Alain C.M. Bourg

Aqueous geochemistry of chromium: A review

The aqueous geochemistry of chromium is reviewed, especially with reference to the recent work of Rai et al. (Report EPRI EA-4544, 1986, Report EPRI EA-5741, 1988). The chromium content of natural waters is generally low whereas rocks exhibit a wide range of Cr concentrations. In natural environments, the most stable oxidation states of chromium are Cr(III) and Cr(VI). The following minerals can control the dissolved chromium content of natural waters: Cr(OH)3(s) and (Fe, Cr) (OH)3 (ss) for Cr(III). Major Cr(III) disolved species are CrOH2+ and Cr(OH)30, as well as several organic and a few inorganic complexes, whereas HCrO4− and CrO42− are the main forms of Cr(VI) in natural waters. The chromium distribution is also controlled by redox processes. Oxidation of Cr(III) to Cr(VI), poor with dissolved oxygen, is very effective in the presence of solid MnO2. Fe(II)- and organic matter-rich environments favour the reduction of Cr(VI) to Cr(III). Interaction with solid phases can also regulate the chromium content of water. Cr(III) exhibits a typical cationic sorption behaviour. Its adsorption increases with pH, but decreases when competing cations are present. Chromium (VI), on the other hand, exhibits a typical anionic sorption behaviour. Its adsorption decreases with increasing pH and when competing dissolved anions are present. The migration of chromium is determined by the competition between complexation, dissolution/precipitation, redox processes and adsorption/desorption mechanisms. Cr(III) will migrate under acidic conditions and/or if present as dissolved organic matter complexes. Hexavalent chromium generally migrates rapidly but its mobility is inhibited when the Fe(II) and organic matter concentrations are high and when sorption processes are favoured (low pH).

Speciation of Fe(II) and Fe(III) in contaminated aquifer sediments using chemical extraction techniques

The iron mineralogy of aquifer sediments was described by chemical extraction techniques. Single-step extractions including 1 M CaCl 2 , NaAc, oxalate, dithionite, Ti(III)-EDTA, 0.5 M HCl, 5 M HCl, hot 6 M HCl, and a sequential extraction by HI and Cr II HCl were tested on standard iron minerals and nine aquifer sediments from different redox environments sampled in a landfill leachate plume. Ion-exchangeable Fe(II) is easily quantified by anaerobic CaCl 2 extraction. A rapid indication of the redox status of a sediment sample can be achieved by a 0.5 M HCl extraction. This extraction gives an indication of the content of amorphous Fe(III) and reduced Fe(II) species such as FeS and FeCO 3 , though the fractions are not quantified

Effect of Solid/Liquid Ratio on the Remobilization of Cu, Pb, Cd and Zn from Polluted River Sediment

The incidence of the solid/liquid ratio on thesolubilization of heavy metals from a polluted riversediment was assessed by batch experiments. Thepercentage of solubilized metal increases stronglywhen the concentration of the sediment-watersuspension decreases from 50 to 0.1 g L-1. Thissolubilization behavior is described by a simpleequilibrium desorption model on the association of themetals and sediment. The association constantscalculated with this model indicate the affinitybetween the metals and the sediment (Pb > Cu > Zn> Cd). If polluted river sediments are in situresuspended, the decrease of the solid/liquid ratioassociated to this natural event could provoke theremobilization of metals trapped in sediments.

Radon-222 and chloride as natural tracers of the infiltration of river water into an alluvial aquifer in which there is significant river/groundwater mixing.

Natural dissolved 222 Rn concentration increases with distance as Lot River water infiltrates into an alluvial aquifer. At distances greater than 150 m from the river, 222 Rn has a constant value of ca. 55 Bq L -1 . This is in agreement with the theory that the increase of 222 Rn along the infiltration path can be described by a radioactive law. In the aquifer investigated, the infiltrating river water mixes with groundwater from the neighboring hills. Radon is used to trace the infiltration of river water into the aquifer. Chloride is used to quantify the mixing of river water with groundwater

Seasonal and spatial trends in manganese solubility in an alluvial aquifer.

The hydrogeochemistry of river water and groundwater in a hydraulic configuration where river water infiltrates into an alluvial aquifer was monitored monthly over a 14-month period. Trends could be seen for temperature, pH, and dissolved O 2 and Mn. Wells located within 15 m of the river show a significant seasonal variation in dissolved Mn. A threshold temperature of 10 o C seems to be necessary in order to trigger and maintain Mn solubility (miaobiologically mediated reactions). Farther from the river, where the temperature is relatively constant, there is little seasonal variation in dissolved Mn. Some of the boreholes contain little Mn (0.5 μmol L -1 ) while in others the Mn concentration is very high (up to 25 μmol L -1 )

Trace metal adsorption modelling and particle-water interactions in estuarine environments

Abstract A brief, critical review of empirical and conceptual metal adsorption models is presented. This covers models conditional to seawater chemistry, as well as models applicable to estuaries or other systems of variable solution chemistry. The conceptual surface complexation approach is used to show why desorption of trace metals does not necessarily occur readily in estuaries, and why in some cases adsorption can even take place. The relevance and weaknesses of this model for the understanding of the mobility and fate of heavy metals in turbid environments, as related to particle dynamics, is discussed using the macrotidal Gironde Estuary, France, as the example.

