environment magazine

Stabilization and Solidification of Contaminated Soil and Waste Part 4: Treatment of Organic Contaminants

By Dr Colin Hills - Directorof the Centre for Contaminated Land Remediation at the University of Greenwich

Scope Soils and wastes contaminated with organic compounds may be toxic, carcinogenic or mutagenic; and may present difficulties in terms of remediation strategies. There is a successful track record for treating many organic contaminants by stabilisation/solidification (S/S) spanning more than two decades and this is discussed in this article.

In this fourth article of an occasional series on the treatment of waste and soil, Dr Colin Hills, Director of the Centre for Contaminated Land Remediation at the University of Greenwich examines the use of cement-based binder systems to treat organic contaminants. He is joined by Edward Bates, recently retired from the US Environmental Protection Agency, and Dr Peter Gunning of Carbon8 Systems Ltd., in assessing S/S as a risk management strategy for organically contaminated soils and wastes.

Introduction Many organic compounds are volatile, or semi volatile, being released slowly as gases. Unlike metals, organic compounds tend not to show solubility minima at higher pH. Indeed, some organic compounds become more soluble at higher pH and an understanding of the nature of their behaviour underpins the successful application of S/S to these compounds. However, it should be noted that organic contaminants are often found mixed with metals and other inorganic compounds.

Key contaminant properties and reactions

Solubility

In general, non-polar, non-ionic organic compounds are insoluble in water. These include petroleum compounds, PCBs, and industrial solvents, including those used for dry-cleaning (trichloroethylene, perchloroethylene, and carbon tetrachloride). Chlorinated pesticides (DDT, lindane and dieldrin) are also insoluble, and bio-accumulate within the environment [10]. However more soluble forms of the compounds have been produced for commercial purposes, such as DDT powder, which has been widely used in the past. In addition, solvents have been widely used in dry cleaning and industrial operations, and it is not uncommon to find sites contaminated with these substances.

Soluble organic contaminants are those with a polar or ionisable structure, and contain water-soluble compounds (such as polar groups) OH and COOH and/or carry a charge; the solubility (of these) is dependant on the pH of the environment in which they are present. Compounds include alcohols, organic acids, and some pesticides.

Volatility Organic compounds can be described by their volatility – the tendency of a compound to become gaseous and transported in the air, which is related to the strength of the intermolecular forces between molecules of the organic contaminate.

Organic compounds are normally divided into three groups [11]: • Volatile organic compounds (VOCs, with a vapour pressure of > 10-2 kPa, • Semi-volatile organic compounds (SVOCs, with a vapour pressure of 10-2 – 10-8 kPa • Non-volatile organic compounds (NVOCs, with a vapour pressure < 10-8 kPa

Volatile organic contaminants include aromatic hydrocarbons, halogenated hydrocarbons, aldehydes and ketones and are considered unsuitable for treatment by S/S without additives (the heat of hydration may cause volatilisation). Semi-volatile organic contaminants include polynuclear aromatic hydrocarbons (PAHs) with four or fewer fused rings and their nitro derivatives, chlorobenzenes, chlorotoluenes, polycholobiphenyls, organochlorine and organophosphate pesticides, and polychlorodibenzo-p-dioxins. Non-volatile organic contaminants include polynuclear hydrocarbons and their nitrogenous and oxygenated derivatives. In general, the larger the compound, and the more polar/ionic it is, the less volatile the compound will be.

Complexation Many organic compounds can form complexes with metals through the reaction with ligands, the reactive groups on the organic compound. As a general rule, organometallic compounds are toxic; and covalently bonded complexes of lead, tin, thallium, and mercury persistent in the environment. Some organometallic ionic compounds such as Methylmercury (CH3Hg+) and gaseous dimethylmercury ((CH3)2Hg) are produced by bacteria in response to exposure to this metal. Soil organic matter (SOM) frequently forms organic complexes containing metals.

Degradability Organic compounds degrade in soil via two processes: biotic and abiotic degradation. Biotic processes (biodegradation) involve chemical reactions undertaken by insects, earthworms, plants and micro-organisms (and is the basis for bioremediation techniques). Abiotic reactions involve reduction/oxidation, hydrolysis and photolytic-degradation on exposed to sunlight.

Many contaminants are subject to biotic degradation, including: 2-4 D, parathion, carbofuran, atrazine, diazinon, volatile aromatic alkyl and aryl hydrocarbons and chlorocarbons, and surfactants [12]. Abiotic degradation mechanisms may involve clay minerals present in soil, and photolysis via exposure to ultraviolet light.

