دانلود رایگان مقاله انگلیسی دیدگاه های پیشرفته جدید افزودنی های کلرید در حذف فلزات سنگین بهبود یافته - الزویر 2018

Abstract

Controlling the removal of heavy metals such as Copper (Cu), Zinc (Zn), lead (Pb) and cadmium (Cd) during the sewage sludge incineration and the recovery of phosphorus (P) from sewage sludge ash (SSA) remain challenging. Herein we aim to investigate the effect of the temperature, retention time and chlorinating agent additives (MgCl2 and KCl) on both the behavior of selected heavy metals and the fixed rate of phosphorus (P) during sewage sludge incineration. Dry sewage sludge was mixed with various amounts of chlorinating agent and treated by a laboratory scale furnace in the temperature range of 800- 1000oC for different retention times (30, 60 and 120 minutes). The results demonstrated that the removal efficiency of heavy metals exhibited an increasing tendency with the addition of chloride, especially in the cases of Cu, Zn and Pb. Moreover, the temperature and retention time demonstrated significant effects on the promotion of heavy metals removal. In the case of Pb 96% was removed at 800oC within 120 minutes, while 86.6% of Cd was effectively removed at 1000oC within 30 minutes. MgCl2 proved to be more effective than KCl in improving the removal efficiency of heavy metals, such that up to 81.6% of Cu, 84.9% of Pb and 73.5% of Zn was removed with the addition of 15%wt Cl- /sewage sludge (SS), At 960oC, the boiling point of cadmium chloride CdCl2 and CdO.Al2O3.2SiO2 retention time and chlorinating agent had no impact on Cd removal. It was also observed that high temperature was beneficial for the transformation of non-apatite inorganic phosphorus (NAIP) to apatite phosphorus (AP). At 900oC this transformation is efficient for Phosphate fixation, which is evidence that apatite phosphorus (AP) has the ability to be more stable than non-apatite inorganic phosphorus (NAIP) at high temperature.

4. Conclusions

The influence of chlorine, temperature and retention time on heavy metals removal efficiency from SSA was investigated. In general, their removal found to be chlorine dependent, especially for Cu, Pb and Zn. In this case, the temperature was appropriate to promote the removal efficiency of heavy metals to some extent. During sewage sludge incineration, increasing the retention time improved significantly the removal efficiency of heavy metals in SSA, this was obvious in the case of lead, which showed 96.0% of the removal after 120min retention time at 800oC without adding any chlorinating agent. As shown in Fig.8, comparing the different parameters that were involved in the removal efficiency of heavy metals their removal efficiency was as follows: Chlorinating agent > Temperature >Retention time.

Chlorinating agents exhibits an obvious influence on the heavy metal removal, especially for the Pb, Cu and Zn, which show a total removal of 84.9%, 81.6% and 73.5%, respectively, and only 29% of Cd can be removed from SSA with MgCl2 15%wt Cl- /SS. During the sewage sludge incineration, all the selected heavy metals Cu, Pb and Zn were positively correlated with MgCl2, except for Cd, which exhibits a removal below 30%. The formation of CdO4 2− decreases the Cd removal efficiency after adding MgCl2, in addition to the formation of (MgAl2O4) compound, which minimizes the influences of Cl- on Cd removal. Moreover, sewage sludge incineration temperature exhibits an obvious influence on phosphate fixation and transformation. Indeed, the total phosphorus (TP) content in treated sewage sludge ash increases from 85.12 mg/g SS at 800oC to 89.91 mg/g SS at 900oC, in addition to its positive effect on the transformation of NAIP to AP at 900oC, in this case more than 50% of NAIP is transformed to AP at 900oC and changed remains significant at 1000oC. The transformation from NAIP to AP is efficient for Phosphate fixation, which indicates that the AP is prone to be stable than NAIP at high temperature as shown in Fig.6. The main mineral phases of AP were Ca8Pb2(PO4 )6(OH)2 and Ca18Cu3(PO4)14 which were detected by XRD and SEM.