| Saponin wastewater is known as the "king of the wastewater", and the main ingredients of saponin wastewater include two categories:one kind is the refractory organic pollutants such as furfural, which caused the high chemical oxygen demand, high colority, and poor biochemical degradation ability; another kind is the residual sulfate with the strong acidity and high concentration during the acid hydrolysis process.The existing technologies for saponin wastewater treatment mainly include adsorption, neutralization, micro-electrolysis, biological augmentation, etc., while the above mentioned technologies are difficult to popularization and application due to the high price and hard regeneration of adsorbent, the large production of excess sludge, the surface passivation of electrode and the high demands of maintenance.As the Zero-valent iron technology has the advantages of low toxicity, simple operation and rational economy, it has caused growing attention and is considered to be one of the most application prospect of technology for water environment restoration and water pollution control. At present, there are many reports about the reduction of heavy metal and refractory organic pollutants using zero-valent iron, while the study for the application of the zero-valent iron in the oxidation system is less, especially the research for "active iron" system composed by zero-valent iron/bivalent iron is much more less. Warm paste is currently the most popular disposable portable heating products in the world. After the using of warm paste, the dark brown solid waste that called as warm paste residue was formed. Iron powder and the adsorption materials such as vermiculite, activated carbon and high absorbent resin are mainly existed in the residue. There are no related reports about the recycling technology of warm paste residue or the secondary utilization of warm paste residue as iron source.Based on the research status, development trend and application prospect of zero-valent iron technology in the field of water treatment, the two kinds of typical pollutants, furfural and sulfate from saponin waste water are taken as treatment objects in the present study, and the combined treatment effects of Adsorption and Fenton-like Oxidation on simulated furfural wastewater are studied by using warm paste residue as adsorbent, catalyst and zero-valent iron source; the removal effect of sulfate from wastewater is also researched by the active iron complex system, which is composed byFe0/Fe2+.Additionally, the removal process and mechanism of Adsorption and Fenton-like Oxidation by sulfate green rust on simulated furfural wastewater are further investigated and explored. The main research results are as follows:1. Research of Adsorption and Fenton-like Oxidation on simulated furfural wastewater using warm paste residue(1) The adsorption performances of commercial activated carbon, self-prepared activated carbon and warm paste residue on simulated furfural wastewater are comparatively studied. The results reveal that the adsorption capacities of both warm paste residue and self-prepared activated carbon on furfural are much better than that of commercial activated carbon; the active adsorption sites distribution on the warm paste residue surface is relatively uniform, and the adsorption mode is mainly dominated by the multilayer adsorption of chemical action; the adsorption process of warm paste residue on furfural follows the pseudo-second-order model, and the adsorption characteristics are consistent with the Freundlich model.(2)The removal efficiency of furfural is investigated by Fenton-like reaction and adsorption/Fenton-like coupling process. As the initial furfural concentrations are from 2 to 40mmol/L and the hydrogen peroxide dosage is 0.176 mmol/L, the furfural removal efficiencies are 93～97% for the Fenton-like reaction, while they are nearly 100% for the adsorption/Fenton-like coupling process.(3) The surface characteristics of the warm paste residue before and after adsorption/Fenton-like reaction are observed using scanning electron microscopy (SEM). The results indicate that the particle size and shape of the warm paste residue show no significant variation after the adsorption, but the particle size is nearly 10μm and approaches the uniform; besides, the specific surface area and the reactivity point number of the warm paste residue are increased due to the continuous dissolution of the iron, aero genesis and exothermic function of Fenton-like reaction.(4) The seven recycle repetition utilized reaction of the warm paste residue and the pure reduced iron powder (AR) declares that the reduced iron powder is only used for Fenton-like reaction with the furfural removal efficiency of 78～88%; the warm paste residue could achieve the combined effect of the adsorption/Fenton-like reaction and be regenerated in situ, at the same time, the inorganic salts or other metal ingredients in the warm paste residue might have a synergistic catalytic role for Fenton reaction, consequently the furfural removal efficiency is as high as 93%.2. Mechanism research of sulfate removal by Fe0/Fe2+ complex systemThe effects of Fe0/Fe2+ complex system on the removal of sulfate are comparatively studied under the conditions of different oxygen supply modes, pH values, dosages and dosing methods of the reactants, moreover, the reaction mechanism and products characteristics of Fe0/Fe2+ complex system and sulfate are further investigated, and the main results are as follows:(1) The oxygen supply modes have significantly effect on the removal performance of sulfate, and the removal efficiency is 93.4%,67.1% and 18.8% for the limited oxygen condition (samples are prepared in open environment, and reactions happened in sealed shaking environment), aerobic condition (samples preparing and reactions are both in open environment)and anaerobic condition(injecting nitrogen and sealed shaking), respectively.(2) The reaction products of Fe0/Fe2+ complex system on the sulfate under the conditions of different oxygen supply are investigated by X-Ray Diffraction (XRD) and SEM analysis. It is found that the blackish green flocculent product under the limited oxygen condition is sulfate green rust, and the final product under the aerobic condition is lepidocrocite, while there is hardly any product under the anaerobic condition. X-ray photoelectron spectroscopy (XPS)is further carried out for the analysis of green rust, and the chemical structural formula is presumed as Fe4、Fe2Ⅲ(OH)12·SO4·8H2O;green rust is extremely sensitive to the environment, and it could be easily oxidized by air; Along with the increase of the exposure level with the air, the oxidation products are transformed gradually from magnetite to goethite and lepidocrocite.