Adsorption of Chromate by Subsurface Soil Horizons

Abstract

Chromate adsorption was measured with and without reactive cosolutes on four subsurface soil horizons differing in pH and mineralogy, and on clay fractions from two of the oxide‐containing subsoils. Chromate adsorption was greatest in lower pH materials enriched in kaolinite and crystalline iron oxides. Over a range in pH, chromate adsorption to subsoil was similar to that observed for pure‐phase oxides. Chromate binding was reversible to pH and was depressed in the presence of SO2‐4 and dissolved inorganic C, which compete for adsorption sites. A surface site density for crystalline Al‐substituted iron oxides in the subsoils was estimated from chromate adsorption on the clay fractions using the model FITEQL and outer sphere surface coordination constants from Al‐goethite. The estimated site density for soil crystalline iron oxides was well below that of clean oxides, suggesting surface saturation by indigenous soil ions. The calculated site density and surface binding constants for Al‐goethite were used in the Triple Layer Model to calculate the effect of ionic strength, cosolutes, solids concentration, and sorbate concentration on CrO2‐4 adsorption. Model calculations were in good qualitative agreement with experimental results.

Sorption Characteristics of Hexavalent Chromium in the Soil Based on Batch Experiment and Their Implications to the Environment

“…Figure 4 shows that the equilibrium adsorption of Cr(VI) on a natural soil conformed well to the Langmuir isotherm at constant pH. Similar results have been noted [32] [33] [36] for the adsorption of chromate on clay minerals, natural soils, and goethite, respectively. The maximum sorption capacity at pH of 8 was 1.527 mg/L soil solution.…”

“…The maximum sorption capacity at pH of 8 was 1.527 mg/L soil solution. This value was lower than the maximum adsorption capacities reported by Zachara et al [32], for the adsorption of Cr(Vl) on two soils for which no inorganic carbon was detected. This compares with 2.32% of total Carbon contents including inorganic cabon (Table 1) for the experimental soil in this study, which might be explained due to their lower adsorption capacities.…”

“…Cr(VI) removal at longer reaction time (phase II) can be attributed to Cr(VI) reduction to Cr(III). It was concluded that phase I was largely due to diffusional transport and adsorption, as has been noted for the adsorption of Cr(Vl) on soil [32], while phase II was largely due to reduction of Cr(VI) to Cr(III).…”

Especiação redox de cromo em solo acidentalmente contaminado com solução sulfocrômica

“…Nessa unidade da Vol. 31,No. 6 Embrapa, assim como em outras, o laboratório de solos utiliza o mé-todo ISO 14235 para a determinação de matéria orgânica no solo e os resíduos de íons cromato gerados pelas análises são armazenados em reservatórios para posterior tratamento adequado.…”

“…5 Crômio(VI) apresenta mais mobilidade que Cr(III), pois seus ânions são facilmente transportados através do solo. 6 Por outro lado, Cr(III) precipita como Cr(OH) 3 ou Fe x Cr 1-x (OH) 3 ou ainda forma quelatos com moléculas orgânicas, tendo portanto menor mobilidade no solo. 7,8 Pela toxicidade do Cr(VI) comparativamente ao Cr(III), a especiação redox de cromo se tornou importante, possibilitando um monitoramento ambiental bem mais adequado que apenas a quantificação do teor total do elemento.…”