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Conventional and experimental methods were studied for the remediation of petroleum contaminated …

Biology Articles » Toxicology » Evaluation of land farming and chemico-biological stabilization for treatment of heavily contaminated sediments in a tropical environment » Introduction

- Evaluation of land farming and chemico-biological stabilization for treatment of heavily contaminated sediments in a tropical environment

Most of the methods currently used to remediate contaminated soils in emerging countries are mere adaptations of technologies originally developed in industrialized regions. However, the imported methods were designed for areas with different kinds of economies, cultural expectations and frequently, different physical and biological environments. The misuse of the imported technologies is often inadequate to the local needs when employed in developing regions. Furthermore, their implementation frequently requires the importation of materials, machinery and know-how. This creates dependence on foreign technology and increases the cost of remediation. Unfortunately, the imported technologies are usually poorly suited to the new socio-economic and physico-biological environments, and fail to succeed (McMillen et al., 2002). Besides, in many developing regions, especially in humid tropical and subtropical environments, physical, chemical or biological processes occur which can be applied to the remediation of contaminated soils (Adams and Castillo, 2000).

An exception to this tendency is the Chemico- Biological Stabilization process for treating hydrocarbon contaminated soils, sediments and drilling cuttings. This process was developed in the southern Gulf of Mexico region and takes advantage of the increased rate of natural biological processes in a humid tropical environment. This method has been proven to be able to treat hydrocarbon contaminated soil and drilling cuttings with reduced labor, machinery and fuel costs rather than other commonly used methods. In the present article, this technology is compared with biostimulation (land farm) and bioaugmentation considering effectiveness, logistics and costs.

This method was developed by and for people and sites in the southern Gulf of Mexico region and can be implemented using locally available materials, machinery and know-how. The first steps in developing this method came from petroleum industry personnel trying to treat old drilling waste and purge pits using simple procedures that had been observed to reduce the oily appearance of contaminated materials, namely the addition on hydrated lime (Ca(OH)2) or quicklime (CaO) plus an organic absorbent (sawdust). These as well as organic rich river levee soil were mixed with the drilling and/or purge wastes and then extended as a thin layer (approx. 20–40 cm) over the surface of the site, and later planted with local grasses or allowed to become naturally colonized with local weedy vegetation. Originally, petroleum industry personnel considered adding the river levee soil to be an important source of microorganisms for biodegradation of the hydrocarbons. Using this method, some sites were partially treated, but the national environmental agency responsible to inspect remediated sites (the Federal Attorney for Environmental Protection, PROFEPA for its abbreviation in Spanish) considered that, due to the large additions of other materials, the process was simply a dilution (at that time, the addition of amendments and soil used by petroleum personnel were quite large). These sites faced serious difficulties in being considered as remediated by the authorities (PEMEX, 1995; Arroyo, 1997).

Subsequently, this method was developed technologically in a local state-run university allround the region. The method was tested systematically, altering: 1) the amount of chemical reagent, 2) the amount of organic amendment, 3) the type of organic amendment, 4) the order of application of chemical and organic amendments and 5) the elimination of exogenous soil (Adams, 2004a). It was found that the order of application was very important and the addition of organic rich soil was unnecessary (Adams, 2004b). Also, other organic amendments worked much better than the sawdust originally employed and important advances were achieved over a mediumterm period with respect to soil fertility (field capacity and water repellency, Adams et al., 2007). Furthermore, toxicity and soil leachates were demonstrated to be considerably reduced in the short term and not to present an unacceptable risk to soil or groundwater (UJAT, 2006).

Material was collected from a sediment “beach” area in a large (320 Ha) dam used to temporarily store waste runoff from a sulfur mine in the southern Veracruz State (Mexico). Sulfur was mined at this site from salt domes using the Frasch process, and acid leachate from the tailings pile was diverted to the holding dam (UNAM, 2002). During the drilling process, bentonite-based drilling fluids were used and at least part of the spent muds were also diverted to the dam, forming a large (approx. 23 ha) sediment “delta” or beach area in which the fine clay settled out (UJAT, 2006). Interlaced with the fine sediments are thin veins of very viscous hydrocarbons which originated from the salt domes and were considered waste material by the mine operators and discharged with the acid water and drilling fluids. This oil is slightly denser than water and has settled on the bottom of the dam just above the sediments. Outside the sediment delta area, there has been very little infiltration of this oil into the native sediments due to its viscosity. Only 13% of the dam area (not including the sediment delta) has hydrocarbons in the native sediments although approx. 60% of this same surface is covered with a layer of oil (UJAT, 2006). The very high viscosity of the oil has prevented further penetration. In gas chromatography characterization (EPA, 1997), the oil was found to be generally midrange (10 - 25 C) but extremely weathered without any peaks arising above the undefined complex mixture in chromatograms. The analytical laboratory, at which samples were processed, sent the following footnote on the analytical report: “appears to be weathered lubricant oil”. The concentration of hydrocarbons in the sediments used in this study was approx. 5–6% (dry weight). The water in the dam had been neutralized with magnesium oxide as part of the site remediation prior to lowering the level in the dam and collecting the sediments used in this study.

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