A composite sorbent (GAC-QPVP) was prepared by coating poly(4-vinylpyridine) onto a granular activated carbon, followed by cross-linking and quaternization processes. The sorbent was characterized by scanning electron microscopy, point of zero charge measurement, and BET analysis. Batch experiments with variable pH, ionic strength, and concentrations of Cr(VI), sorbent, and competing anions were conducted to evaluate the selective sorption of Cr(VI) from aqueous solutions. The results showed that Cr(VI) sorption rates could be described by a reversible second-order kinetics, and equilibrium uptake of Cr(VI) increased with decreasing pH, decreasing ionic strength, and increasing sorbent concentration. The estimated maximum equilibrium uptake of chromium was 53.7 mg/g at pH = 2.25, 30.7 mg/g at pH = 3.65, and 18.9 mg/g at pH = 6.03, much higher than the maximum capacity of PVP-coated silica gel, an adsorbent for Cr examined previously. When compared with the untreated granular activated carbon, sorption onto GAC-QPVP resulted in much less Cr(VI) reduction and subsequent release of Cr(III). The effect of phosphate, sulfate, and nitrate was minor on the selective sorption of Cr(VI). An ion exchange model that was linked with aqueous speciation chemistry described Cr(VI) sorption reasonably well as a function of pH, ionic strength, and Cr(VI) concentration. Model simulations suggested that sorbed Cr(VI) was partially reduced to Cr(III) on the sorbent when pH was less than 4. The presence of Cr(III) on the sorbent was confirmed by the X-ray photoelectron spectroscopic analysis. Overall, the study has demonstrated that GAC-QPVP can effectively remove Cr(VI) from aqueous solutions under a wide range of experimental conditions, without significant Cr(III) release associated with the virgin GAC treatment.

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ABSTRACT. Chlorination is the most widely practiced form of disinfection in the US. However, there is concern that the disinfection by-products (DBPs) formed during chlorination might be carcinogenic. Because of this increasing concern in water supply systems, there is a need for models that can be used to predict chlorine residuals and optimize the disinfection practices. This paper presents a chlorine decay model based on the possible chlorine decay mechanisms. To evaluate this model, four raw surface and alum treated waters (Chester, Garden City, Maysville, and Lake Vandalia) were used. The chlorine residual at the end of the study period was maintained at the same concentration to avoid effects of chlorine concentration difference. Results show that this model predicts the chlorine residual extremely well, consistently yielding correlation coefficients greater than 0.98. Alum treatment substantially increased the fraction of rapidly reacting functional groups by 24% and decreased the specific chlorine demand (SCD) by an average of 14.4%. Therefore, alum coagulation processes may preferentially remove natural organic matter (NOM) having a slower reaction rate (with chlorine), higher specific chlorine demand, and higher chlorinated DBPs production.

This research involved the synthesizing of a reactive polymer, long alkyl quaternized poly(4vinylpyridine) (PVP), and coating it on the surface of silica gel to produce a granular sorbent to remove Cr(VI) from water. Batch experiments were conducted to determine the kinetics, sorption isotherm, pH effects, and influence of other anions on the chromium adsorption onto the coated silica gel. The research demonstrated that the synthesized PVP-coated silica gel (referred to as coated gel) could successfully remove chromium(VI) from solution. The adsorption of Cr(VI) by the coated gel was strongly influenced by the pH. The maximum sorption occurred at about pH 4.5-5.5 under the test conditions. The removal efficiency was 100% when the initial Cr(VI) concentration was 2.5 mg/L with 2.5 g/L of coated gel at pH 5.0. The concentrations of Cr(VI) had a pronounced effect on the rate of sorption. Compared to ion exchange, the sorption kinetics of Cr(VI) were fast (about 5h). The equilibrium sorption data fitted the Langmuir isotherm model. Chromium adsorbed on the coated gel was easily recovered under certain conditions.

A new sorbent has been prepared by coating alkyl quaternized poly(4-vinylpyridine) (PVP) on the high surface area of silica gel. The resulting granular sorbent was used to remove Cr(VI) from water. Batch experiments were conducted to determine the kinetics and sorption rate. The research demonstrated that the modified PVP coated silica gel (referred to as coated gel) could successfully remove chromium(VI) from solution and that the sorbent provided fast kinetics. A finite-bath diffusion-control model with changing bulk concentration was derived analytically for application to the coated gel sorption of Cr(VI). This model fitted the experimental data well. The sorption kinetics on modified PVP coated gel was found to be diffusion-controlled with a value of 5.8 × 10 -7 cm 2 /s for the product of the distribution coefficient and the effective diffusivity in the sorption of Cr(VI) on the coated silica gel.

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