Reduction of Hexavalent Chromium Concentrations using a Fixed Bed Bioreactor

Nicole Levy and Dorota Rek

California Polytechnic State University
Department of Civil & Environmental Engineering
San Luis Obispo, CA 93407

(Nicole Levy pictured at right in front of research apparatus.)

Abstract
The presence of heavy metals in water supplies and wastewater threatens the environment and the health of humans. Over the past decade, the emergence of bioremediation studies have proven micro-organisms to be effective in adsorption of heavy metals such as uranium, lead, and copper successfully. In some cases, the micro-organism uptakes the heavy metal then transforms it to a less toxic and/or immobile state. Chromium(VI) is of particular concern because it is a highly soluble and toxic ion that has relatively few bacterial species which can adsorb it efficiently. This study focuses on the author's current investigation of bioadsorption and reduction of hexavalent chromium in a fixed bed reactor. The objective of the experiment is to find optimal conditions considering pH, temperature, flow rates, nutrients, oxygen, and chromium concentrations, for hexavalent chromium to be adsorbed or reduced by the biomass.

Introduction
Bioremediation is becoming a widely utilized abatement technology for environmental pollutants because of its effectiveness in mineralizing organic pollutants and adsorption and reduction of heavy metals. Biomass is used to degrade organics to non-toxic compounds. Bioremediation can also be accomplished through the adsorption of leaching metals which lead to changing the metal to the less harmful 3+ oxidation state. The versatility of bioremediation in terms of aerobic/anaerobic, air/water/soil applications, thousands of species in the biomass, temperature, and pH make this process a preferred alternative. This option is used especially when environmental impact, range of pollutants, costs, and feasibility are in question.

Chromium naturally occurs in the environment as chromium(III) or chromium(VI). Chromium is also produced as a waste stream from processes such as metal finishing, petroleum refining, chrome tanning, textile manufacturing, and wood preserving. Natural oxidation of chromium(III) could also occur to produce chromium(VI). Chromium toxicity and mobility depend on its oxidation state. The trivalent forms are relatively immobile, more stable, and much less toxic than the hexavalent forms.[1] Determination of Cr speciation is critical for effective environmental treatment strategies. Many remediation and treatment technologies such as activated carbon adsorption and cation exchange are ineffective in treatment of Cr(VI).[2] Vitrification is ineffective in the presence of Cr(VI), as well. Therefore, it is essential to remediate Cr(VI) before the use of these treatments for other contaminants. Although heavy metals are essential for life, at high concentrations heavy metals are toxic. Chromium(VI) contamination is particularly imperative to remediate because of the severe carcinogenic and toxic affects on humans and wildlife.

Definitions
The definition of bioremediation has appeared in many forms in the literature. One such form reiterates the definition of bioremediation in the context of organics as the acceleration of the biodegradation process through the addition of nutrients and other materials to contaminated media using techniques such as aeration, venting and temperature control.[3] In terms of heavy metals, bioremediation can be defined as the adsorption, transformation, or immobilization of heavy metal by a biomass.

Materials and Methods
A sample was obtained from the aeration basin at the San Luis Obispo Wastewater Reclamation Center. A fraction of this sample was mixed into a nutrient solution containing sodium phosphate, potassium phosphate, ammonium sulfate, magnesium sulfate, ferric chloride, manganous sulfate, and glucose as a carbon source. The sample was placed on a shaker table to provide adequate aeration. The objective is to culture the chromium resistant microbial strains which are used within the bioreactor. A one mL sample of this culture solution was extracted and placed into a new nutrient broth containing chromium. The process was repeated every 3 to 5 days for a month. This process allowed for the selection of microbes that were capable of surviving in the chromium rich environment.

The experiments are performed in the KONTES Airlift Bioreactor. The bioreactor packing material consists of small plastic chips obtained from the recycling center at Santa Clara University. The chips maximize surface area for microbial attachment and nutrient exchange. Chromium solution, in the form of potassium dichromate, is fed into the bottom of the reactor, and effluent is collected from the top. Chromium feed concentrations range from 5 to 50 ppm. Aeration is accomplished by a pump attached to a bottom inlet. A water jacket encloses the outside of the reactor to keep the temperature at a constant 30°C.

