CHROMIUM (III) ADSORPTION FROM AQUEOUS SOLUTIONS BY CHITOSAN Loris Pietrelli1, Iolanda Francolini2 and Antonella Piozzi2 1. ENEA, CR Casaccia, Via anguillarese,301 00123 Rome Italy 2. Department of Chemistry, Sapienza University of Rome, P.le A. Moro 5, 00185 Rome Italy
[email protected] Introduc>on The removal of heavy metal ions from aqueous soluAons, either for polluAon control or for raw material recovery, have been taking on increasing importance in recent years. A number of separaAon techniques (ion exchange, selecAve precipitaAon, nanofiltraAon, adsorpAon, etc.) and processes have been developed to remove heavy metals. AdsorpAon-‐based technologies have proven to be more viable alternaAves proposed for the treatment of industrial wastewater containing heavy metals as a result of the low cost of processing and instrumentaAon, simple operaAon, and the availability of different types of low-‐cost and environmentally friendly adsorbents (Fu & Wang, 2011). A wide range of materials including acAvated carbon (El-‐Shafey et al, 2002), metal oxides, carbon nanotubes, polymers, agricultural residues ( Garcia Reyes and Mendez, 2010), and natural and modified clays (Chen et al 2011) have been successfully used to adsorb heavy metals from water soluAons. A very promising and cheap material is chitosan (poly-‐β -‐(1→ 4)-‐2-‐amino-‐2-‐deoxy-‐ D -‐glucose), a nitrogenous polysaccharide prepared from chiAn by parAally deacetylaAng its acetoamine groups using strong alkaline soluAons at about 70°C. Chitosan has high potenAal in the metal ions adsorpAon since it has both amine and hydroxyl groups that can serve as chelaAng sites for metal ions. One of the most interesAng advantages of chitosan is its versaAlity: the material can be readily modified preparing different polymer form such as beads (Chiou & Li, 2003), membrane (Pietrelli & Xingrong, 2004), sponge (Ko et al., 2010) for many applicaAons. CriAcal reviews of published data about chitosan applicaAons appeared in 2000 (MajeA and Kumar), 2003 (Babel & Kurniawan) and in 2004 (Guibal). PolluAon by chromium is of considerable concern as the metal has found widespread use in electroplaAng, leather tanning, nuclear power plant, texAle industries (Rengaraj et al 2001). Chromium is found in either III and VI oxidaAon states, as all other oxidaAon states are not stable in aerated aqueous media (Fendorf 1995). The trivalent state is the most stable form under reduced condiAons and is present as a caAonic species with the first or second hydrolysis products dominaAng at pH values from 4 to 8. The low solubility of Cr(OH)3 (log k=-‐16.19) greatly limits aqueous Cr3+ concentraAons at pH values greater than approximately 5. Considering that the equilibrium analysis is the most important fundamental study required for evaluaAng the affinity of a sorbent, therefore in this study the ability of chitosan to remove chromium III by adsorpAon was studied to assess its suitability for applicaAon in the field of texAle wastewater treatment.
9.43
0.9747
Intrap. Diff. model ki R2 (g/mg min) 0.5928 0.9002
Effect of pH Figure 2 shows the effect of pH on adsorpAon of Cr3+ onto chitosan flakes. It can be noted that pH of the soluAon strongly affects the metal ions uptake, the adsorpAon increased with increasing pH of the soluAon. In acidic condiAons probably more protons will be available to protonate amine groups (R-‐NH3+) to form a sort of polycaAon and then reducing the number of binding sites for adsorpAon of Cr3+. On the contrary, since the free electron doublet of nitrogen is responsible for the caAon adsorpAon, high pH will favour the adsorpAon of caAons. Considering that kps= [Cr3+]x[OH]3=6.7x10-‐31 the chromium hydroxide, Cr(OH)3, begins to precipitate at pH≅6.5 therefore for pH higher than 3.8 a significant reducAon of Cr3+ fracAon in favour of Cr(OH)2+ and Cr(OH)2+ hydrolysed complex occurs (Fendorf 1995) as shown in the Figure 3. The result is an increase of adsorpAon of chromium due mainly to the hydrolysed forms. Therefore, the metal ions adsorpAon is due mainly to electrostaAc interacAons between two counter ions. AdsorpJon isotherms To determine the maximum adsorpAon capacity of Cr3+ onto chitosan, a study of adsorpAon isotherm was achieved comparing the commonest models, therefore data were analyzed using the Langmuir and the Freundlich equaAons: qe=Q°bCeq/( l+bCeq) or, linearized 1/qe = (1/Q°kL) (1/Ceq) + 1/Q° (1) qe=kFCeq 1 /n or, linearized log qe = log kF + 1/n log Ceq (2) where "qe" (mg/g) is the amount of element on the solid phase at equilibrium, Ceq (mg/L) is the equilibrium concentraAon of the element in the aqueous phase. According to Langmuir expression, Q° (mg/g) and kL correspond, respecAvely, to complete coverage available sites or limiAng adsorpAon capacity when the surface is fully covered with dye molecules and to an empirical coefficient related to the affinity of the binding sites. While according to the Freundlich equaAon, kF and l/n are empirical constants determined, respecAvely, by intercept and slope of the Freundlich equaAon on a logarithmic plot and represenAng the sorpAon capacity and adsorpAon intensity respecAvely. The essenAal characterisAcs of the Langmuir equaAon can be expressed in terms of a dimensionless separaAon factor RL which is defined by McKay (1982) as: RL=1/[1+(kLCo)] where Co is the highest iniAal Cr3+ ion concentraAon (mg/ml). RL indicates the shape of the isotherm and the adsorpAon is unfavourable if RL>1, favourable if 0
