TOXICOLOGICAL PROFILE FOR CHROMIUM - Davidborowski.com

CHROMIUM 2965. POTENTIAL FOR HUMAN EXPOSUREmay exist in the aerobic zone of some natural soil. The oxidation of chromium(III) to chromium(VI) insoil is facilitated by the presence of low oxidizable organic substances, oxygen, manganese dioxide, andmoisture. Oxidation is also enhanced at elevated temperatures in surface soil that result from brush fires(Calder 1988; Cary 1982). Organic forms of chromium(III) (e.g., humic acid complexes) are more easilyoxidized than insoluble oxides. However, oxidation of chromium(III) to chromium(VI) was not observedin soil under conditions of maximum aeration and a maximum pH of 7.3 (Bartlett and Kimble 1976). Itwas later reported that soluble chromium(III) in soil can be partly oxidized to chromium(VI) bymanganese dioxide in soil, and the process is enhanced by pH higher than six (Bartlett 1991). Becausemost chromium(III) in soil is immobilized due to adsorption and complexation with soil materials, thebarrier to this oxidation process is the lack of availability of mobile chromium(III) to immobilemanganese dioxide in soil surfaces. Due to this lack of availability of mobile chromium(III) tomanganese dioxide surfaces, a large portion of chromium in soil will not be oxidized to chromium(VI),even in the presence of manganese dioxide and favorable pH conditions (Bartlett 1991; James et al.1997).The microbial reduction of chromium(VI) to chromium(III) has been discussed as a possible remediationtechnique in heavily contaminated environmental media or wastes (Chen and Hao 1998). Factorsaffecting the microbial reduction of chromium(VI) to chromium(III) include biomass concentration,initial chromium(VI) concentration, temperature, pH, carbon source, oxidation-reduction potential and thepresence of both oxyanions and metal cations. Although high levels of chromium(VI) are toxic to mostmicrobes, several resistant bacterial species have been identified which could ultimately be employed inremediation strategies (Chen and Hao 1998). Elemental iron, sodium sulfite, sodium hydrosulfite, sodiumbisulfite, sodium metabisulfite sulfur dioxide and certain organic compounds such as hydroquinone havealso been shown to reduce chromium(VI) to chromium(III) and have been discussed as possibleremediation techniques in heavily contaminated soils (James et al. 1997; Higgins et al. 1997). Thelimitations and efficacy of these and all remediation techniques are dependent upon the ease in which thereducing agents are incorporated into the contaminated soils.5.4 LEVELS MONITORED OR ESTIMATED IN THE ENVIRONMENTReliable evaluation of the potential for human exposure to chromium depends in part on the reliability ofsupporting analytical data from environmental samples and biological specimens. In reviewing data onchromium levels monitored or estimated in the environment, it should also be noted that the amount ofchemical identified analytically is not necessarily equivalent to the amount that is bioavailable.