Samples from the influent and effluent solutions are taken once a day. The samples are frozen to prevent any further reactions from occurring. They are thawed at the end of the week and tested for total chromium and hexavalent chromium concentrations. Total chromium is measured by an atomic absorption spectrophotometer, and hexavalent chromium is measured by a colorimetric spectrophotometer. Influent and effluent concentrations are compared to determine the concentration that has been adsorbed and the concentration that has been reduced.

Toxicity tests at varying concentrations of chromium(VI) were performed on the bacterial consortia. The concentrations varied from 5 to 100 ppm Cr(VI). A computerized respirometer was utilized to measure the oxygen demand and carbon dioxide production of the biomass. The oxygen demand correlates to the activity rate of the microbial consortia. The concentration at which chromium becomes toxic to the biomass is determined by evaluating these activity rates.

The nature of this experiment involves varying different parameters in order to obtain maximum removal efficiency. These parameters consist of temperature, pH, and flow rates. A temperature between 25 to 30°C is critical for the optimum biological growth rate and enzyme activity. The pH can facilitate adsorption by increasing the solubility of Cr. The flow rate of the solution is of importance for a sufficient nutrient supply along with adequate contact time for adsorption. These parameters will be explored in future experiments. Varying concentrations of chromium(VI) solutions will be fed into the reactor while varying the flow rate, temperature, and pH. The goal is to provide optimal conditions for the microbial consortia to reduce and adsorb the chromium(VI) ions in solution.

Results
Reduction of the chromium(VI) has been observed when comparing our 10 ppm influent and effluent concentrations at ambient conditions. Approximately 20% of the chromium(VI) was reduced. However, there was a negligible reduction in the total chromium concentrations. It was concluded that this set of experiments were performed at temperatures that were too low for optimal growth and enzyme activity. The ambient temperature in the laboratory varied from 19 to 22°C. These experiments are being repeated because of the minimal bacterial growth. The second set of experiments are being performed with the water jacket at 30°C and an increased carbon supply. Activity rates increase with temperature, but above 35°C the reactor becomes too warm and the biomass dies off. Results from the toxicity tests indicate that Cr(VI) concentrations above 50ppm were toxic to the microbes. Data from the 5ppm solution indicated an active culture, and the 20ppm solution indicated a slight decrease in activity. These will indicate the maximum allowable chromium feed concentration. Freezing the samples until measurements can be taken, help to prevent any reaction outside of the experiment from occurring. Further studies involving pH, temperature, and flow rate while feeding different Cr(VI) concentrations are scheduled for later this month.

Given proper conditions, chromium(VI) can be reduced. Microbial growth can be stimulated through adjustment of pH, addition of nutrients, adequate oxygen, and flow rates. It is predicted that with the proper conditioning the micro-organisms will reduce and adsorb the chromium at increasing percentages. Chromium can be potentially recovered by extracting the accumulation of Cr in the biomass, and chromium that has been reduced to 3+ will potentially precipitate or adhere to soil particles. Micro-organisms from an aeration basin and recycled plastic media can be obtained at minimal costs. Material costs which involve aeration basin effluent and recycled plastic, would be kept at a minimum. This would keep chromium out of the environment and possibly save companies money in disposal and remediation costs.

References:

1. Cifuentes, F.R., W.C. Lendemann, and L.L. Barton. 1996. Chromium sorption and reduction in soil with implications to bioremediation. Soil Science. 161:233-241.
2. Pal, Nirupam. 1996. Reduction of Hexavalent Chromium to Trivalent Chromium by Phanerochaete Chrysosporium. Battelle International Symposium.
3. Hoff, R. Z. 1993. Bioremediation: an overview of its development and use for oil spill cleanup. Marine Pollution Bulletin. 26:476-481.

Dr. Nirupam Pal, Assistant Professor / Project Advisor
California Polytechnic State University, San Luis Obispo
Last Update: April 27, 1997.