Chromate Desorption Research Articles

A multi-scale assessment of the impact of salinity on the desorption of chromate from hematite: Sea level rise implications

The solubility and transport of Cr(VI) is primarily controlled by adsorption-desorption reactions at the surfaces of soil minerals such as iron oxides. Environmental properties such as pH, ionic strength, and ion competition are expected to affect the mobility and fate of Cr(VI). Sea level rise (SLR), and consequent seawater intrusion, is creating a new biogeochemical soil environment at coastal margins, potentially impacting Cr(VI) retention at contaminated sites. We employed in-situ ATR-FTIR spectroscopy and DFT calculations to investigate at the molecular level the adsorption of Cr(VI) on the hematite surface and its desorption by sulfate, as a function of pH and ionic strength. We further used a batch experiment to assess Cr(VI) desorption at varying artificial seawater (ASW) concentrations. IR results demonstrate the complexity of Cr(VI) adsorption, showing a combination of monodentate inner-sphere complexation at high pH and dichromate outer-sphere (∼75%) at low pH. The Cr(VI)-complexes exhibited desorption induced by increasing pH values (58% of desorption) and sulfate competition (∼40% desorption). ASW desorbed ∼20% more Cr(VI), even at just 1% concentration. Our findings provide insight into Cr(VI)-adsorption complexation that controls the retention and remobilization of Cr(VI) on Fe-oxide minerals. The results point to an elevated risk of Cr(VI) mobilization in contaminated soils affected by SLR.

Efficient Retention of Chromate from Industrial Wastewater onto a Green Magnetic Polymer Based on Shrimp Peels

Magnetic chitosan grafted with Schiff’s base polymer (M-Schiff’s-Chit) was prepared as a green sorbent starting from shrimp peels for the effective removal of hexavalent chromium. Chitosan extraction involved three main stages as preconditioning, demineralization deproteinization and deacetylation. The grafting process is confirmed by elemental analysis, Fourier transform infrared spectroscopy. Adsorption experiments were carried out in batch sorption mode to investigate the effect of pH, contact time, sorbent dose, concentration of Cr(VI) and evaluating the thermodynamics. Sorption kinetics are effectively modeled using the pseudo-second order rate equation while Langmuir equation successfully fits sorption isotherms. The sorption is endothermic, spontaneous and entropic. Chromate desorption can be successfully performed with 2 M of NaCl in 0.5 M NaOH and the sorbent can be recycled for at least three sorption/desorption cycles without significant loss in sorption/desorption performances. Furthermore, the environmentally friendly and low-cost M-Schiff’s-Chit could be applied as an effective sorbent to remediate Cr(VI) contamination from tannery effluent.

Regeneration of HDTMA-modified minerals after sorption with chromate anions

Surfactant-modified minerals have been widely used for the sorption of anionic and non-polar organic contaminants. This paper investigates the regeneration of various hexadecyltrimethylammonium bromide (HDTMA)-modified natural minerals saturated with chromate anions using different solutions for the extraction of chromates that has been sorbed at optimum conditions. The examined minerals include HDTMA-modified zeolite (clinoptilolite), exfoliated vermiculite, bentonite and attapulgite (palygorskite). Deionized water and sodium chloride, potassium chloride and sodium cyanide solutions at different concentrations (0.1–3 M) were employed as chromate desorption solutions. The HDTMA/Cr(VI)-mineral type, the desorbing solution type and its concentration and the mineral–solution contact time are key parameters that affect the process. From our results it was deduced that regeneration and reuse of HDTMA-modified minerals saturated with chromate is feasible. Bentonite exhibited greater stability than the other minerals studied in adsorption/desorption processes, maintaining 73% of its initial adsorption capacity after 14 successive regeneration cycles. The regeneration performance of the examined minerals follows the order: Bentonite > Attapulgite > Zeolite > Vermiculite. Kinetic study revealed that desorption was faster than adsorption, while the pseudo-second-order equation simulated better the experimental data.

