Remediation concepts and applications

Chemistry

Remediation Chemistry at Chromium Contaminated Sites

Chromium is a ubiquitous soil and water contaminant that is produced from both natural and anthropogenic sources. Of significant concern, however, is its release into the environment as an industrial byproduct, since it is a common element in many industrial processes: ore refining, steel production, pigment manufacturing, corrosion inhibition, leather tanning, wood preservation, and the combustion of coal or oil. Chromium contamination from industrialization poses a significant threat to the integrity of soil and groundwater resources (Yolcubal and Akyol 363; Pagilla and Canter 243).

The purpose of this study is to evaluate some of the predominant methods of remediation of chromium contaminated sites. In general, the methods employed can be divided into two types: removal from the soil (in which various chemical extractants are used to flush chromium from the site) or immobilization in the soil (in which reducing agents are used to break down dangerous forms of chromium) (Pagilla and Canter 243). After an examination of several of the more promising method of remediation currently being employed, I will conclude with a discussion and analysis of these methods in terms of their real-world applicability.

Remediation Methods

In order to understand the most common chemical remediation options available for dealing with chromium contaminated sites, it is first important to understand a little about chromium and how it occurs in the environment. Chromium is most commonly found in two stable forms in the environment: trivalent chromium Cr (III) and hexavalent chromium Cr (VI). The former variety of chromium is generally naturally occurring. It is insoluble and an important element essential for many biological processes. The latter form of chromium, Cr (VI), is more commonly produced through anthropogenic means and is not nearly so beneficial to life as Cr (III). Cr (VI) is very toxic as well as carcinogenic and mutagenic. It has been demonstrated to cause liver and kidney problems in human beings, not to mention significant respiratory problems (Yolcubal and Akyol 363). Combined with its high solubility, these factors make Cr (VI) a significant and dangerous environmental contaminant that must be controlled in order to preserve the health of soil and groundwater resources.

Remediating a chromium-contaminated site is dependent upon a number of factors, not least of which are the soil and water characteristics of the site. A determination of the most suitable remediation method to use is dependent upon a careful analysis of the site conditions beforehand. For example, Cr (VI) is highly anionic and thus strongly absorbed by slays and iron and manganese oxides and hydroxides. This absorption occurs most strongly in acidic media. In other words, an ideal site for remediation might be one in which the contamination occurred in acidic clay soil with a high mineral content of iron oxide (Yolcubal and Akyol 363). Though an extreme example, this point highlights the importance of site conditions in determining the correct course of remediation methods. In studies that have considered multiple methods for remediating chromium-contaminated sites, two more promising methods emerge: using extractants, chemical or otherwise, to flush the soil free of chromium; and immobilization, which employs reducing agents to break Cr (VI) down into a Cr (III) precipitate (Pagilla and Canter 247).

One common way of extracting Cr (VI) from contaminated sites is heap leaching, which under certain conditions has been shown to be effective (Kapoor and Viraraghavan 366). It has the potential to be an extremely practical and cost-effective means for removing chromium from the site through the same methods that are employed by industrial miners who often cause chromium contamination in the first place. Heap leaching involved placing contaminated materials in piles that can be irrigated with a leaching solution that dissolves the metals that are desired. In this case, by irrigating contaminated soils with chemicals known to remove chromium it is feasible to extract the chromium by essentially "washing" the contaminated site. This method may not be as effective in field conditions as the study demonstrates it was in the lab because of more variable conditions, but it represents a sort of standard in extraction-based remediation.

However, other forms of chemical extraction are available for use that may seem unorthodox but are actually less invasive and damaging to the site than chemical washes. Field trials conduced from 1999 to 2001 employed nine different types of common crops to remove metals from the soil (Ciura et al. 17). It is through well-known processes that plants remove minerals and metals from soils in order to fuel their own growth. Plants will even remove metals that are not beneficial to their growth, though this may have an adverse effect on their growth rates and final crop yields. The process of using this aspect of plants to remove heavy metal contaminants such as chromium is known as phytoremediation.

To effectively remove chromium from contaminated sites, any crop chosen must be able to accumulate significant amounts of the metal from the soil, grow fast, produce a large biomass, be easily harvested and processed, suited to the conditions of the site. Interestingly, of the nine crops that were tested in these field trials, four demonstrated a significant capacity for removing chromium from the soil. The top performer was maize, which was able to remove 16.6mg of Cr (VI) per square meter per year. Field pumpkins removed 14.8mg/m2/year, barley 14.3mg/m2/year, and alfalfa 10.3mg/m2/year. Unfortunately, the majority of the Cr (VI) accumulation occurred in the roots of the plants, meaning that for the remediation to be successful, the roots of each plant would have to be removed during the harvest Ciura et al. 19). Nonetheless, this method of extraction seems to be well suited to remediating contamination while simultaneously improving the site.

Of course, chemical flushing of a site is not always feasible, based on the site conditions. In these cases, it can be more useful to use immobilization methods that transform Cr (VI) into Cr (III). Many of these techniques are referred to as bioreductive because they employ bacteria and other microorganisms to reduce the level of Cr (VI) in soils. These methods, especially those that use Cr-resistant forms of bacteria have been found to be extremely effective at reducing levels of Cr (VI) in contaminated soils (Camargo et al. 569). These methods can be exceptionally useful in cases were conventional physical or chemical remediation, such as heap leaching or phytoremediation, are not cost-effective or will not work on the scale required. These forms of bioreduction can work in aerobic or anaerobic conditions through microbial metabolism or a bacterial metabolite (like H2S). Either of these techniques were shown extremely effective at reducing Cr (VI) into precipitates of Cr (III) (Camarago et al. 569). Studies into the effectiveness of these remediation methods demonstrate that using biological methods for chromium remediation are quite effective, especially when using cell-free enzymes that are unaffected by growth inhibitors, predators, or toxins (Camargo et al. 572).

One of the significant issues with using biological remediation methods, however, is the fact that the microorganisms used are actually alive and require nutrients other than chromium in order to live and reproduce. Thus, it is important that critical nutrients be introduced into the contaminated sites. Unfortunately, this can be extremely difficult using conventional methods, especially in field situations. Alternative methods have had to be developed that can feed cultures of microorganisms that are capable of reducing Cr (VI) into Cr (III). One method that has demonstrated qualified success is the electrokinetic introduction of critical nutrients into the soil (Reddy, Chinthamreddy, and Saichek 931). While the technique can be effective at introducing the necessary nutrients into the soil by inserting negatively and positively charged electrodes into the contaminated site, it is crucial that the electrokinetic process be tailored to meet the optimal nutrient, pH, and environmental conditions required by the strains chosen. Bioreduction has been demonstrated to be an effective means of remediation of chromium contaminated sites, but new methods such as electrokinetics must be developed in order to help encourage the ideal conditions needed to encourage growth of the strains chosen.