Processes affecting the remediation of chromium-contaminated sites.

Abstract

The remediation of chromium-contaminated sites requires knowledge of the processes that control the migration and transformation of chromium. Advection, dispersion, and diffusion are physical processes affecting the rate at which contaminants can migrate in the subsurface. Heterogeneity is an important factor that affects the contribution of each of these mechanisms to the migration of chromium-laden waters. Redox reactions, chemical speciation, adsorption/desorption phenomena, and precipitation/dissolution reactions control the transformation and mobility of chromium. The reduction of CrVI to CrIII can occur in the presence of ferrous iron in solution or in mineral phases, reduced sulfur compounds, or soil organic matter. At neutral to alkaline pH, the CrIII precipitates as amorphous hydroxides or forms complexes with organic matter. CrIII is oxidized by manganese dioxide, a common mineral found in many soils. Solid-phase precipitates of hexavalent chromium such as barium chromate can serve either as sources or sinks for CrVI. Adsorption of CrVI in soils increases with decreasing chromium concentration, making it more difficult to remove the chromium as the concentration decreases during pump-and-treat remediation. Knowledge of these chemical and physical processes is important in developing and selecting effective, cost-efficient remediation designs for chromium-contaminated sites.

Chromium-Containing Organic Fertilizers from Tanned Hides and Skins: A Review on Chemical, Environmental, Agronomical and Legislative Aspects

“…Inorganic Cr(III) form is limited at pH values less than 5, while above pH 5 Cr(III) is hydrolyzed and the cationic species deriving from the hydrolysis are characterized by low solubility: this is the main reason why, in normal soil conditions, Cr(III) has low mobility and bioavailability [15,17,21,23].…”

“…CrO  above pH 6), is highly water soluble and is a strong oxidizing agent due to its high positive reduction potential. As a consequence, it is a highly mobile and toxic form of Cr [17,21,23]. Adsorption is less influenced by organic matter content and pH than Cr(III) [24], even if adsorption is favored under acidic to neutral pH because positive charges on soil colloids increase [25,26] and ends when pH is above 8.5 [20].…”

“…In fact in natural environment Cr(III) can be oxidised to Cr(VI) only in the presence of electron acceptors (strong oxidants), such as O 2 and high-valence Mn oxides [23]. However it was found that Mn oxides represent the only naturally occurring oxidant of Cr(III) [17,23]; as a consequence their presence is essential because they serve as electron acceptors in the oxidation reaction [20].…”

“…In fact in natural environment Cr(III) can be oxidised to Cr(VI) only in the presence of electron acceptors (strong oxidants), such as O 2 and high-valence Mn oxides [23]. However it was found that Mn oxides represent the only naturally occurring oxidant of Cr(III) [17,23]; as a consequence their presence is essential because they serve as electron acceptors in the oxidation reaction [20]. However oxidation should occur with the simultaneous presence of other factors, such as low organic matter content, alkaline pH and high redox potential values [17,23]: all these conditions limit greatly the possibility of oxidation that represents a serious hazard for the environment.…”

“…However it was found that Mn oxides represent the only naturally occurring oxidant of Cr(III) [17,23]; as a consequence their presence is essential because they serve as electron acceptors in the oxidation reaction [20]. However oxidation should occur with the simultaneous presence of other factors, such as low organic matter content, alkaline pH and high redox potential values [17,23]: all these conditions limit greatly the possibility of oxidation that represents a serious hazard for the environment. Moreover oxidation of Cr(III) to Cr(VI) is a phenomenon occurring very rarely even in the presence of Mn oxides and favorable pH conditions, due to the poor availability of mobile Cr(III): exchangeable trivalent Cr(III) is practically inexistent in soil where the pH overcomes 5 [25,26].…”