Preliminary Studies On Reduction Of Nitroaromatic Compounds By Natural Pyrrhotite

Nitrobenzene is a common organic pollutant. It is chemically stable and toxic to microorganism, so the biological system would be destroyed when it enters. In order to remove NB from wastewater the effective process is to reduce NB to aniline (AN) first using chemical methods and then to mineralize AN completely using biological methods. The natural pyrrhotite is one of the most common sulfides in nature. It is mainly composed of ferrous sulfide. Though the synthetic ferrous sulfide has showed excellent performance on transformation of heavy metals and chlorinated organic, the processing cost is high. In this study, the feasibility of reduction of nitrobenzene with natural pyrrhotite is investigated by the batch experiment. And the effects of pH, initial NB concentration, temperature, particle size, etc on the NB removal efficiency were studied. At the same time, the reductive transformations of representative NBs by natural pyrrhotite were also studied. Furthermore the mechanism of the reaction between nitrobenzene and the pyrrhotite was discussed. Based on batch experiments, a pyrrhotite fixed bed reactor was constructed to treat wastewater containing NBs and the effects of HRT, pH, temperature, etc on the NB removal were explored with wastewater pumped into the reactor continuously. Conclusions were drawn as follows:(1) Nitrobenzene can be reduced to aniline by natural pyrrhotite and the reaction followed pseudo-first-order kinetics. NB removal reached 100% after 144h at the condition of 100 mg/L nitrobenzene and 0.23 kg/1 pyrrhotite.The NB removal was over 50% after 72h in the pH range of 3 to 11. NB removal gradually decreased with initial NB concentration and pyrrhotite dosage increasing. The optimal reaction rotary speed was 40rpm in this study. The reduction efficiency gradually increased firstly and then kept steady with particle size decreasing and 76.5% of NB was reduced when the mineral particle size was 150 meshes. NB removal increased from 58.9% to 91% when the temperature is increased from 25℃ to 45℃.There is little difference among the NB removals of different reuse times of pyrrhotite.(2) It can be found that the variations of SO42-, Fe2+ and pH in the experiment of NB reduction and Control Experiment were nearly the same. It suggested that the reaction between NB and pyrrhotite has little influence on the concentrations of SO42-, Fe2+ and pH. Furthermore, according to the analysis of SEM、XRF、XPS, the reaction equation between the pyrrhotite and NB is suggested as follow:(3) 4-MeNB、4-ClNB and 1,3-DNB can be reduced to 4-MeAN、4-ClAN、1,3-DAN respectively, by natural pyrrhotite. And all of the reactions followed the pseudo-first-order kinetics. The maximum rate constant is 0.093 hour-1 for 1,3-DNB and the minimum is 0.014 hour-1 for 4-MeNB while the rate constant of 4-ClNB is 0.093 respectively. We can find the nitro group was reduced to amino group by pyrrhotite and the nitrobenzene compounds were reduced to corresponding anilines. Order of the reaction rate is 1,3-DNB> 4-ClNB>NB> 4-MeNB when these nitrobenzene compounds existed in the same wastewater were reduced by pyrrhotite. At the same time, the rate constant of nitrobenzene compounds has little difference between the single and mixed wastewater. When the tannery wastewater containing 28.30 mg/L from a chemical plant in Zhengzhou was treated by pyrrhotite, the NBs removal efficiency reached 82.6% as reaction time was 6d. And the BOD5/CODcr of wastewater was increased from 0.073 to 0.204, thus providing good conditions for further biodegradation.(4) NB could be converted to AN continuously in the pyrrhotite reactor. And the concentrations of heavy metal ions in effluent were below the national discharge standards. When the influent concentration of NB is 20 mg/L and HRT is 24h, the NB removal rate is 86%. High removal efficiency about 90% can be attained when the initial pH range from 2 to 11. NB removal constantly decreased with increasing influent concentration. NB removal increased with increasing temperature and reached 100% when the temperature increased to 55℃. When the pyrrhotite packed column reactor is applied to treat the wastewater containing 15.94 mg/L NBs from a chemical plant, the NBs concentration in effluent reduced to 0.2 mg/L under the conditions of HRT 24 h, initial pH 2.18.