Arsenate and Chromate Retention Mechanisms on Goethite. 2. Kinetic Evaluation Using a Pressure-Jump Relaxation Technique

Grossl, Paul R.; Eick, Matthew J.; Sparks, Donald L.; Goldberg, Sabine; Ainsworth, Calvin C.

doi:10.1021/es950654l

Cited by 336 publications

(229 citation statements)

References 17 publications

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“…These results were in accordance with previous studies, suggesting that high-chemisorbed water molecules prevented the bidentate complexes from forming, as noted for the complexation of chromium with ferrihydrite [48]. A similar substitution of water molecules by HCrO 4 − anions could take place at a higher chromium concentration, even at hydration equilibrium, explaining the formation of bidentate surface species when there is a high surface coverage, observed for the adsorption of Cr (VI) or a similar anion on ferrihydrite [29]. It should be noted that the C 10 ≡ > S(HCrO 4 )H 0 complex could also be expressed as an outer-sphere > SOH + 2 − HCrO − 4 species, since we could not distinguish between the different complexes' formulations or structures, based on one H 2 O molecule.…”

Section: Surface Complexes and Effect Of Ph And Contact Time On H 3 Osupporting

confidence: 91%

“…The variation in the extraction efficiency with the solution's pH could be related to the protonation/deprotonating of both surface groups, and to the acidity of H 2 CrO 4 (pK a1 = 0.2 and pK a2 = 6.5) [27][28][29]. According to the chromium speciation pH-diagram, the chromium (VI) was adsorbed as hydrogen chromate (HCrO 4 − ) at pH ≤ 5.0 (≥95%), as chromate (CrO 4 2− ) at pH ≥ 7.6 (≥95%), and as a mixture of these species between pH 5.0 and 7.6.…”

Section: Effect Of Ph On Chromium Adsorptionmentioning

confidence: 99%

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Adsorption of Chromium (VI) on Calcium Phosphate: Mechanisms and Stability Constants of Surface Complexes

El-Yahyaoui¹,

Ellouzi²,

Zabadi

et al. 2017

Applied Sciences

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Abstract:The adsorption of chromate on octacalcium phosphate (OCP) was investigated as a function of contact time, surface coverage, and solution pH. The ion exchange method was adapted to establish the interaction mechanism. Stoichiometry exchange of H + /OH − was evaluated at a pH range of 3-10, and obtained values ranged between 0.0 and 1.0. The surface complexes formed between chromate and OCP were found to be > S(HCrO 4 ) and > S(CrO 4 ). The logarithmic stability constant logK 1-1 , and the logK 10 values of the complexes, were 6.0 in acidic medium and 0.1 in alkaline medium, respectively. At low pH and low surface coverage, the bidentate species > S(HCrO 4 ) 2 with logK 10.5 of about 2.9, was favored at a hydration time of less than 150 min. The contribution of an electrostatic effect to the chromium uptake by the OCP sorbent, was also evaluated. The results indicate that the adsorption of chromate on OCP is of an electrostatic nature at a pH ≤ 5.6, and of a chemical nature at a pH > 5.6.

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Section: Surface Complexes and Effect Of Ph And Contact Time On H 3 Osupporting

confidence: 91%

Section: Effect Of Ph On Chromium Adsorptionmentioning

confidence: 99%

Adsorption of Chromium (VI) on Calcium Phosphate: Mechanisms and Stability Constants of Surface Complexes

El-Yahyaoui¹,

Ellouzi²,

Zabadi

et al. 2017

Applied Sciences

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“…49,50 Some of these surface hydroxyls are exchangeable, and oxyanions, such as chromate, can form both monodentate surface complexes and biatomic-bidentate surface complexes where an oxygen is shared between the oxyanion and a surface iron atom. [51][52][53] The net surface charge of a hydoxylated iron surface is pH dependant (protons readily exchange with the surface groups to produce OH 2 + , OH, or O -depending on pH. 50 For most iron-containing minerals, the solution pH value that results in no net charge on the surface (i.e.…”

Section: Sem Analysis Of Cr-reacted Zvi Couponsmentioning

confidence: 99%

Chromate Reduction in Highly Alkaline Groundwater by Zerovalent Iron: Implications for Its Use in a Permeable Reactive Barrier

Fuller

Stewart

Burke

2013

Ind. Eng. Chem. Res.