(3)The sulfate green rust could be obtained as Fe0/Fe2+ complex system reacted with the sulfate radical under the limited oxygen condition, and it is a new preparation method for green rust. There is no precise control of oxygen supply conditions or pH values in the present method, and the shortcomings in traditional preparation methods such as oxidation and coprecipitation are avoided.(4) The bivalent iron plays an important role in the reaction system of Fe0/Fe2+ and sulfate. On the one hand, the bivalent iron directly involves in the reaction and then the green rust is generated, and the bivalent iron could be catalytic and keep the concentration gradient and the reaction rate in the reaction; On the other hand, the bivalent iron could promote the dense lepidocrocite layer that adhered closely to the zero-valent iron surface transform into progressive structure magnetite, and ensure the active reaction point number of the zero-valent iron surface and excite the zero-valent iron. In addition, adjusting the pH value of the reaction liquid to acidic is conducive to the reaction, and it is mainly due to that the zero-valent iron could be converted into bivalent iron in the acidic condition.(5) Increasing the reduced iron powder means that the total surface area of zero-valent iron and the active reaction point number of zero-valent iron surface is increased, and this is advantageous for the sulfate removal.(6) The initial open condition is good for the oxidation of part of the zero-valent iron, and subsequently the airtight reaction could promote the production of green rust, which is generated by the reaction of iron hydroxide ions and sulfate under the anaerobic condition; besides, the concentration gradient of the reactants could be increased by separate dosing the reagents. The effects of dosing methods such as bivalent iron and acid on sulfate removal are also investigated, and the results show that the order of different dosing methods from good to bad is:Airtight separate dosing>Airtight single dosing>Open separate dosing.3.Treatment of the actual saponin wastewater byFe0/Fe2+ complex system(1)The simulated sulfate radical wastewater with the initial sulfate concentration of 1000 mg/Land pH value of 6.15 is treated by four different iron sources including the coarse warm paste residue (manual grinding), the fine warm paste residue (the particle size was closed to iron powder), waste iron scraps and iron powder (AR), the treatment effect order from good to bad was:iron powder> the fine warm paste residue> waste iron scraps> the coarse warm paste residue, and the maximum and the minimum removal efficiency was 78.5% and 43.9%, respectively; while for treating the actual saponin wastewater with the initial sulfate concentration of 11426 mg/L and pH value of 0.65, the treatment effect order was:the fine warm paste residue>the coarse warm paste residue>iron powder> waste iron scraps, the minimum removal efficiency was also reached 61.6%,while the maximum removal efficiency was nearly 100%. The sulfate removal efficiencies for the saponin wastewater are much better than that for the simulate wastewater.(2)The experiments of recycle using of iron source indicate that the sulfate from actual saponin wastewater could be effectively removed by both the fine warm paste residue and the iron powder, while the removal attenuation rate of the fine warm paste residue on sulfate was much higher than that of the iron powder. In consequence, the warm paste residue could be applied for the actual saponin wastewater treatment, while the supplement of regular iron source should be considered during the treatment processing.4. Treatment of the simulated furfural wastewater by sulfate green rustThe effect of sulfate green rust on simulated furfural wastewater is carried out according to the study of Adsorption and Fenton-like Oxidation on simulated furfural wastewater by warm paste residue, and the main results were as follows:(1) The adsorption isothermal characteristics of sulfate green rust on furfural are in accordance with the model fitting results of both Temkin and Freundlich isothermal models, and the process is mainly due to the multilayer complex adsorption of the non-uniform surface. As the initial furfural concentration is 10,20 and 30 mmol/L respectively, the equilibrium adsorption of the sulfate green rust on furfural is 60.5,110.7 and 162.7 mg/g, while the equilibrium adsorption of the warm paste residue on furfural is 12.9,19.8 and 26.0 mg/g respectively. Although the adsorption dynamics characteristic of the sulfate green rust on furfural is similar to that of the warm paste residue, the adsorbing effect of the sulfate green rust is much better than that of the warm paste residue.(2) As the same results of warm paste residue, the removal effect of the sulfate green rust on furfural with the fully adsorption of furfural followed by Fenton-like reaction is better than that with the direct Fenton-like reaction. The catalytic effect of traditional dissolved Fe2+on hydrogen peroxide is far lower than that of the combined Fe2+ from the sulfate green rust. As the adsorbent for furfural removal and the iron source for Fenton-like reaction, the sulfate green rust is much more efficient than the warm paste residue. Under the same conditions that the furfural removal efficiency was closed to 100% and with the initial furfural concentration of 10 mmol/L, the consumption of the sulfate green rust (8.24 g/L) is only 16.5% of that of the warm paste residue (50 g/L); in addition, the consumption of hydrogen peroxide for the sulfate green rust treatment (0.039 mol/L) is only 22.2% of that for the warm paste residue treatment (0.176 mol/L).(3) As comparing the XPS results of the sulfate green rust before and after Fenton-like reaction, it is found that Fe2+ from the sulfate green rust(Fe4Ⅱ FeⅢ2(OH)12SO4·8H2O) is oxidized to Fe3+, and hydroxyl or combined water occurred to the deprotonation effect; moreover, the hydroxyl relative contents in the reaction product (Fe6ⅢO2(OH)12SO4)are sharply reduced while the O2- contents are significantly increased. |
PDF Full Text Request