Chromate sorption

Sullivan, E.J., Hunter, D.B., and Bowman, R.S., Fourier transform raman spectroscopy of sorbed HDTMA and the mechanism of chromate sorption to surfactant-modified clinoptilolite, Environ. Sci. Technol., 32, 1948, 1998. 

TABLE 5.3. Rate and Quantity of Chromate Sorption in the Presence of Silicic Acid as a Function of pH... 

Competing anions have a drastic effect on Cr(VI) adsorption. Chromate sorption in the natural environment is highly influenced by electrostatic conditions imposed by common anions (Cl , NO , SO ", HCO , etc.), which are bound to the soil or sediment surface. Accordingly, adsorption of Cr(VI) is minimal in soil. 

Feltz, A. Martin, A. (1987) Solid-state reactivity and mechanisms in oxide systems. 11 Inhibition of zinc ferrite formation in zinc oxide - a-iron(lll) oxide mixtures with a large excess of a-iron(lll) oxide. In Schwab, G.M. (ed.) Reactivity of solids. Elsevier, 2 307—313 Fendorf, S. Fendorf, M. (1996) Sorption mechanisms of lanthanum on oxide minerals. Clays Clay Miner. 44 220-227 Fendorf, S.E. Sparks, D.L. (1996) X-ray absorption fine structure spectroscopy. In Methods of Soil Analysis. Part 3 Chemical Methods. Soil Sd. Soc. Am., 377-416 Fendorf, S.E. Eick, M.J. Grossl, P. Sparks, D.L. (1997) Arsenate and chromate retention mechanisms on goethite. 1. Surface structure. Environ. Sci. Techn. 31 315-320 Fendorf, S.E. Li,V. Gunter, M.E. (1996) Micromorphologies and stabilities of chromiu-m(III) surface precipitates elucidated by scanning force microscopy. Soil Sci. Soc. Am. J. 60 99-106... 

Silicate, chromate, molybdenate, and humic acids also may create significant interferences with the sorption of As(V) and As(III) on zerovalent iron (Su and Puls, 2001b Giasuddin, Kanel and Choi, 2007). Interferences from borate and sulfate, however, were negligible with As(V) and only minor with As(III). Some interferences with As(III) and As(V) sorption occurred with carbonate and nitrate (Su and Puls, 2001b). Similarly, Farrell et al. (2001) concluded that carbonate and nitrate could hinder the efforts of zerovalent iron to lower As(V) concentrations in water to below 5 pgL-1. 

For the second pilot test, the 14-40 SMZ was excavated from the frame, a nylon screen on the barrier frame was removed, and two sections of the frame were refilled with 8-14 mesh (2.4-1.4 mm) SMZ. The remaining one-third of the frame was filled with iron/SMZ pellets as part of another project. After steady water flow was reestablished, chromate and PCE were injected over a period of eight weeks. No plume deflection occurred in the test with the 8-14 SMZ. The SMZ fully intercepted the contaminant plume and prevented migration of contaminants downgradient of the barrier. Near the end of the test, chromate and PCE were detected in samplers installed in the upgradient portion of the SMZ. The estimated retardation factors for chromate and PCE in the pilot test were 44 and 39, respectively. These retardation factors are very close to the values of 42 and 29 for chromate and PCE predicted from laboratory sorption isotherm experiments. 

The grain size distributions of the two batches of SMZ were determined by sieve analysis. Chromate and PCE sorption isotherms for each batch of SMZ were prepared using methods described earlier (Li and Bowman 1997 Li and Bowman 1998). 

The mean hydraulic conductivities for the aquifer sand determined using the two laboratory methods (constant-head and falling-head permeameters) were 5 10"4 m sec"1 and 2 1 O 4 m sec"1, respectively. The isotherm results showed that the sand had negligible sorption capacity for either chromate 01 PCE. 

The retention of both chromate and PCE is similar for the two size fractions of SMZ, with the 14-40 material showing about 20-30% greater sorption and retardation. This greater retention by the 14-40 material is consistent with its 30% greater loading ofHDTMA. 

Using the above equation we calculated retardation factors of 44 for chromate and 39 for PCE. These pilot-test retardation factors are very close to the estimates of 42 and 29 calculated in section 3.1 for chromate and PCE based upon their laboratory sorption isotherms. The pilot test results thus confirm that the field barrier performance of SMZ may be predicted reasonably well from laboratory characterization of contaminant interactions. 

Surfactant-modified zeolite can be manufactured in multi-ton quantities at a cost of about 460 per cubic meter ( 460 per metric ton). The bulk-produced SMZ has physical and chemical properties essentially identical to SMZ prepared in the laboratory. In particular, the contaminant (chromate and PCE) sorption characteristics of bulk- and laboratory-produced SMZ are the same. 

Haggerty, G. M., and Bowman, R. S. (1994). Sorption of chromate and other inorganic anions by organo-zeolite. Environ. Sci Technol., 28, 452-458. 

Anions (including metal oxyanions such as chromate) can also be adsorbed, as can anionic ligands (such as phosphates and sulfates), and dissolved or suspended humic substances (such as fulvic acids). The ionic strength and pH influence the sorption of anions since they compete with the OH- ions... 

Many adsorbates and/or adsorbents are redox sensitive. The specific adsorption in such systems depends on the redox potential, which is very difficult to measure or control, thus, systematic studies in this direction are rare. On the other hand some practical implications are well known, e.g. the uptake of chromates by soils and sediments in enhanced on addition of Fe(II) salts [27] as an effect of a redox reaction, in which Cr(VI) is reduced to Cr(III). A few examples of redox reactions accompanying sorption processes are reported in the column results". The changes of oxidation state in the sorption process are probably more common than it is apparent from literature reports, but they are often overlooked, namely, analytical methods must be specially tailored to observe these changes. 

At 2x10" and 8x10" mol dm" of both ions presence of oxalate does not affect sorption of chromate, but sorption of oxalate is depressed in the presence of chromate. The uptake of chromate is depressed at a 4x excess of oxalate. 

The sorption mechanism of chromate is unclear. Zachara et al. (1989) suggested that chromate forms an outer-sphere complex on the surfaces of Fe and Al oxides. However, spectroscopic studies have shown that chromate forms inner-sphere complexes (both bidentate and monodentate) on goethite (Fendorf et al., 1997). This anion has a smaller shared charge than do arsenite and arsenate. 

Mesuere and Fish (1992) found that oxalate diminished the sorption of chromate onto goethite most effectively at low pH and when sorbate concentrations were near surface-saturation levels. Chromate significantly inhibited oxalate sorption over a wide range of pH (from 4 to 10). Balistrieri and Chao (1987) suggested that for a given anion concentration ratio, the competition sequence... 

Hiemstra T, Venema P, VanRiemsdijk WH (1996) Intrinsic proton affinity of reactive surface groups of metal (hydr)oxides The bond valence principle.. J Col Inter Sci, 184 680-692 llton ES, Veblen DR, Moses CO, Raeburn SP (1997) The catalytic effect of sodium and lithium ions on coupled sorption-reduction of chromate at the biotite edge-fluid interface. Geochim Cosmochim Acta 61 3543-3563... 

Hollow fibers have recently received attention with the objective of performing the simultaneous sorption and desorption of the target metal. Hollow chitosan fibers were prepared and the system was used for the recovery of chromate anions. The hollow fibers were immersed in the chromate solution while an extractant was flowed through the lumen of the fiber. Chromate anions adsorbed on the fiber were re-extracted by the solvent extractant. The hollow fiber acts simultaneously as a physical barrier that can make the extraction process more selective. (Vincent, 2000, 2001)... 

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