Valentine Nzengung

Research Areas:

Environmental Geosciences

Research Interests:

Main Research Interests

My research focuses on the development and evaluation of innovative technologies for the cleanup of water, sediment, and soils. Particular emphasis is placed on sorption and degradation of contaminants and immobilization of metals. Specifically, my ongoing research focuses on the following five technologies:

Phytoremediation Technologies:

Cleanup of soils, sediment, water and air using green plants and root zone associated mechanisms.

Microbial Mats Treatment System:

Treatment of contaminated waters using green photosynthesizing microbial mats dominated by cyanobacteria.

· Abiotic transformation of organic contaminants:

Chemical treatment of contaminated soils and water. Emphasis is placed on the applications of activated bulk reductants in remediation of oxidized compounds, including the immobilization of metals.

· In-situ Bioremediation of Contaminants in Soils and Groundwater:

Preparation and characterization of surfactant modified (organo-modified clays or Clay nanocomposites) as cost-effective sorbents/filtration media for perchlorate and organic contaminants.

· In-situ bioremediation of soils and groundwater:

Biostimulation and enhancement of contaminants degradation in soil and groundwater using innovative techniques to deliver carbon and electron donors to the subsurface. This approach avoids expensive dig-and-treat of contaminated source area soils in the vadose zone.

Phytoremediation Technologies

Phytoremediation is the use of green plants (both aquatic and terrestrial) and the root zone associated microorganisms to cleanup hazardous waste sites. Phytoremediation involves a multitude of processes, thus the name phytoremediation technologies. A rigorous design of phytoremediation identifies and enhances the most applicable processes for the field site of interest. Phytoremediation is effective in treating livestock and poultry wastes, munitions constituents (e.g., perchlorates and nitroaromatics), organic contaminants, and metal contaminated hazardous waste sites. Compared to other remediation approaches, phytoremediation is more acceptable to the public and regulatory agencies because the ecosystem health is significantly improved during cleanup of the contaminants of concern. The trees also remove greenhouse gases from the atmosphere to produce biomass, which could be utilized as raw material for biofuels production.

My research group has screened many species of terrestrial and aquatic plants for the purpose of identifying the most effective species for phytoremediation of the different groups of contaminants. The data generated from our laboratory studies, so far, has been valuable in the development of better environmental fate models by regulatory agencies, design, and field application of phytoremediation technologies. We have coupled laboratory and field data from our analysis of plants used in a demonstration project at the Carswell Air Force base in Fort Worth, Texas, to provide a better understanding of phytoremediation processes and pathways for chlorinated organic solvents. Also, at a Naval Facility in Florida, we have successfully coupled phyto-processes and natural attenuation to accelerate the cleanup of a shallow chlorinated solvent plume upwelling into a wetland and recreational lake. Our research also involves engineered phytoremediation; an innovative approach for biostimulation and enhancement of rapid rhizodegradation of degradable contaminants such as nitrate, perchlorate, explosives constituents, chlorinated solvents, pesticides, and petrochemicals. Rhizodegradation enhancement significantly reduces the cleanup time and minimizes uptake and accumulation of degradable contaminants in plant tissues. Thus, if rhizodegradation can be successfully achieved, there is no need to dispose of the plant matter as hazardous waste when the soils and water cleanup goals have been achieved.

Microbial Mats Treatment System

Microbial mats are naturally occurring stratified microbial communities, composed of a complex consortia of bacteria dominated by photoautrophic cyanobacteria (also referred to blue-green algae). Mats generally include anoxygenic photoautrotrophs (purple bacteria) as well as sulfur-reducing microorganisms. They are tightly annealed together within a negatively charged polymeric matrix of gel. The 3.5 billion-year survival of mats testifies to their successful capacity in altering hostile environments through cellular and community-­mediated activities.

