Introduction and Literature Review
1.6 State of the art
1.6.3 Application of composite membranes
1.6.3.1 Separation of chromium from aqueous solution
Chromium is a toxic chemical species involved in severe environmental pollution problem. It is mostly available in hexavalent (Cr (VI)) and trivalent states (Cr (III)), where Cr (VI) is more toxic in comparison to Cr (III) (Muthukrishnan and Guha 2008). Cr (VI) may exist in various forms such as HCrO4-
, CrO42-
, and Cr2O72-
and stability of all chromate ions depends on pH of the system (Gzara and Dhabhi 2001; Cassano et al., 1996; Fabiani et al., 1996). The main human toxicities are lung cancer, kidney damage, liver and gastric problem (Kimbrough et al., 1999). Besides, chromium affects plants and animals. According to World Health Organization (WHO), the discharge limit of Cr (VI) into water bodies is 0.05 mg/L (Kozlowski and Walkowiak 2002). The Ministry of Environment and Forest, Government of India, has fixed the minimal national standards (MINAS) as 0.1 mg/L containing Cr (VI) for the safe disposal of effluent in surface water.
Chromium species are particularly present in the wastewater from various manufacturing units including, metal plating, paintings and pigments, galvanization, printing inks, textile and dye industries and leather tanning (Religa et al., 2011; Lakshmipathiraj et al., 2008;
Sacmaci et al., 2012; Nataraj et al., 2007). Therefore, it is essential to recover chromium from waste disposal for the environmental eco-friendly system. Different technologies have been established for the removal of Cr (VI) such as adsorption (Garg et al., 2004;
Namasivayam and Yamuna 1995; Sarma et al., 2005; Salgado-Gomez et al., 2014), precipitation (Golder et al., 2011), photocatalytic (Das et al., 2006; Mohapatra et al., 2005), ion-exchange (Galan et al., 2005), extraction using solvent (Alonso et al., 1994), bio- reduction (Han et al., 2007; Sahinkaya et al., 2012) and membrane based separation process (Neelakandan et al., 2003; Sanchez et al., 2011). The separation of inorganic pollutants with membrane processes has several advantages such as, no requirement or addition of chemicals, simple alteration of modular membrane area as per the effluent load, lower energy
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requirement, maximum recovery and higher yield of purified water. According to the membrane separation techniques cited in the literature, microfiltration (MF) (Sondhi et al., 2000), ultrafiltration (Kishore et al., 2003; Shukla and Kumar 2004; Pugazhenthi et al., 2005), nanofiltration (Hafiane et al., 2000), reverse osmosis (Hafez and El-Manharawy 2004) and electrodialysis (ED) (Frenzel et al., 2005) have been frequently used for the removal of chromium from aqueous solution.
Neelakandan et al., (2003) studied the removal of Cr (VI) using modified and unmodified poly (methyl methacrylate-ethylene glycol dimethacrylate) copolymer ultrafiltration membrane. The rejection of unmodified and modified membrane was found to be 68 %, and 67 %, respectively. The hydrophilicity and the solute flux of the membrane were increased after modification with NOx. In another work, the unmodified carbon membrane displayed a maximum Cr (VI) rejection of 96% whereas nitrated and aminated carbon membrane demonstrated around 84%, and, 88%, Cr (VI) rejection, respectively (Pugazhenthi et al., 2005). Shukla and Kumar (2007) investigated the removal of Cr (VI) using modified zeolite composite ultrafiltration membranes. The membranes were modified using NOx (Z2) and further by hydrazine hydride (Z3). The results showed that the observed rejection of the modified membranes increased from ~20 % (unmodified) to 40 % (Z2) and ~50 % (Z3). Also, the observed rejection displayed anomalous trend with variations in pressure while the intrinsic rejection increased with increasing pressure, which is typical for the separation of electrolyte by charge membrane. Arthanareeswaran et al., (2007) investigated the removal of Cr (VI) with ultrafiltration process using cellulose acetate and sulfonated poly (ether ether ketone) ultrafiltration membrane from aqueous solution. The role of various factors such as concentration of solute, pH, concentration of PVA, applied pressure and composition of blending membranes on the rejection and permeate flux was examined. The authors reported
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(2008) prepared styrene acrylonitrile composite membrane on a ceramic support by coating a prepolymer solution obtained using a dual initiator system. The prepared membrane was chemically modified by gas phase nitration followed by amination reaction to make charged.
The modified composite membrane is employed for the separation of chromic acid at different pressures, feed concentrations and pH. The mean pore size of the unmodified, modified with NOx and aminated membrane were 6.26, 8.32 and 11.2 nm, respectively. The rejection was found to be 90% with modified membranes for the concentration of 1000 ppm at pH 3. Ren et al., (2010) fabricated asymmetric poly (m-phenylene isophthalamide) nanofiltration membrane by phase inversion method for the removal of Cr (VI) from wastewater. The highest retention of 98% was achieved at pH 8 among the studied parameters (applied pressure, feed concentration and pH). Sanchez and Rivas (2011) fabricated cationic hydrophilic polymers coupled to the ultrafiltration membrane for the retention of Cr (VI). They reported that the highest rejection capacity of all the membranes was obtained at operating pH of 9. Hafiane et al., (2000) employed the removal of hexavalent chromium by nanofiltration and the process was studied as function of pH as well as ionic force. The authors reported the rejection of 80% at a feed concentration of 1 mM and nanofiltration is a promising candidate with the charged for the removal of hexavalent chromium. Aroua et al., (2007) investigated the removal of Cr (III) and Cr (VI) from aqueous solution by polymer-enhanced ultrafiltration using polysulfone membranes. The complete removal of chromium was achieved for the feed concentration of 10 ppm. Vasanth et al., (2012) prepared ceramic microfiltration membrane by uni-axial dry compaction method from low cost clays and tested for the separation of Cr (VI) using baker’s yeast biomass. They reported that the rejection increased with increasing concentration of biomass and the highest rejection was obtained at pH = 1. The Table 1.7 represents the summery of literatures on various membranes for the removal of Cr (VI).
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Table 1.7 Performance characteristics of various membranes for the removal of Cr (VI) from aqueous solution
Membrane Method of filtration
Concentration (ppm)
Rejection
(%) Reference
PMMA-EDGM Dead-end
UF 1000 68 Neelakandan et al.,
(2003) Cellulose acetate Dead-end
UF 200 93 Arthanareeswaran et
al., (2007)
Carbon Dead-end
UF 1000 96 Pugazhenthi et al.,
(2005) Ceramic membrane Cross flow
filtration 1000 99 Sondhi et al., (2000)
Zeolite-clay Dead-end
filtration 1000 66 Shukla and Kumar
(2007)
Styrene acrylonitrile Dead-end
UF 1000 90 Sachdeva and Kumar
(2008) Composite
polyamide NF 1000 99 Muthukrishna and
Guha (2008) Poly (m-phenylene
isophthalamide) NF 5 98 Ren et al., (2010)