Study On Biodegradability Of Typical Sulfide Mineral Flotation Collectors

Sulfide mineral flotation collectors are the most widely used reagents in mineral flotation. The residual sulfide mineral flotation collectors in flotation wastewater have brought more and more serious environmental problems. It is known that even small concentration of these reagents in flotation wastewater is toxic to water life, besides their deleterious influence on the end stream processes during recycling. Groundwater pollution due to tailing dams is a worldwide problem. At present, studies of the biodegradability of sulfide mineral flotation collectors have seldom been reported at home and abroad. The flotation wastewater pollution is controlled mainly by physical and chemical methods. Bioremediation techniques have brought attention internationally as they are found to be versatile, inexpensive, stable and environmentally benign techniques in wastewater treatment. Therefore, studies on biodegradation law and mechanism of sulfide mineral flotation collectors in flotation wastewater and investigation on the feasibility of bioremediation technology treatment of flotation wastewater will provide some theoretical guidance for developing effective, low toxicity and environment-friendly flotation collectors and effective treatment of flotation wastewater.The biodegradability of four kinds of typical sulfide mineral flotation collectors such as sodium diethyldithiocarbamate, ammonium butyl-dithiophosphate, n-butyl xanthate and ethylthionocarbamate has been evaluated by BOD5/CODcr, static flask screening tests, oscillating culture method and Modified Sturm Test (OECD301B), aiming to propose a new appropriate evaluation method for evaluating the biodegradability of sulfide mineral flotation collectors. The biodegradation behaviors of difficultly biodegradable collector ethylthionocarbamate under the aerobic, facultative aerobe-aerobic, conventional anaerobic and nitrate, sulfate and ferric reduction conditions have been studied. Additionally, the biodegradation behaviors of ethylthionocarbamate under three different oxygen conditions were compared, and the optimal biodegradation conditions of ethylthionocarbamate were also obtained. Furthermore, the biodegradation behaviors of plant flotation wastewater containing ethylthionocarbamate under aerobic and ferric reductive conditions were discussed. Meanwhile, the influence of molecular structures on biodegradability was investigated, and the quantitative structure-biodegradability relationship (QSBR) model of typical sulfide mineral flotation collectors was established. Finally, the biodegradation mechanisms of the two most representative collectors were explored. From the present investigation, the main conclusions are as follows:(1) The BOD5/CODcr of sodium diethyldithiocarbamate, ammonium butyl-dithiophosphate, n-butyl xanthate and ethylthionocarbamate is0.46,0.32,0.21and0.14, respectively. And the biodegradation extent (D) can reach85.49%,45.93%,38.88%and37.91%, respectively. The index of biodegradability (IB) can reach202.9867,140.5366,99.0013and63.1683, respectively. Coincident conclusions have been obtained through those methods, indicating that sodium diethyldithiocarbamate is a readily biodegradable collector whereas ammonium butyl-dithiophophate is partially biodegradable. However, n-butyl xanthate and ethylthionocarbamate are poorly biodegradable collectors. Besides, the magnitudes of the biodegradation rate constants (k) are in the following order:Ksodium diethyldithiocarbamate>Kammonium butyl-dithiophophate>-butyl xanthate> Kethylthionocarbamate-(2) The primary biodegradation extent (PBD) of sodium diethyldithiocarbamate, ammonium butyl-dithiophosphate, n-butyl xanthate and ethylthionocarbamate can reach97.05%,93.70%,81.76%and37.32%, respectively in8d. And their biodegradation follows the first order reaction kinetics equation as follows: Ct=29.54e-0.4512t, Ct=29.91e-0.3463t, Ct=27.30e-0.2168t and Ct=28.66e-0.0557t, respectively.(3) The Modified Sturm Test (OECD301B) indicated that the suppressed time of sodium diethyldithiocarbamate, ammonium butyl-dithiophosphate and n-butyl xanthate attained4,7,12d, respectively. However, the PCD curve of ethylthionocarbamate is consistently located below the PCD curve of endogenous respiration, indicating that ethylthionocarbamate is refractory and toxic to the growth of microorganisms. Besides, their ultimate biodegradation rate constants (KCO2) are0.1817,0.1588,0.1315and0.1205, respectively. And their ultimate biodegradation follows the biodegradation kinetics equation proposed by Joel Blin and Diederik Schowanek as follows:BCO2=0.8022(1-e-0.1817(t-4)), BCO2=0.5554(1-e-0.1588(t-4)), BCO2=0.3912(1-e-0.1315(t-4)) and BCO2=0.2496(1-e-0.1205(t-8)),respectively.(4) The inoculum and dissolved oxygen concentration are two important factors that influence the biodegradability of ethylthionocarbamate under the aerobic condition. The biodegradation rate of ethylthionocarbamate can be significantly improved when ferric salt added. Therefore, low concentration (less than10mg/L) of ethylthionocarbamate could be rapidly degraded by the microbe.