Sorption The sorption of organic pollutants involves uptake by soils and sediments via: • mineral surfaces • partitioning into the organic matter component of soils

Mineral surfaces can attract or repel contaminants from their actives sites, whereas sorption onto SOM may account for a large proportion of the uptake in the soil ‘system’. The soil adsorption coefficient can be used to estimate the partitioning of organic contaminants in both soil and sediments [13].

As SOM consists of long chained carbon molecules (with OH- and COOH- functional groups) at neutral/acidic pH the protonated molecule attracts hydrophobic organic contaminants. At alkaline pH, negatively charged SOM attracts hydrophobic organic contaminants.

Many ionisable organic pollutants (including phenols and chlorophenols) are polar, and negatively charged at environmental pH’s, and thus, remain mobile.

Hydrolysis The interaction of organic pollutants with water can lead to hydrolysis, where: RX is an organic compound, X a functional group (e.g., Cl) and (ROH) is the newly formed compound. Hydrolysis reactions can be catalysed in both, basic or acidic conditions.

Deprotonation Some organic compounds behave as weak acids and can lose a hydrogen atom. The resultant charge makes the ionised compound more water-soluble and more mobile in the environment, or in an S/S waste form.

Reduction/Oxidation (Redox) Many organic compounds can form reduced or oxidised species, with biological or ecological impacts. It is possible to predict whether an organic compound will be present in its oxidised or reduced form to allow prediction of behaviour in soil, or in an S/S treated product.

Soil-Binder Interference The use of cement or lime (alone) in the treatment of organic-contaminated soils and wastes can be problematic, as many organic compounds interfere with hydration reactions. Interference can result in the retardation of set and lower strength. The mechanisms causing retardation have been hypothesised [14,15]: • Adsorption on the surface of cement particles, forming a protective skin which slows down the hydration reactions; • Adsorption on calcium hydroxide nuclei, poisoning their growth, prolonging the induction period indefinitely; • The formation of calcium-organic complexes  discouraging the formation of nuclei of calcium hydroxide; • Precipitation of insoluble compounds around cement  grains, forming a protective skin; • Incorporation of the organic molecule in the protective membrane around cement particles reducing its permeability and the continued growth of hydration products.

As organic compounds often do not form chemical interactions with cementitious phases, their retention is often physical rather than chemical in nature. Retention involves entrapment pore structure of the waste form.

Aromatic compounds

Aromatic compounds contain a benzene ring and are generated by coal conversion plants (town gas plants), wood preserving facilities (creosote), oil and petrochemical industries, and leaks from underground storage tanks. They tend to be persistent in the environment as they are hard to biodegrade due to the stability of the benzene ring. Phenol is a common contaminant, resulting from the breakdown of coal tars and is toxic. Pentachlorophenol (PCP) as used in some wood preserving operations, is very toxic, as is benzene itself. In an S/S system both phenol and o-chlorophenol inhibit the setting time and strength development of cement [17], and complexes with Ca(OH)2 were identified. Chlorophenol, p-chlorophenol (PCP) and p-bromophenol (PBP) have also been found to interfere with hydration, including the conversion of ettringite to monosulphate. The formation of crystalline calcium-phenol complexes reported by most workers may be questioned because of the high levels of phenol leaching in cement-based systems [24].  Work by Cote et al investigated the relative chemical or physical containment in an S/S system, and predictions for the long-term containment of organic contaminants were made [25]. 

Non-aromatic compounds

A large number of non-aromatic compounds exist. The retarding effect of ethylene glycol on the hydration of cement has been studied by Eaton et al. [26]. The formation of hydrogen bonds, between the contaminant and C-S-H was suggested as responsible. However, Sheffield et al. found that at high concentrations the setting of PC significantly decreased but upon leaching, 80% of the ethylene glycol could be recovered [27]. Methanol and trichloroethylene inhibited the growth of ettringite. El Korchi et al. hypothesised the number of hydroxyl, carboxyl, and carbonyl groups in the organic molecules could be correlated with the degree of retardation of hydration [28].

Volatile organic compounds

Volatile organic compounds have been investigated by Arocha et al. who showed that low w/c ratios were necessary to limit VOC loss after S/S [29]. The retention of toluene was improved when PC and sodium silicate was used, leading to reduced VOC loss. The use of clay additives has also been examined. The mobility of nitrobenzene (NB) and 1,2-dochlorobenzene (DCB) was investigated, and a reduction of 85 and 90 % respectively was recorded [30]. However, unacceptably high VOC concentrations during remedial treatment (i.e. mixing and curing) have necessitated the use of an extraction hood and carbon filter trap [31].

