Remediation Of Typical Contaminated Soil In Chemical Industry By Microwave Radiation

With the industrialization and urbanization and the performance of "Stockholm International Convention", a large number of chemical enterprises to be closed or retired are presented in our country. The abandoned chemical industry sites will be developed for commercial and residential use. The residues of the multi-type, high load, high toxic pollutants in contaminated sites pose potential risk to ecological environment and public health. Thus, it is necessary to remediate the contaminated sites effectively.Thermal remediation technology was widely used in contaminated sites remediation. In conventional heating method, heat transfers by thermal conduction, convection and radiation, which leads to slow heat transfer velocity and low remediation efficiency. Besides, conventional heating is an integral heating process, which consumes more energy. In recent years, microwave (MW) heating was used to instead of the conventional heating method for soil remediation. Soil can be heated rapidly and selectively by MW, and the pollutants in the soil were desorbed, degraded or stabilized. MW technology has shown great potential for the remediation of contaminated site of chemical industry due to its non-selective for the pollutants, high heating rates and great control of the treatment process.In this paper, three typical contaminants, antibiotic of pharmaceutical industry, nitro-compound of organic chemical industry and heavy metal of inorganic chemical industry, were selected as the treatment targets by MW technology. The operation conditions were optimized and the removal mechanisms were investigated. Then pilot-scale MW equipments were designed and used to explore the prospect of industrial application of MW technology. The main conclusions are drawn as follows:(1) Chloramphenicol (CAP), a typical antibiotic, was treated by MW radiation. The catalytic effect of the MW absorbents followed granular activated carbon (GAC)>Fe>H2O>MnO2. When GAC was used as MW absorbent, large MW power and GAC dosage should be used for a completed decomposition of CAP. The remediation efficiencies increased with the soil mass in the range from 0.5 g to 3.0 g. For 3 g soil, 93% of CAP remediation efficiency was reached in 5 min, whereas for 0.5 g soil, it was only 80% even in 20 min MW radiation. The effect of initial CAP concentration was minute. CAP experienced carbon-carbon bond rupture and oxidation reactions in MW radiation, and a part of CAP fragment reacted with the soil organic matter and formed new compounds, but the concentration of each product was extremely low. No dechlorination process occurred during this process.(2) MW technology was used to treat simulated 4-nitrophenol (4-NP) contaminated soil. GAC shows the best catalysis effect among the MW absorbents. Degradation efficiency of 4-NP increased when water was added into the soil, but the increment was less than 10%. The addition of GAC obviously increased the degradation efficiency of 4-NP in real soil. For 1 g real soil mixed with 0.3 g GAC,70% of 4-NP remediation efficiency was reached in 20 min at the MW power of 700 W. The 4-NP remediation efficiency was above 96% for 2 g soil mixed with 0.6 g GAC. The degradation efficiency increased with the MW power, the 4-NP initial concentrations had minute effect on the degradation efficiency.4-NP did not evaporate from the simulated soil by MW radiation, it was probably thermally decomposed to CO2 and H2O, or decomposed to fragments, which were further tightly bound to the soil.(3) Cr (Ⅵ) could be reduced by soil organic matter (SOM), Cr (Ⅵ) was reduced fastly at the beginning state, its concentration in soil decreased from 100 mg·kg-1 to 31.73 mg·kg-1 in the first 3 days. Then the reduction rate decreased. At a certain SOM concentration, the reduction rate of Cr (Ⅵ) decreased with the initial Cr (Ⅵ) concentration, but the reduction amount was increased. MW radiation posed a great reduction effect for Cr (Ⅵ) in the soil, and about 85% of the Cr (Ⅵ) could be reduced in 1-3 min at the MW power of 700 W. GAC dosage was the most important impact factor for the reduction process, the effects of radiation time and MW power were minute. When the initial concentration ranged from 7.34-610.48 mg kg-1, more than 75% of Cr (Ⅵ) could be reduced in 3 min. Several ten days was needed to reduce 85% of Cr (Ⅵ) by SOM in the soil, but only 3 min was needed by MW radiation. MW enhanced reduction of Cr (Ⅵ) showed great efficiency, which is of fleetness and high performance.(4) The pilot-scale MW soil remediation equipment showed great effect for the contaminated soil. For 200 g 4-NP contaminated diatomite with initial concentration of 100 mg kg-1, the removal of 4-NP reached 68-82% in 50 min MW radiation. The equipment was run stable and convenient to operate, and no MW leakage was detected. Nevertheless, the problem of recovery and utilization of the MW absorbent needs to be solved. Summarily, MW radiation can be considered as a promising technology for the site remediation of chemical industry. A pilot-scale MW reactor was developed to remove ammonia nitrogen in coke-plant wastewater, the output power of the reactor was 4.8 kW and the handling capacity was about 5 tons per day. Low ambient temperature and higher flow rate reduced the effectiveness of the MW reactor, initial concentration had minute influence on ammonia removal, and the removal could be enhanced about 9-10% with aeration. Ammonia removal efficiency of the MW pilot-scale system could reach 74-84% for real coke-plant wastewater. The cost of the MW equipment was a litter lower than conventional steam-stripping method.