Introduction

The magnitude of the effort required for remediation of hazardous waste sites throughout the country is staggering. The number of underground storage tanks that are registered with the U.S. Environmental Protection Agency (EPA) alone is estimated at between 1.8 million and 2 million, of which 25% are assumed to be leaking [1], The costs associated with cleanup of our hazardous waste problems are even more appalling. The remediation of underground storage tanks (USTs) alone is estimated to cost $67 billion, assuming that current regulatory policies do not get any more stringent. The cost of cleaning up all hazardous waste sites could reach $1.7 trillion by the year 2020 [2]. Numbers of this magnitude will make the remediation of hazardous wastes one of the most pressing and vital matters for the next several decades. Cost/benefit analysis will become the driving force in the environmental community; no longer will we be able to pay exorbitant prices for soil remediation, nor will we have to. The responsible party (RP) is now more educated, more cost-conscious, more aware of the available options, and much more receptive to new and innovative methods. Such will be the focus of the next generation, a move away from the "dig and haul" mentality to treatment and elimination of the problem.

One of the low-cost soil and sludge remediation methods that has been widely used for many years is the solidification or stabilization of heavy metals. These techniques are generally based on the addition of a cementitious material to the contaminated soil and the formation of a solid monolith. Due to the multitude of reactions that take place both within the cementitious material and between the heavy metals and the cement, this process can work well, with a few drawbacks. The most significant downside of well-solidified heavy metal-contaminated soil is the large volume increase required to provide an adequate degree of nonleachability. This volume increase can be as little as 15% or as much as 150% of the original contaminated soil volume [3,4]. The advantages of solidification/stabilization, however, most often far outweigh these disadvantages. High volume throughput (100 yd3/hr is common) and low cost make this an attractive process for heavy metal solidification.

It is these distinct advantages that have led many people to consider and actually use this type of process for the remediation of hydrocarbon-contaminated soils. These attempts have met with varying degrees of success, from outright failure [5] to some measure of solidification that is usually contaminant-dependent [6], These inconsistent results have led EPA to state that "immobilization of organics is uncertain" [7,8] and that the most stringent tests for total hydrocarbons should be used in assessing the results of any organic solidification/ stabilization process.

The main reason for poor success is the fact that all of the conventional solidification, stabilization, or fixation processes use some type of cementitious or pozzolanic material as the mainstay of the process. Materials such as portland cement, fly ash, kiln dust, and lime (the four major cementitious or pozzolanic materials used) all undergo hydration reactions as part of the curing process. It is well known within the concrete industry that the setting of concrete can be retarded by adding a small amount of diesel fuel prior to pouring the concrete. The hydrophobic diesel fuel absorbs onto the crystal faces of the pozzolanics and effectively blocks the infusion of water and subsequent hydration and thus the curing of the cement. This dichotomy of trying to solidify something that inherently prevents solidification has given solidification of organics its "uncertain" reputation and more recently has led to vendors looking at additives such as carbon and clays to improve the retention of hydrocarbons within a solidified matrix [9].

The Siallon process for the microencapsulation of organics and hydrocarbons is a unique approach that does not use any cementitious or pozzolanic materials. It is a simple two-step chemical reaction that results in the hydrocarbon being encapsulated within a micrometer-sized particle of silica [10,11]. As there are no large amounts of pozzolanics used, there is no large volume increase with treated soil and no attenuated cure time, and results are always determined using total concentration analysis.

0 0

Post a comment