Elsevier

Water Research

Volume 44, Issue 6, March 2010, Pages 1927-1933
Water Research

Water purification from metal ions using carbon nanoparticle-conjugated polymer nanocomposites

https://doi.org/10.1016/j.watres.2009.11.041Get rights and content

Abstract

The paper deals with a novel method of obtaining nanocarbon-conjugated polymer nanocomposites (NCPC) using nanocarbon colloids (NCC) and polyethylenimine (PEI) for water purification from metal ions. Size of NCC, process of NCPC synthesis, its chemical characteristics, ratio of NCC and PEI in NCPC, speed of coagulation of NCPC, mechanism of interaction of metal ions with NCPC, ability of removing metal ions from water by NCPC against pH have been studied. NCPC has a bonding capacity of 4.0–5.7 mmol/g at pH 6 for most of the divalent metal ions. Percent of sorption of Zn2+, Cd2+, Cu2+, Hg2+, Ni2+, Cr6+ ions is higher than 99%. Lifetime of NCPC before coagulation in the treated water is 1 s–1000 min and depends on the ratio of polymeric molecules and carbon nanoparticle concentrations. Results of laboratory tests of the method are described.

Introduction

An extensive use of metals and chemicals in process industry has resulted in generation of a large amount of effluent containing high levels of toxic heavy metals. Furthermore mining and mineral processing procedures are known to generate toxic liquid waste. The presence of different organic and heavy metal contaminants in waste water has a large environmental, public health and economic impact. Most of the traditional methods such as solvent extraction, activated carbon adsorption, ion exchange, biological degradation and precipitation techniques are rather efficient, but often are costly and/or time-consuming. Rumeau et al. (1992) were the first to introduce the method of separation by coupling ultrafiltration and complexation of metallic species with industrial water-soluble polymers. This approach implies that polymeric molecules operate as a sorbent and a carrier of metal ions. In the method metal ions are primarily bound to water-soluble polymers. The unbound ions pass through the membrane, whereas the polymers and their complexes are retained. Within this approach various weakly basic, water-soluble polymers including chitosan, polyethylenimine (PEI), poly(diallyldimethylammonium chloride), sodium polystyrene sulfonate (PSS) etc. have been used for removal of metal ions from water (Juang and Chiou, 2000; Molinari et al., 2004, Vieira et al., 2001, Steenkamp et al., 2002, Abderrahim et al., 2006). The main advantages of the method lie in relatively low energy requirements of the ultrafiltration process and high removal efficiency due to effective binding of metal ions with polymers (Juang and Shiau, 2000). On the other hand, there is a difficulty of removal of the carrier of metal ions from concentrated solution and the separation of polymeric molecules and metal ions.

Application of carbon nanoparticles and nanotubes for removal of metal ions (Cd2+, Cu2+, Ni2+, Pb2+, Zn2+, etc.) from water have been described recently (Rao et al., 2007, Hudson et al., 1997). Within this approach nanoparticles or nanotubes operate as a sorbent and a carrier of metal ions. Electrolytically generated nanocarbon colloids (NCC) have functional groups such as carbonyl, hydroxyl and carboxyl groups forming on the surface of carbon nanoparticles (Peckett et al., 2000, Khaydarov et al., 2009, Hsu et al., 1996, Kim et al., 2008). The sorption capacities of these particles are high, up to 7 mmol/g (Khaydarov et al., 2009), and is purely comparable with that of cation-exchange resins. On the contrary, the sorption capacities of raw nanotubes for metal ions are very low, because walls of carbon nanotubes are not reactive. But their fullerene-like tips are known to be more reactive, and the sorption capacity significantly increases after oxidation by HNO3, NaOCl and KMnO4 solutions due to the generation of –COOH, –OH, or –CO groups (Li et al., 2002, Li et al., 2003, Li et al., 2006, Li et al., 2005, Lu and Liu, 2006, Lu and Chiu, 2006, Lu et al., 2006, Liang et al., 2004, Lu et al., 2007, Chen and Wang, 2006). The sorption capacities of carbon nanotubes are not greater than 1 mmol/g (Rao et al., 2007) that is 2–5 times less than those of cation-exchange resin. The main disadvantage of this method is a complexity of separation of metal ion carriers (i.e. nanoparticles with metal ions) from water.

In order to improve the separation of carriers of metal ions from treated water, the metal ions can be bound to polymeric molecules and carbon nanoparticles forming nanocarbon-conjugated polymer nanocomposites (NCPC) in water that is able to precipitate rapidly. The idea of the technique described in the paper is to add water-soluble polymeric molecules and carbon colloids to water in such a way as to bind metal ions and simultaneously form NCPC. That leads to significant increase of the size of NCPC species with follow-up formation of precipitates. This sediment can be easily removed from water by filtration or centrifugation with follow-up procedure of extraction of the metals.

Section snippets

Materials and equipment

Copper sulphate pentahydrate (CuSO4·5H2O), cobalt nitrate hexahydrate (Co(NO3)2·6H2O), nickel nitrate hexahydrate (Ni(NO3)2·6H2O), cadmium nitrate tetrahydrate (Cd(NO3)2·4H2O), zinc nitrate hexahydrate (Zn(NO3)2·6H2O), potassium chromate (K2CrO4), mercury chloride (HgCl2), poly(ethylenimine) (molar weight of 10,000 and 200,000–350,000) from Sigma Aldrich, UK were used in the as-received condition.

NCC was prepared by the electrochemical method that we described earlier (Khaydarov et al., 2009).

Results and discussions

The process of metal ion removal from water comprises the following stages: i) capture of metal ions by PEI and NCC and simultaneous formation of NCPC, ii) sedimentation of NCPC, iii) removal of sediments containing NCPC with metal ions by filtration.

Conclusion

The method of obtaining nanocarbon-conjugated polymer nanocomposites using nanocarbon colloids and polyethylenimine has been studied and a simple, rapid and reliable method of water purification from metal ions was developed.

The process of metal ion removal from water solutions involves the following stages: i) sorption of metal ions by PEI and NCC with simultaneous formation of NCPC, ii) coagulation of NCPC containing metal ions and iii) removing the coagulated NCPC by filtration or

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