Special Issue on Contaminant Mixtures: Fate, Transport, and Remediation

There are thousands of sites across the United States and worldwide where soils and groundwater have been contaminated by multiple contaminants, most commonly organic compounds [such as polycyclic aromatic hydrocarbons (PAHs), benzene, and phenols] and heavy metals (such as lead, zinc, and nickel) together. Such mixed contaminants pose unacceptable risk to public health and the environment; therefore, urgent cleanup actions are required to reduce the risk to below the acceptable levels (Sharma and Reddy 2004). This special issue is devoted to focus on recent studies dealing with various aspects of assessing and remediating contaminant mixtures in soils and groundwater.

The fate and transport of mixed contaminants in subsurface environments can be quite complex because of various physical, chemical, and biological processes. In-depth understanding of geochemistry that influences the distribution, speciation, and transport is needed to understand the risk and develop remedial strategies. Joo et al. performed batch experiments to evaluate the sorption of six nonpolar and six polar organic compounds as mixtures to both hydrophilic mineral surfaces [uncoated, iron (hydr)oxide-coated, and aluminum (hydr)oxide-coated sands] and humic acid–mineral complexes with different fractions of organic carbon. The mixture effects on sorption of the nonpolar compounds were found to be not significant, whereas they were found to be significant for polar compounds depending on the mixture compositions and the type of sorbent. This implies the need for accurately quantifying and modeling mixture effects on fate and transport of multiple organic contaminants present at the site. Poly and Sreedeep performed batch sorption experiments and found that the presence of multiple contaminants affect the sorption of individual contaminants. The contaminant fate prediction based on linear isotherm is influenced by the range of concentration for single and multiple contaminants. For lower ranges of concentration, the contaminant fate prediction based on the Freundlich isotherm is greater than that of Langmuir, and vice versa for higher concentration range. Further, the difference in contaminant fate prediction based on Freundlich and Langmuir nonlinear isotherms decreases with an increase in concentration. The fate and transport parameters of soils vary in space because of soil heterogeneity, and such conditions require transport modeling using stochastic approaches. Nezhad and Javadi present an advective and diffusive-dispersive contaminant transport, using a stochastic finite-element approach. The need for expanding such approaches to multiple contaminant conditions is warranted.

Previous research on the advancement of remediation technologies, such as soil washing, chemical oxidation, bioremediation, thermal desorption, incineration, and pump and treat had targeted either organic contaminants or metals, with very little attention to the mixed contaminants. As a result, conventional technologies are found to be ineffective or costly for the complex mixed contaminant conditions. Reddy et al. evaluated the effectiveness of various flushing agents (chelant, surfactant, and cyclodextrin) to enhance the remediation of a manufactured gas plant soil contaminated with PAHs and heavy metals. Bench-scale experiments were conducted under two different hydraulic gradient conditions. It is shown that the contaminant removal in soil flushing systems depends on the flushing solution affinity and selectivity toward the target contaminant as well as the existing hydraulic gradient condition. Soil flushing can be effective only in high-permeability soils; however, it is difficult to induce substantial flow in low-permeability soils under common hydraulic gradient conditions. Wan et al. investigated the remediation of hexachlorobenzene (HCB) contaminated kaolin (low-permeability soil) by the electrokinetics (EK) couple with permeable reactive barrier (PRB) using microscale Cu/Fe particles. Surfactant (Triton X-100) was used to enhance the solubility of the contaminant. This study showed removal of more than 82% of HCB passing by the PRB.

An alternative to removal is the immobilization of the contaminants within the soil. Karachalios et al. investigated simultaneous stabilization of copper, lead, and tungsten in eight representative contaminated firing range soils in the United States using various amendments. The amendments included granulated ferric oxide, granulated titanium dioxide, pahokee peat soil, gascoyne leonardite soil, elliot silty loam soil, calcium phosphate monobasic, and apatite. The results showed significant retention of the contaminants by the amendments. In a similar study, Grubb et al. demonstrated the use of steel slag fines to immobilize arsenic in both arsenite ($A{s}^{3+}$) and arsenate ($A{s}^{5+}$) forms in dredged material. The leachability was examined by the toxicity characteristic leaching procedure and synthetic precipitation leaching procedure. X-ray diffraction and scanning electron microscopy studies were used in combination with MINTEQ modeling to examine the mechanisms responsible for the arsenic immobilization. Vaishya and Gupta used quartz sand coated with iron and manganese (known as mixed oxide coated sand) to immobilize $A{s}^{3+}$ in groundwater.

Bioremediation is considered a green and sustainable remediation option. Li et al. investigated aerobic biodegradation to mineralize mixed contaminants (BTEX, TPH, TCE, and DCE) in groundwater. Different environmental conditions (pH and temperature) using indigenous microorganisms isolated from potentially contaminated regional sites were tested. The highest TPH bioremoval efficiencies ($<50\%$) occurred at 25°C and under neutral/alkaline conditions. TCE was cometabolized with toluene and/or $o$-xylene provided as growth substrates. The pH and temperature impacted the mineralization of compounds. This study shows the potential for applying bioremediation technology to mixed contaminants at field sites.

Though advances have been toward understanding the fate, transport, and remediation of mixed contaminants in subsurface, there are many technical issues yet to be resolved. Fundamental synergistic mechanisms need to be further explored, and also effective, inexpensive, green, and sustainable remediation technologies are yet to be developed and tested.