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It is not currently known if the widely used reaction of zero valent iron (ZVI) and Cr(VI) can be used in a permeable reactive barrier (PRB) to immobilise Cr leaching from hyper alkaline chromite ore processing residue (COPR). This study compares Cr(VI) removal from COPR leachate and chromate solution by ZVI at high pH. Cr(VI) removal occurs more rapidly from the chromate solution than from COPR leachate. The reaction is first order with respect to both [Cr(VI)] and the iron surface area, but iron surface reactivity is lost to the reaction.Buffering pH downwards produces little change in the removal rate or the specific capacity of iron until acidic conditions are reached. SEM and XPS analysis confirm that reaction products accumulate on the iron surface in both liquors, but that other surface precipitates also form in COPR leachate. Leachate from highly alkaline COPR contains Ca, Si and Al that precipitate on the iron surface and significantly reduce the specific capacity of iron to reduce Cr(VI). This study suggests that although Cr (VI) reduction by ZVI will occur at hyper alkaline pH, other solutes present in COPR leachate will limit the design life of a PRB. 4

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“…According to literature data, two mechanisms of arsenic binding on the surface of iron oxi-hydroxide are: the mechanism of surface precipitation and the mechanism of surface complexation. Surface complexation mechanism can be monodentate, dominant at a low surface coverage of modified zeolite, or bidendate at higher surface coverage when iron forms complexes with arsenic [42][43][44]. …”

mentioning

confidence: 99%

Natural Zeolites in Water Treatment – How Effective is Their Use

Margeta¹,

Logar²,

Šiljeg³

et al. 2013

Water Treatment

171

150

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Additional information is available at the end of the chapter http://dx.doi.org/10.5772/50738 IntroductionNatural zeolites are environmentally and economically acceptable hydrated aluminosilicate materials with exceptional ion-exchange and sorption properties. Their effectiveness in different technological processes depends on their physical-chemical properties that are tightly connected to their geological deposits. The unique treedimensional porous structure gives natural zeolites various application possibilities. Because of the excess of the negative charge on the surface of zeolite, which results from isomorphic replacement of silicon by aluminum in the primary structural units, natural zeolites belong to the group of cationic exchangers. Numerous studies so far have confirmed their excellent performance on the removal of metal cations from wastewaters. However, zeolites can be chemically modified by inorganic salts or organic surfactants, which are adsorbed on the surface and lead to the generation of positively charged oxihydroxides or surfactant micelles, and which enables the zeolite to bind also anions, like arsenates or chromates, in stable or less stable complexes. Natural zeolites have advantages over other cation exchange materials such as commonly used organic resins, because they are cheap, they exhibit excellent selectivity for different cations at low temperatures, which is accompanied with a release of non-toxic exchangeable cations (K + , Na + , Ca 2+ and Mg 2+ ) to the environment, they are compact in size and they allow simple and cheap maintenance in the full-scale applications. The efficiency of water treatment by using natural and modified zeolites depends on the type and quantity of the used zeolite, the size distribution of zeolite particles, the initial concentration of contaminants (cation/anion), pH value of solution, ionic strength of solution, temperature, pressure, contact time of system zeolite/solution and the presence of other organic compounds and anions. For water treatment with natural zeolites, standard procedures are used, usually a procedure in column or batch process. Ion exchange and adsorption properties of natural zeolites in comparison with other chemical and biological processes have the advantage of Water Treatment 82removing impurities also at relatively low concentrations and allows conservation of water chemistry, if the treatment is carried out in the column process [1]. Subject of further academic and industrial research should be to improve the chemical and physical stability of modified zeolites and to explore their catalytic properties, which would allow their use in catalytic degradation of organic pollutants. More careful consideration of their superb metal removal properties and awareness of possible regeneration or further use of contaminant/metal-loaded forms can considerably increase their environmental application possibilities, with a focus the reduction of high concentrations of cations and anions in drinking water and wastewater, for surface, und...

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Arsenate and Chromate Retention Mechanisms on Goethite. 2. Kinetic Evaluation Using a Pressure-Jump Relaxation Technique

Cited by 336 publications

References 17 publications

Adsorption of Chromium (VI) on Calcium Phosphate: Mechanisms and Stability Constants of Surface Complexes

Adsorption of Chromium (VI) on Calcium Phosphate: Mechanisms and Stability Constants of Surface Complexes

Chromate Reduction in Highly Alkaline Groundwater by Zerovalent Iron: Implications for Its Use in a Permeable Reactive Barrier

Natural Zeolites in Water Treatment – How Effective is Their Use

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