(5) When adding a small amount of glucose as co-metabolism substrate during the treatment of flotation wastewater containing ethylthionocarbamate, it can greatly enhance the biodegradation efficiency of ethylthionocarbamate, and greatly shorten the biological treatment period. The results showed that co-substrate metabolism is one of the most effective ways to enhance aerobic biodegradability of ethylthionocarbamate.(6) The biodegradability of ethylthionocarbamate under facultative aerobe-aerobic condition was significantly inferior to aerobic conditions. The biodegradation rate of ethylthionocarbamate was only37.48%in30d under facultative aerobe-aerobic condition, but the biodegradation rate was up to75.57%in15d under co-metabolism conditions. The biodegradation of ethylthionocarbamate under facultative aerobe-aerobic conditions can be accurately described by Sigmoidal kinetics. The corresponding kinetics equation is as follows:(7) Under anaerobic conditions, the enriched mixed bacteria could accelerate the biodegradation of ethylthionocarbamate when nitrate, sulfate and ferric were the terminal electron acceptors. It can be seen that with different electron acceptors, the biodegradation rates are comparatively different. Under these electron acceptor conditions, the order of the biodegradation rate of ethylthionocarbamate was:ferric> nitrate> sulfate.(8) Ferric was the most favorable electron acceptor compared to nitrate and sulfate under anaerobic conditions. Under different electron acceptor conditions, ethylthionocarbamate degradation was coupled to nitrate, sulfate and ferric reduction, respectively.Under sulfate reducing and ferric reducing conditions, the measured mass ratios between terminal electron acceptor and ethylthionocarbamate consumption were slightly lower than the theoretical ratios. When nitrate was a terminal electron acceptor, and the measured value between nitrate and ethylthionocarbamate consumption was3.557, which was higher than the theoretical mass ratio of2.784that was expected assuming the complete reduction of nitrate to nitrogen with ethylthionocarbamate complete mineralization, but was lower than the theoretical ratio of6.959that was calculated according to the assumption that nitrate was only reduced to nitrite. So we can conclude that nitrate was reduced to nitrite, but only part of the nitrite was further transferred to nitrogen.(9) When nitrate, sulfate and ferric were the terminal electron acceptors, the anaerobic biodegradation of ethylthionocarbamate can be accurately described by first order exponential decay kinetics. The order of the decay intensity constants (A) of ethylthionocarbamate is as follows:AFe3+>ANO3->ASO42-·(10) Satisfactory results were obtained for biological treatment of plant flotation wastewater containing ethylthionocarbamate under the conditions of aerobic and ferric reducing. The results demonstrated that anaerobic treatment of ethylthionocarbamate wastewater under ferric reducing condition is a new efficient wastewater treatment technology.(11) The electrical parameters such as ELUMO,μ,TE,(ELUMO-EHOMO),(ELUMO+EHOMO) and (ELUMO-EHOMO)2were the dominant factors affecting the biodegradability of sulfide mineral flotation collectors. However, the steric parameters and hydrophobic parameters had a little impact on biodegradability of the collectors.(12) The (QSBR)pri model of sulfide mineral flotation collectors was established for primary biodegradability prediction by multiple linear regression as follows: logKb=0.3242ELUMO-0.08653μ+7.228×10-4TE+316.1784×(ELUMO-EHOMO)-20.7715×(ELUMO-EHOMO)2-1210.496(R2=0.970, P<0.0001, n=7). Meanwhile, the (QSBR)ult for ultimate biodegradability prediction was also obtained as follows:PCD=0.3211μ- 5.4717×10-4TE-1416.2314×(ELUMO-EHOMO)-1.7989x (ELUMO+EHOMO)+94.4990×(ELUMO-EHOMO)2+5288.1325(R2=0.998, P<0.0001, n=7).(13) The (QSBR)pri and (QSBR)ult models showed that the calculated values fit well with the experimental data, the error between the experimental and predicted values of sulfide mineral flotation collectors is less than4.56×10-2, proving that the QSBR models have favorable predicting ability and can serve as a general predictor for unknown flotation collectors.(14) The biodegradation pathway of sodium diethyldithiocarbamate shows that the cleavage of C-N bond takes place firstly resulting in the formation of triethylamine, and then demethylation, trimethylamine are thus formed. Trimethylamine demethylation and further oxidation occur to form small molecules CO2and H2O. Meanwhile, ethanol is also produced, which is gradually oxidized to form ethanal and acetic acid. And acetic acid could be further oxidized to CO2and H2O. Finally, carbon disulphide as intermediate is also generated in biodegradation process of diethyldithiocarbamate. The biodegradation pathway of ethylthionocarbamate shows that the cleavage of C-N bond takes place firstly, with the formation of thiocarbonyl isopropyl ether and ethylamine. Ethylamine is gradually oxidized to form ethanol, which is further oxidized to form ethanal and acetic acid. Finally, acetic acid is oxidized to CO2and H2O. There are two pathways for biodegradation of thiocarbonyl isopropyl ether. One is thiocarbonyl isopropyl ether oxidize to methanthiol and acetone, another is thiocarbonyl isopropyl ether oxidize to dimethylmethane and carbonyl sulfide, and then further oxidized to CO2and H2O.