Applications

So how is all this reduced to practice in applying S/S for treatment of organic contaminants? Compared to inorganic contaminants, the treatment of organic contaminants is: • Chemically more complex and less well understood; • Often heavily dependent on creation of a very low permeability, monolithic matrix; • Dependent upon the experience and intuition of the binder-system formulator; • Requires the careful application of bench-scale treatability studies (often necessitating the testing and application of multiple formulations); • Prone to interferences of the cementitious reaction; • Reliant on the use of special reagents such as activated carbon, modified clays, lime, fly ash, ground blast furnace slag, etc., to either bond the organic molecules or to lower the permeability of the matrix.

Volatile compounds

Volatile compounds do not immobilize well. If the contamination consists only of volatile organic compounds, then an alternative treatment such as soil vapour extraction, oxidation, or biodegradation may be both more effective and of lower cost. In the case where there are either metals or non-volatile organic contaminants, co-mingled with a small amount of volatile contamination, then activated carbon or organoclays can be used to adsorb the volatiles, which are then locked into a low permeability matrix. However, if the volatile component is very high, then the use of these special reagents becomes cost prohibitive. In such a case, it may be possible to first treat the volatiles via another technology (by stripping or biodegradation), before treating with S/S.

Semi-volatiles and non-volatile toxic compounds

Soils contaminated with pentachlorophenol, dioxins, and PCBs can be effectively treated using S/S and often at a far lower cost than with other technologies. The American Creosote case study (Jackson, Tennessee) involved full-scale treatment of PCP and dioxins at low cost, and has been published [6]. An ex-situ pugmill was used to treat the PCP, creosote, and dioxin contaminated soil using a formula containing Portland cement, fly ash, and activated carbon.

Petroleum hydrocarbons

Short-chain petroleum hydrocarbons are often volatile and difficult to treat with S/S while solvents, fuels, and cutting oils can greatly interfere with the setting and hardening reactions making them difficult to treat with S/S. Sometimes the addition of lime, especially hydrated quick lime (CaO) can supress the interference of oils with the cementitious reactions. However, quick lime itself is dangerous to handle and the high heat of hydration can create a risk of spontaneous combustion. Long-chain hydrocarbons, especially those that have cured (been baked in the sun or been heavily oxidized) can often be effectively treated.

Coal tars and creosotes

Coal tars and creosotes can be effectively treated with S/S, provided that they do not contain high levels of benzene. Figure 1 depicts the successful treatment of about 107,000 m3 of soil contaminated with coal tar sludge from a manufactured gas plant site in the USA using insitu augers. The formula developed after extensive treatability tests, both at bench and at full scale, used a blend of Portland cement and ground blast furnace slag.

Acid oily sludge

Acid oily sludge, such as those produced by refining processes and waste oil recycling processes present special challenges. The sludges often contain high levels of organics with high molecular weight and (sometimes) toxic metals, including lead.  Figure 2 depicts a non-acidic waste oil sludge containing lead being treated in a two-step process. First TSP (triple super phosphate) was mixed into the excavated sludge using an excavator in order to immobilize the lead, then the pre-treated sludge was fed into a pugmill and mixed with Portland cement. Note that some sludges are very acidic and adding highly alkaline reagents such as Portland cement can cause a violent exothermic reaction, unless first neutralized with a mild alkaline reagent, such as ground limestone.

Summary

Generally organic compounds present a greater challenge than inorganic contaminants for S/S, and it is important to be aware that they can interfere with the hydration and setting of the waste form. The retarding effect of organics on the hydration and setting can be correlated with the number of hydroxyl, carboxylic, and carbonyl groups in the organic molecules, and compounds that are volatile, water soluble and easily dissociated may present leaching and/or interference problems.

Nevertheless, the use of additives such as activated carbon, rice husk ash, shredded tyre particles containing carbon and organoclays (sorbents) increase the chemical containment of the contaminant, via either the physical or the chemical immobilisation. The use of silica fume and fly ash can improve the physical retention of organic compounds through lowered waste form porosity and permeability.

Many organic compounds have been successfully treated using S/S for over two decades. However the science and understanding is less well developed than for inorganics, making the experience of the formulator critical. The use of bench-scale treatability studies to develop case-specific formulations for organic contaminants is critical if successful treatment by S/S is to be realised.