influence of ph on decomposition of total petroleum hydrocarbon in soil slurry-sequencing batch reactors

the contamination of soil and water with petroleum hydrocarbons is a widespread environmental problem. the treatment requirement and removal of this pollutant are becoming more important in these days. total petroleum hydrocarbon (tph) has been reported to be toxic, carcinogenic, and an endocrine disrupter widely used in developing countries. various physical, chemical, and biological methods were used for removing tph in water treatment. soil slurry – sequencing batch reactor (ss-sbr) is one of the important in-situ or ex-situ technologies which were able to bio-remediate the soils containing high levels of organic matters. in fact, in this condition the pollutant depletion rates depend mainly on the degradation activity of the microorganisms available in the system. the results obtained generally reflect the actual biological depuration potential of the soil. the application of aerobic ss-sbr is predominant for bio-remediation of soils. a large number of successful laboratory, pilot and full-scale studies and cases of aerobic ss-sbrs have been reported for bioremediation of soils polluted with polycyclic aromatic hydrocarbons (pahs), pesticides, diesel, and explosives. slurry-phase bioreactors (spb) are well-stirred tanks in which soil and water are mixed with air, microbial cells, and nutrients. soil is sieved to produce a 1mm particle, approximately, before feeding it to the reactor. many factors influence the feasibility and effectiveness of bioremediation. some of these are the presence of suitable microorganisms, the availability of nutrients, temperature, and ph. consideration of these factors is critical for a successful implementation of bioremediation systems. soil ph is an important process control parameter. the optimum ph for soil biodegradation lies between 6 and 8; however, effective biodegradation can be found outside this range. in this research, the ex-situ methods have been selected. in comparison with in-situ methods, they are faster, simpler and more controllable and can be applied for treating and removing more pollutants and soils. among ex-situ techniques, slurry phase has been chosen, where contaminated soil is combined with water and other additives in bioreactors and then mixed. nutrients and oxygen are added, and conditions in the bioreactor are controlled to create the optimum environment for microorganisms to degrade the contaminants. this technique offers two advantages. first, these experimental conditions maximize the contact between the solid and the aqueous phase, thus enhancing the mass transfer and, as a consequence, the biodegradation rate. in addition, slurry-phase degradation experiments give the results that can be promptly transferred to a full-scale process. the biodegradation of oils in a spb has a higher degradation rate than other biological treatment methods. various modes of spb operation have been tested in laboratories and pilot-scale plants, and one of the most common and best performing modes involves an ss-sbr. this research has investigated the optimum ph that afforded best degradation of tph in ss-sbr with a variation of ph and other fixed conditions. materials and methods the hplc grade n-hexane and dichloromethane have been used for extraction solvents. anhydrous granular (na2so4) has been provided. sulfuric acid and sodium hydroxide were used for ph adjustment. nh4cl, k2hpo4, and kh2po4 as nutrients have been used for balancing the c:n:p ratio as 60:2:1. all of these chemicals were purchased from merck, germany. the soil sample has been collected from azimabad region in the south of tehran refinery. the soil main characteristics are shown in table 1. table 1. the soil main characteristics pah (mg/kg) tph (mg/kg) phosphate (mg/kg) nitrate and nitrite (mg/kg) ph total n (% w/w) toc (% w/w) composition not detected 67000