equation, considering both linear and nonlinear adsorption equilibrium. Although the nonlinear local equilibrium model (NLLE, RMSE = 0.273) predicted the data better than the linear local equilibrium model (LLE, RMSE = 0.317), As(V) breakthrough occurred more rapidly than predicted by either model due to adsorption nonequilibrium. However, due to the presence of an irreversible or slowly desorbing fraction, the peak aqueous As(V) concentration (0.624 mg L(-1)) and the total amount of As(V) recovered (44%) was lower than predicted based on the two equilibrium models (NLLE and LLE). For the conditions used in this study [1 mg L(-1) As(V), pH 4.5 and 9,0-0.25 mM PO4, 0.53-1.6 cm min(-1) pore water velocity], the effect on As(V) mobility and recovery increased in the order pH < pore water velocity < PO4.