| CN-containing heavy metal wastewaters contain both of cyanide and heavy metals,which leads toxicity to environment and human health. Therefore, research on thedevelopment of new and efficient methods for CN-containing heavy metalwastewaters treatment technology is of great significance to achieve cyanide andheavy metals zero emissions. Based on our previous work, we suggested a novelstrategy-Ni2+-assisted layered double hydroxide (LDH) and NiZn bimetal cyanidecomplexes precipitation to simultaneously remove aqueous CN and Zn/Ni metals. Theremoval ratio impact factors of cyanide and heavy metals were dissussed and thenchose the best conditions in the practical wastewater purification. Moreover, theremoval pathways were also studied. Results in this paper are as follows:Since the chemical forms of cyanide in CN-containing heavy metal are free CN-and metal cyanide complex anions, the treatment of wastewaters is quite difficultbecause the strong CN-metal binding must be first broken before chemicalprecipitation. We used Ni solution added into Zn/CN solution first before LDHprecipitation. The removal ratio of total CN and heavy metals were95%and80%,much higher than that without Ni addition. The results of XRD and FTIR also showedthat the fomation of Ni(CN)42-and intercalated into ZnAl-LDH to formZnAl-Ni(CN)4-LDH.The influence factors (such as CN/Al, Ni/CN molar ratio, reaction time, layeredmetals and final pH) were studied and the optimum conditions were obtained. Themain results were as follows: the optimum CN/Al molar ratio was2:1and Ni/CNmolar ratio was1:4for cyanide removal; reaction time had obvious influence on theremoval of cyanide and heavy metals, the optimum reaction time was10min; layeredmetals had great influence on the structure of the co-precipitation products and theremoval of cyanide and heavy metals, and the optimum layered metals was ZnAl;final pH had obvious influence on the removal of cyanide and heavy metals, theoptimum final pH was8. Second treatment can also improved the removal ratio tomeet the discharge standards. In this section, the removal mechanism of cyanide in the co-treatment of Zn/CNand Ni solution was studied. FTIR for liquid-phase samples gave a powerful proof forthe conversion of Ni(CN)42-from Zn(CN)42-in Zn/CN solution. Determination of thecyanide and metals concentration, further proved above process.In this section, the optimum reaction conditions were applied into practicalwastewater treatment. Through Ni rich wastewater was added to Zn/CN wastewaterfirst, addition of MgAl salts forming LDH, the removal of total CN and Ni were88%and85%. LDH was formed with different initial CN concentration. FTIR showedobvious stretching vibration of free CN-and Ni(CN)42-, which indicated that theremoval of CN is partial through intercalated into LDH and partial adsorbed on thesurface of LDH. On the other hand, increased the dosage of metal salts and secondtreatment can also help improve the removal ratio with above90%.Since the addition of Al salts increased the cost and treatment steps, we designed anew strategy that only Ni addition to co-treatment of cyanide and heavy metals.Different Ni/CN molar ratio was applied. The main results were as follows: the totalCN decreased from108mg/L to0.3-0.4mg/L, the removal ratio of Ni and Zn were80%in stimulated Zn/CN solution. The results of XRD and FTIR showed that thereaction product was NiZnCN, thus indicating that the removal mechanism wasforming a kind of bimetal cyanide complexes through co-precipitation.In this section, the way to remove cyanide and metals was applied in the realwastewaters. Differently from the stimulated cases, the removal of free CN-was90%,complex CN was only35%-50%, zinc was almost removed completely and Ni was85%. The results of XRD and FTIR showed that the products were mainly metalhydroxides, the removal mechanism was mainly by adsorbed on the surface ofhydroxides through co-precipitation. Especially, the XRD of products showed thesame peak as that in stimulated cases at Ni/Zn=6:1, which indicated that bimetalcyanide complexes was also partial formed with the increasing of Ni dosage... |
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