Diurnal variations in the water chemistry of a river contaminated by heavy metals: natural biological cycling and anthropic influence

A chemical survey of the water of the Lot River (Southwestern France) was conducted for 26 h. Water temperature, pH, dissolved oxygen and zinc exhibited a diurnal cycle. These variations are caused by biological activity in the river (photosynthesis during the day and respiration during the night). The Zn diurnal cycle was related to the pH variations. The solubility of Mn seemed to follow a photoreductive diurnal cycling pattern. An increase in conductivity due mainly to higher dissolved Ca, Mg and sulfate concentrations is attributed to an increase in the solute input from a small tributary which drains smelter tailings. Even though the river sediments are significantly contaminated with Zn, Pb and Cd, the heavy metal dissolved concentrations are always low.

TRENDS IN THE HEAVY METAL CONTENT (Cd, Pb, Zn) OF RIVER SEDIMENTS IN THE DRAINAGE BASIN OF SMELTING ACTIVITIES

Abstract The geochemical and granulometric characteristics of sediments from three sites in the Lot River basin contaminated by heavy metals (cadmium, lead and zinc) were studied over a period of three years. Total heavy metal concentrations show little variation with time. However, a correlation between the heavy metal concentration and the fraction of sediment particles smaller than 20 μm in the Lot River at Capdenac-Gare, a moderately polluted site, shows the occurrence of yearly variations: concentrations are highest during the winter and spring, periods of high water discharge. Correlations between various components of the sediments and heavy metals indicate that the heavy metal content is probably not controlled by a single geochemical component but by several. Depending on the degree of contamination of the site (no pollution, moderate pollution or high pollution) and the heavy metal investigated, high metal concentrationsmay be associated to iron, manganese, organic and inorganic carbon, calcium, sulfide and small particles. Heavy metal transport in the Lot River, downstream from the confluence with the polluted Riou Mort, takes place mainly in particulate form. Zinc and Cd input is definitely related to the Riou Mort.

A review of the attenuation of trichloroethylene in soils and aquifers

Abstract The transport, transformation, sorption and volatilization of trichloroethylene (TCE) as a non-aqueous phase liquid (NAPL) and as a dissolved aqueous constituent, under both laboratory and field conditions, are reviewed. NAPL-TCE is expected to migrate quite rapidly in soils and other water-unsaturated conditions, leaving droplets of organic liquid in the pore spaces. At the water table level, TCE may stop its downward movement and diffuse laterally, together with a slow migration downwards. From there on NAPL-TCE will slowly dissolve in groundwater. However, some specific hydraulic characteristics may induce rapid transport downwards away from the water table. TCE may appear in soils and aquifers as a result of its wide industrial use and also, possibly, as a degradation by-product of other chlorinated hydrocarbon solvents. TCE can undergo abiotic and (mostly) biotic, aerobic and (mostly) anaerobic transformations leading to less chlorinated ethenes. A half-life of 300 days was observed under field conditions. Batch and column experiments show that TCE is weakly sorbed on soil and aquifer solids with linear partition coefficients. Retardation factors in the range of l to 9 are reported from laboratory and field investigations. The adsorption is almost fully reversible. Gaseous TCE is more adsorbed than aqueous TCE, the extent of uptake being greatly influenced by the moisture content of the solid. Soil gas surveys appear as a promising technique for the study of soil and groundwater contamination by TCE. NAPL-TCE contamination should be quite effectively removed by steam injection. Because of its low adsorption and weak degradation under aerobic conditions, the diffuse contamination of aquifers by dissolved TCE can be extensive. In the case of localized and more severe pollution, approaches for the restoration of aquifers based on pumping followed by treatment at the surface may be effective but they usually are not economical. In situ treatment by stimulating the biotrans-formation is considered as a favourable alternative.

Migration of chlorinated hydrocarbon solvents through Coventry sandstone rock columns

The migration in the Coventry sandstone aquifer system of aqueous solutions of various chlorinated hydrocarbon solvents (CHSs) (1,1,1-trichloroethane (TCA), trichloroethylene (TCE) and tetrachloroethylene (TeCE)) was studied by means of percolation experiments with undisturbed rock cores. The retardation factor (Rf) was very low (always less than 2 for TCA, less than 3 for TCE and less than 5 for TeCE). The Rf varied slightly according to the nature of the solid sample but could not be related to the solid organic carbon content. The solvent adsorption was always almost completely reversible and little affected by the percolation flow rate (0.08–0.56 m day−1). The CHS breakthrough curves were typical of non-equilibrium processes (caused by sorption by the solids and possibly by diffusion between mobile and immobile water). The Rf values obtained experimentally were much lower than those calculated from batch adsorption studies, showing that percolation experiments are necessary to evaluate the filtering properties of consolidated rocks. These results indicate that in the Coventry sandstone aquifer (relatively high vertical and, especially, lateral hydraulic conductivities), two-carbon CHS aqueous solutes should migrate readily with water with very little retardation.