“Constructed” microbial mats grown using a standard technique that is very inexpensive and can be accomplished with minimal training is used at the bench and field scale to treat complex waste streams and produce biomass. The mats are very effective for sequestering or precipitating metals/radionuclides by surface absorption or by altering the surrounding chemical environment, thus they bioconcentrate the metals/radionuclides in a small volume (<5% of wet mass). The microbial mats are used to mineralize organic and inorganic contaminants, including pesticides, munitions constituents (i.e., perchlorate and explosive compounds), petrochemicals, and chlorinated solvents. Because microbial mats use greenhouse gases to produce biomass and are among the fastest growing photoautotrophs, they show great promise in the production of biomass for bioenergy. My research group continues to develop new applications for microbial mats in bioenergy, bioremediation, and sequestration of greenhouse gases.

In-situ Bioremediation of Contaminants in Soils and Groundwater

Contaminated soils are often the long-term sources of contaminants leached into surface and ground waters. Most soils will normally contain natural bacteria capable of degrading the contaminants of concern. As a result, the persistence of degradable contaminants in soils and groundwater is commonly caused by the limited supply of nutrients and suitable environmental conditions to support the growth of microorganisms capable of degrading the contaminants of concern. Building on results of multiple bench scale tests, we have demonstrated at field sites the efficacy of in-situ bioremediation by applying suitable carbon and electron donors at the soil surface and progressively mobilizing into the groundwater below. The amendments mixed in with the surface soils create suitable conditions for growth of significantly high numbers of indigenous microorganisms capable of degrading the contaminants of concern. The successful infiltration of selected suitable carbon and electron donors through the vadose zone into the site groundwater apparently creates favorable environmental conditions for the growth of very high numbers of the microorganism capable of degrading the contaminants. Even more important, the latter approach achieves a larger foot print of bioremediation than injection systems. This Surface Application and Mobilization of Nutrient Amendments process has been successfully applied at the field scale to cleanup perchlorate, munitions, pesticides, and petrochemicals in vadose zone soils and groundwater. Since there is no dig-and-treat involved, the technology costs are an order of magnitude less than for competing technologies, such as windrow composting, soil vapor extraction system, soil washing, pump-and-treat, thermal treatment.

Abiotic Transformation of Organic Contaminants and Sequestration of Metals

My research group has been studying the application of oxidation-reduction reactions of iron and other redox sensitive metals in soils and sediments in remediation of contaminated soils, sediment and waters. Our research has focused on the role of ferric (Fe(III)) and ferrous (Fe(II) iron because Fe(III) is the fourth most abundant element in earth materials and the most important electron acceptor following the onset of anoxic conditions in groundwater, soils and sediment. Completed and ongoing research involves the enhanced degradation of oxidized organic contaminants (e.g., chlorinated aliphatics and aromatics, explosives, PCBs, DDT, etc.) using the free radicals generated during the decomposition of bulk reductants under suitable environmental conditions. Aquifer material and soils are chemically treated with bulk reductants (e.g., dithionite) to create reducing conditions. Specific interest is placed on the reaction kinetics, identification of degradation products and determination of mass balance. The advantages offered by this innovative approach are: (1) in-situ redox manipulation using dithionite enhances reductive dechlorination of chlorinated organics and immobilization of metals, (2) rapid degradation of explosive compounds in soils and groundwater at Military Ranges and other Defense facilities, (3) the system can be easily optimized and regenerated, (4) no toxic end-products are generated, and (5) remediation costs are less than those of most conventional abiotic and biotic techniques.

Preparation and characterization of surfactant modified clays (clay nanocomposites)

Building on my previous research on sorption and degradation of organic pollutants, we are developing unique formulations of surfactant modified clay (SMC) for sorption and treatment of waste water. The SMC (also called organo-modified clays or organoclays) are effective sorbents for filtration of nitrate, perchlorate, metals and organic contaminants from water. My research also involves studying the effects of organic cosolvents, solute properties and organoclay structure on the mechanism of sorption of hydrophobic organic chemicals and development of nonselective organoclays. Since the raw clays are widely available and cheap, SMC can be utilized commercially as low cost use-and-toss sorbents.