ANION EXCHANGE CAPACITY OF CHROMATE ON MODIFIED ZEOLITE CLINOPTILOLITE WITH HDTMA-Br AND ITS REGENERATION

Zeolite Clinoptilolite from Lampung, located in South of Sumatra, had been modified with surfactant hexadecyltrimethylammonium bromide (HDTMA-Br) as chromate anion exchanger. Surfactant modified zeolite (SMZ) Clinoptilolite in particle size range of 1.5 - 2.0 mm, which contained 196.7 mmol HDTMA-Br/kg zeolite, was used for anion exchange of chromate at neutral pH. This experiment was conducted in a glass column filled with 5 gram SMZ. The breakthrough chromate exchange capacity was found 1.262 mg/g SMZ, while the total capacity was found 2.107 mg/g SMZ. The regeneration of SMZ saturated with chromate was conducted using a mixed solutions of 0.28 M Na2CO3 and 0.5 M NaOH, compared with using a solution of 0.01 M Na2S2O4. The desorption of chromate achieved 92% with the mixed solutions of Na2CO3 and NaOH and 90% with the Na2S2O4 solution. The regenerated SMZ with Na2CO3-NaOH solutions was prior washed with HCl solution to remove the carbonate from SMZ, before being used for chromate sorption again. Its breakthrough capacity was reduced to 1.074 mg/g SMZ, and to 0.724 mg/g SMZ when regenerated with Na2S2O4 solution. These results indicated that regeneration of SMZ affected its exchange capacity for anion chromate. However, it is still could be acceptable, when Na2CO3/NaOH solutions were used for the regeneration of SMZ saturated with anion chromate. Keywords: anion exchange capacity, chromate, regeneration, SMZ, zeolite clinoptilolite

Adsorption of chromate on variable charge soils as influenced by ionic strength

The adsorption behavior of chromate on two variable charge soils (Oxisol and Ultisol) was investigated through batch experiments at different ionic strengths and pH values. The adsorption of chromate on the variable charge soils was found to be strongly dependent on the pH of the soil solutions. A characteristic pH was observed, which corresponds to the intersection of the chromate adsorption—pH curves at different ionic strengths. The characteristic pH values are 5.50 for Oxisol and 5.04 for Ultisol, close to the point of zero salt effect (PZSE) of these soils. The zeta potentials measured for these soils provide the evidences to support the interpretation of the effect of ionic strength on the adsorption of chromate on these variable charge soils. The adsorption behavior of chromate was interpreted by a schematic representation of chromate distribution at increasing ionic strength. The chromate desorption–pH curves were also found to intersect at pH of 5.15 and 4.89 for the Oxisol and Ultisol, respectively. It is considered that chromate adsorption by the variable charge soils was mainly determined by the electrostatic potential on the adsorption plane, which was controlled by the ionic strength of the soil solutions.

Retardation of chromate through packed columns of surfactant-modified zeolite

In this study, zeolite aggregates with particle size <0.4, 1.4–2.4, and 3.6–4.8 mm were modified by the cationic surfactant hexadecyltrimethylammonium (HDTMA) bromide to a surfactant loading level of 80, 130, and 250 mmol kg −1, respectively. The modified and unmodified zeolites were subjected to column tests to study the chromate transport and retardation as affected by particle size. At an input concentration of 11–15 mg L −1, unmodified zeolite did not retard chromate transport for all three particle size ranges. In contrast, the observed retardation factor for chromate, defined as the number of pore volumes passed when the output concentration equals to half of the input concentration, was 55, 50, and 500 for the columns packed with 3.6–4.8, 1.4–2.4, and <0.4 mm modified zeolite, respectively. Prolonged tailing of chromate desorption from the modified zeolite was the most striking feature after the feeding solution was switched from chromate to water at full breakthrough. Monitoring of HDTMA and counterion bromide concentration in the effluent revealed that slow but persistent desorption of HDTMA and bromide occurred throughout the transport experiment, which resulted in stripping off of the upper layer of the surfactant bilayer formation on zeolite. The change of HDTMA surfactant surface configuration from bilayer to monolayer resulted in a loss of functionality to absorb and immobilize chromate on the modified zeolite surfaces.

The mechanism of chromate sorption by three variable charge soils

Adsorption of chromate and desorption of the pre-adsorbed chromate were studied using three representative variable charge soils from the south of China. The mechanisms of the adsorption were discussed based on the hydroxyl release and the change of ζ potential during the chromate adsorption. The adsorption and desorption of chromate followed the same order: the Hyper-Rhodic Ferralsol > the Rhodic Ferralsol > the Haplic Acrisol. The adsorption and the desorption both increased with elevation of the equilibrium chromate concentration and decreased with increasing of the soil solution pH. The percentage of the specific adsorption of chromate was 54.0–59.4%, 54.3–60.3%, and 43.9–46.2% for the Hyper-Rhodic Ferralsol, the Rhodic Ferralsol, and the Haplic Acrisol, respectively; the percentage of the electrostatic adsorption was 40.0–46.6%, 39.7–45.8%, and 50.8–56.5% for the three soils, respectively. These findings suggest that both the specific adsorption and the electrostatic adsorption contributed to the chromate adsorption by the variable charge soils. The hydroxyl release during the chromate adsorption shared the same trend with the adsorption envelopes, and decreased with increasing of pH. This is attributed to the exchange of chromate with the hydroxyl on the soil particle surfaces and the formation of a chemical bond between chromate and the surface. Our results indicate that the adsorption of chromate resulted in a shift of ζ potential-pH curves of the soil colloids to negative values, which suggests that the adsorption increased the negative surface charge and decreased the surface potential of the soil colloids.

