| Hexavalent chromium is identified as a cancerogenic substance. Because of its high solubility, it is easy to permeate from the initial contaminated sites to the deep soil aquifer and contaminate environment. The technologies for Cr(VI) treatment developed by researchers at home and abroad include chemical reduction method, electrochemical method, photocatalytic reduction method, absorption method, separation membrane and biological treatment, etc, which have their merits and defects. Most of them are targeted for the treatment of Cr(VI) in the water, while there is little focus on the treatment of Cr in the sludge. Chemical reduction method was chosen as the technology for Cr(VI) treatment in this paper. To solve the problems, such as the slow reaction rate at the neutral and weak alkaline condition and the large yield of sludge, the ligands or an external magnetic filed was used to enhance the reducibility of zero-valent iron(ZVI) for the aqueous Cr(VI) treatment at the neutral and weak alkaline condition, and the alkali-activated slag was used to immobilize the chromium-bearing sludge and residue.Using Cr(VI) as the object, the influence of the initial p H, temperature, ZVI dosage, ion strength and co-existing cations and anions on the removal efficiency and reaction kinetics was investigated. The results show that the influence of ZVI dosage, temperature and initial p H on the Cr(VI) removal are great, the higher ZVI dosage and temperature as well as the lower initial p H were in favour of Cr(VI) removal, and the process of Cr(VI) reduction by ZVI coincided the pseudo-first order reaction dynamic model. When initial p H is 3.0, the concentration of Cr(VI) is 5mg/L, ZVI dosage is 5g/L, the most removal rate of 98% was achieved for 25 min. The ion strength in the range of 2-50mmol/L had little influence on the Cr(VI) reduction. The removal rate of Cr(VI) was decreased in the presence of magnesium and aluminum ions, while the co-existing nitrate, sulfate and phosphate improved the removal rate of Cr(VI).To improve the reduction rate of Cr(VI) by ZVI at the neutral condition, phosphate, citrate, nitrilotriacetic acid(NTA) and ethylene diamine tetraacetic aicd(EDTA) are added into the system. The influence of phosphate, citrate, NTA and EDTA on the Cr(VI) by ZVI at initial p H 7.0 was investigated. The addition of phosphate, citrate, NTA and EDTA improved the reduced rate of Cr(VI) at initial p H 7.0. The removal rate of 27.4% without ligands was increased to 77.7%, 93.4%, 98.4% and 97.1% with ligands, respectively. There is no removal rate of Cr(VI) were obtained by these four ligands alone for 24 h. The illumination and Fe(III) only affected the performance of Cr(VI) reduction by citrate, which showed that the reductants were ZVI and Fe(II) generated in the presence of phosphate, NTA and EDTA.The influence of illumination on the absorbance value of Fe(II)-phenanthroline complex in the presence of Fe(III) and citrate at 510 nm was investigated. The results show that the yield of Fe(II) was not controlled by citrate concentration but Fe(III) concentration, while citrate play an important role in electron transmission in the photocatalytic reaction. The influence of the above ligands on the Fe(II) concentration change without Cr(VI) in the Fe0/H2 O system was investigated, at the same time the existing form of Fe(II) in the Fe0/H2 O system was simulated using MINTEQ soft. In the presence of these four ligands, the main existing form of Fe(II) in the system were the Fe(II)-complexes, which showed that the reasons of the Cr(VI) removal rate improved by ligands were the formation of Fe(II)-complexes and the increase of soluble Fe(II) concentration.The addition of ligands can improve the performance of Cr(VI) by ZVI at the neutral condition. However, the concentrations of total iron and total chromium are very high in the effluent. Hence, a physical method is used to improve the performance of Cr(VI) by ZVI. The removal rate and the removal performance of Cr(VI) by ZVI are related to magnetic field. The influence of magnetic field on the Cr(VI) removal performance by ZVI at the different initial p H and initial Cr(VI) concentration was investigated. The results show that there was little influence of magnetic on the Cr(VI) removal at initial p H 3.0 and 4.0, while with the initial p H increased to 8.0 the influence of magnetic on the reducibility of ZVI for Cr(VI) was increased. The effect of magnetic field type and strength on the performance of Cr(VI) by ZVI are great. Without magnetic field the removal rate of Cr(VI) was below 20% at the initial p H 7.0, with the initial Cr(VI) concentration increased from 2.5mg/L to 10mg/L, the removal rate of Cr(VI) decreased from 93.3% to 46.4% with magnetic field at the initial p H 7.0, while which was higher than that without magnetic field at the same initial Cr(VI) concentration. In addition, the influence of magnetic field on the removal of Cr(VI) by aged ZVI was also studied. The results showed that comparison with the reacted ZVI in presence of magnetic the reacted ZVI lost the reducibility quickly without magnetic field under the neutral condition, the reducibility of the aged ZVI was recovered partly after the magnetic field was impressed, while which was inferior to that of aged ZVI with magnetic field all the time. The reducibility of ZVI was affected by the corrosion rate and passivation, the impressed magnetic field relieved the passivation, promoted the ZVI corrosion rate and improved the reducibility of ZVI for Cr(VI) reduction.To control the release of Cr in the settling sludge again, the solidification by alkali-activated slag is used to treat the settling sludge. The results show that the solidified sludge with the sludge content of 20-45% can be disposed in municipal solid waste landfills and general industrial solid waste landfills, the cumulative fraction of Cr leached from the solidified sludge for 5d are lower than 0.002%, the diffusion coefficients ranged from 10-15 to 10-14cm2/s, which show that Cr in the settling sludge immobilized by alkali-activated slag was very stable, the control mechanism of Cr release from the settling sludge was wash off. They all had LX values of released Cr higher than 9.The reduction/solidification of chromium ore process residue(COPR) by ZVI and alkali-activated slag was investigated. In the reduction process, comparison with the direct mixing way, the indirect mixing way was favour of the reduction of Cr(VI) in COPR. In addition, the higher liquid/solid ratio was not suggested to be adopted. The toxicity leaching results showed the treated COPR by ZVI alone cannot meet the relevant standard. The lower ZVI dosage, the higher acid dosage and COPR content were favour of the compressive strength of solidified COPR. The Cr in the reduced/solidified COPR was stable, which can be used in “controlled utilization”, and the control mechanism of Cr release from the reduced/solidified COPR was diffusion and wash off.In this paper the method about the combination of chemical reduction and solidification is developed to solve the chromium-bearing wastewater and residue once, which supplied the basic data for the remediation of chromium pollution. |
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