Advances in In Situ Biological and Chemical Groundwater Treatment

Groundwater contamination generically refers to modifications in biological, physical, or chemical characteristics; radioactivity; or the presence of undesirable solutes at significant concentrations. In terms of undesirable solutes, inorganic or organic chemical mixtures frequently occur, including metals and semi-metals, such as chromium and arsenic, and volatile chlorinated hydrocarbons (e.g., tetrachloroethene, trichloroethene). “Pump and treat” is a common method for cleaning up groundwater contaminated with dissolved chemicals. Groundwater is pumped from wells to an above-ground treatment system that removes the contaminants. Pump and treat may last from a few years to several decades, with the actual cleanup time being long when the concentrations of the contaminants are high, the pollution source has not been completely removed, or the groundwater flow is slow. The increasing availability of scientific studies has progressively drawn attention to in situ technologies for groundwater remediation. Most of them are innovative compared to the pump-and-treat approach, allowing the remediation time to be reduced and the remediation sustainability to be increased. In situ bioremediation of groundwater involves the encouragement of indigenous bacterial populations to metabolize target contaminants through the addition of various amendments, or the use of selected strains of bacteria in the subsurface to help treatment. Bacteria perform coupled oxidation/reduction reactions to live, and bioremediation exploits all these reactions to remove contaminants from groundwater. Aerobic bioremediation most commonly takes place in the presence of oxygen, and it is most effective in treating non-halogenated organic compounds. Anaerobic reductive bioremediation takes place in the absence of oxygen and promotes the bioreduction of oxidized contaminants such as chlorinated solvents. Microbes or their enzymes may also effectively remediate toxic heavy metal contamination via their metal-resistance mechanisms, including the transformation of metals into less toxic species, biosorption to the cell wall, entrapment in extracellular capsules, or precipitation. Nanotechnology is a multidisciplinary field that has gained significant momentum in recent years. The use of nanomaterials, such as zero-valent iron and carbon nanotubes, in the cleanup of groundwater is relatively new and has a great potential for providing efficient, cost-effective, and environmentally acceptable solutions to face the increasing requirements of stringent quality standards. The large surface area of these nanoparticles results in high sorption capacity, along with the ability to be functionalized for the enhancement of their affinity and selectivity. Nanoand microplastics have received widespread attention in recent years as they can sorb various organic contaminants.