Sorption and desorption of chromate by wood shavings impregnated with iron or aluminum oxide

This study examined the impregnation of hybrid poplar shavings with Fe or Al oxide and their reactivity with Cr(VI). The shavings were impregnated with an average of 1.24 ± 0.18 mmol Fe g −1 or 1.51 ± 0.17 mmol Al g −1, and the impregnated Fe or Al oxide was highly amorphous. Capacity of Cr(VI) sorption by impregnated shavings was high and rate of the sorption was rapid. While the freshly sorbed Cr on the Fe oxide-shavings was readily desorbed in 0.1M NaOH, nine weeks of ageing reduced the desorbability to only 6.5% of the sorbed Cr. Extraction with 0.1 M NaOH remained feasible to recover most sorbed Cr from the Al oxide-shavings over the same ageing period. Impregnation of the shavings with Fe or Al oxide turns the wood waste into a product for removing Cr(VI) or other oxyanions from wastewater.

Competitive sorption and diffusion of chromate and sulphate in a flow system with goethite in gel beads

Column experiments and model simulations were employed to evaluate the processes involved in multicomponent solute transport in a system with heterogeneous flow. Column experiments were performed with goethite embedded in polyacrylamide gel beads. The gel forms an immobile water region that can be accessed by diffusion. A two-region transport model with diffusion into spheres was combined with a surface complexation model to predict reactive transport in the goethite–gel bead system. Chromate and sulphate breakthrough curves were measured in a set of transport experiments, along with corresponding changes in the pH of the effluent. Sorption and transport of sulphate and chromate in separate columns were predicted from independently measured sorption parameters. The model overestimated the pH changes in the effluent, possibly because of proton buffering by the polyacrylamide gel. The effect of competitive sorption on transport was examined in experiments with both anions present. The model predicted the effect of competition very well in a system initially equilibrated with sulphate, followed by infiltration with chromate. However, when sulphate was infiltrated after equilibration with chromate, chromate desorption and sulphate adsorption were clearly overestimated by the transport model. The exchange between the more strongly bound chromate and the sulphate added subsequently may be too slow to cause a substantial chromate peak in the effluent. This suggests that the local equilibrium assumption was not applicable in this case.

Adsorption of chromate by clinoptilolite exchanged with various metal cations

Unmodified zeolite surfaces show no affinity for anions, due to the fact that zeolites are negatively charged. Thus, adsorption of anions by zeolites has not been given much attention. In this work, after modification of clinoptilolite by different cations, the mineral was found to adsorb a considerable amount of the divalent anion chromate. Chromate adsorption was proportional to the K(sp) of the chromate precipitate and the amount of the exchangeable cation. The amount of chromate adsorbed was maximized when the Pb-exchanged form was used. Chromate desorption in deionized water indicated that between 2.50% and 18.60% of the adsorbed chromate was released depending upon the exchangeable cation. Some of the exchanged forms are candidate materials for adsorption and immobilization of chromate.

Kinetics of Chromate Adsorption and Desorption at Fused Quartz/Water Interfaces Studied by Second Harmonic Generation

It is now recognized that an understanding of hexavalent chromium pollution in the environment is tied to an understanding of how hexavalent chromium binds to geosorbent surfaces. We have applied the nonlinear optical laser spectroscopy surface second harmonic generation (SHG) to study the adsorption and desorption kinetics of submonolayer amounts of chromate interacting with the fused quartz/water interface at pH 7 and at room temperature. The chromate concentrations are varied between 10-6 and 10-5 M. The adsorption and desorption behavior of chromate at the fused quartz/water interface can be described by a Langmuir adsorption and a first-order chromate desorption model with an adsorption rate constant of 3(1) × 103 s-1 M-1 and a desorption rate constant of 0.9(7) × 10-3 s-1. At 300 K and pH 7, the resulting equilibrium constant for chromate binding is in good agreement with equilibrium constants obtained from Langmuir isotherm measurements carried out between pH 4 and 9. Thus, thermodynamic and kineti...