(137b) Removal and Recovery of Heavy Metals from Industrial Waste Streams by Means of a Hybrid-Precipitation and Polymer Enhanced Ultrafiltration

REMOVAL AND RECOVERY OF HEAVY METALS FROM INDUSTRIAL WASTE STREAMS BY MEANS OF A HYBRID-PRECIPITATION AND POLYMER ENHANCED ULTRAFILTRATION

Sezin Ýslamoglu ,Levent Yýlmaz Department of Chemical Engineering, Middle East Technical University

One of the main causes of industrial pollution is the discharge of effluents containing heavy metals. Heavy metals, especially cadmium can have serious effects on human and animal health. Beside the health effects, heavy metals are non-renewable resources. Therefore, effective recovery of heavy metals is as important as removal of them from waste streams.

Wastewater produced during electroplating is one of the main causes of contamination of the natural environment with metal ions. The negative impact of waste streams containing heavy metals upon the environment, gives rise to increasingly strict regulations. According to recent regulations of EPA, for common metals facilities discharging 38,000 liters or more process wastewaters (resulting from the process in which a ferrous or nonferrous basis material is electroplated with copper, nickel, chromium, zinc, tin, lead, cadmium, iron, aluminum, or any combination thereof ) per day should not exceed 4.5 ppm for Cu, 4.1 ppm for Ni, 7.0ppm for Cr, 4.2ppm for Zn, 0.6ppm for Pb and 1.2 ppm for Cd.

Amoung the industrial waste effluents containing heavy metals, treatment of electroplating wastes is a serious worldwide problem, because of their high content of economically valuable heavy metals.

For the fractional separation of heavy metals from electroplating waste streams, the coupling of two or more techniques may result in better performance than using either unit operation individually. By combining the membrane techniques with other physical and chemical processes the effectiveness of the operation can be improved. Precipitation of sparingly soluble metal compounds and micro-or ultrafiltration is the suitable and economical hybrid operation for the removal and recovery of heavy metals from waste streams.

Precipitation of heavy metals with lime, sulfide and caustic soda are the most commonly used economical methods for metal removal. Besides the cost effective properties, its appropriateness as a first step of a hybrid separation processes makes the precipitation widely used method.However, heavy metals can not be removed efficiently by hydroxide precipitation. Higher degrees of removal can be achieved by sulfide precipitation, but in this case high volumes of toxic sludges which are difficult to dispose are produced [1]. More sustainable, effective and economic method could be rectified for heavy metal removal and recovery if a process scheme which can achieve selective separation of metals and produce reusable pure metal sludges could be developed.

In this study, for selective removal and recovery of the metals like Cd, Cu, Fe in cadmium electroplating bath (containing high amounts of Cd, Zn, Cu, Fe and small amounts of Ni, Co, Mn), hybrid precipitation- polymer enhanced ultrafiltration based separation scheme was developed.

The precipitation scheme comprised three consecutive steps: 1)Acid treatment with nitric acid (HNO3). 2) Alkali precipitation by sodium hydroxide (NaOH). 3) Sulfide Precipitation by sodium sulfide (Na2S). Dosage of precipitating agent addition, pH sequencing are the parameters of the precipitation experiments.

Results of the precipitation experiments may be summarized as follows: Cyano-metal complexes were decomposed. Whole Fe content and almost half of the Ni were removed in acidification step.Then by adding sodium hydroxide heavy metals were precipitated and pure Cd(OH)2, was obtained which is a valuable product used in electroplating industry. In sulfide precipitation, pH is an important parameter, such as: Addition of sodium sulfide in alkali pH range leaded to cadmium precipitation whereas copper is totally precipitated in acidic pH range.

Depending on the quality and quantity of the desired solution obtained at the end of the precipitation experiments, different acidification, alkalination and sulfide precipitation paths were followed. Cd-rich samples were obtained by making sulfide precipitation in acidic pH range where as Cu-rich samples were obtained by making sulfide precipitation in basic pH range.

After precipitation experiments, PEUF was applied. Polymer enhanced ultrafiltration (PEUF) is one of the complexation enhanced ultrafiltration processes based on the complexation between a water-soluble polymeric binding agent and the target component, which is desired to be separated from the aqueous solution by an ultrafiltration process [2]. Binding capacities of PEI with Cd, Cu, Zn and Ni strongly depend on pH. As a consequence, pH plays a significant role in the retention of heavy metals [3].

For Cu-free, Cd-rich solutions, at pH 3 and pH 4, retention of zinc and nickel were higher than the retention of cadmium. As the pH value increases, retention of Cd increases which means Cd could be removed from solution more effectively as compared to zinc and nickel. Relatively low retention in low pH values may be due to more protonation of nitrogen donor groups on the polymer at low pH.PEI exists in two forms in aqueous solutions, neutral and protonated. In neutral form free imine sites of PEI are available to form complexes with metal ions whereas in the protonated form, protons bind on PEI. Decreasing the pH enhances the binding of H+ ions to form a complex with PEI [4].

Therefore, it can be concluded that at low pH values protons compete with Cd ions and inhibit them to bind to PEI. Zn and Ni retentions were higher than Cd retention at low pH values. Therefore it can be suggested that, at low pH values Zn and Ni ions are capable of replacing with H+ ions to form a complex with PEI.

Two experiments were performed at pH 3 and pH 6.5 with 0.02 loading by Cu rich samples. Retentions of Cd, Zn, Ni and Cu increase when pH was increased from pH 3 to pH 6.5. Retention of Cd changed drastically as compared to retention of other metals, this result is consistent with the results of the previous studies in which it was shown that binding capacity of PEI and Cd is maximum at pH 6.5 [5]

It can be concluded that, by adjusting the precipitation parameters (pH, time, precipitation agents), together with the parameters of PEUF experiments (pH and loading (metal/polymer ratio)) selective separation of heavy metals from industrial wastes can be achieved. Process sequencing can be suggested for the removal and recovery of heavy metals, especially for Cd and Cu, from electroplating industry's waste effluents.

REFERENCES: 1.Peters, R.W.; Ku, Y. AIChE Symposium Series: Separation of heavy metals and other trace contaminants.1985, p.9. 2.Mulder M., Basic Principles of Membrane Technology, Kluwer Academic Publishers, 1st ed.,1991, Netherlands. 3.Mundkur S.D., Watters J.C., Polyelectrolyte Enhanced Ultrafiltration of Copper from a Waste Stream, Separation Science and Technology, 28,pp.1157,1993. 4.Geckeler K.E., Volchek K., ?Removal of Hazardous Substances from Water Using Ultrafiltration in Conjuction with Soluble Polymers?, Critical Review, Environmental Science and Technology, 30(3), 1996. 5.Müslehiddinoglu J.,Uludag Y., Önder H.Ö., Yýlmaz L.,?Effect of Operating Parameters on Selective Separation of Heavy Metals from Binary Mixtures via Polymer Enhanced Ultrafiltration?, Journal of Membrane Science, 140,p.251-266., 1998.