AND USES

The present invention is a bi-functional product comprising proton exchange membrane and its composites for the reduction of chromium(VI) to chromium(III) and recovery of Chromium(III) present in industrial wastewater, and the method for synthesis of the said bi-functional product.

BACKGROUND OF THE INVENTION

Immense efforts have been executed to decontaminate Cr (VI) by means of conventional and classical methods such as chemical reduction, adsorption, solvent extraction, ion-exchange, chemical precipitation, membrane filtration, reverse osmosis, biological treatments. All these traditional methods have their individual benefits as well as detriments such as consumption of concentrated or more reagents, the inefficiency of removal/reduction/recovery, cost effectiveness, energy requirements depending on the source and concentration of the contaminant. Photo-catalytic semiconductor mediated reductions have been widely studied for the same. Off recently, use of nano structured semiconductors for the above process has received an extreme importance because of their tunable and desirable photo-catalytic, optical and electronic properties. These properties can be tailored through very classic techniques like doping of metal ions, forming composites, immobilization on appropriate substrates and also by thermal treatments. The energy of visible and ultraviolet lights is sufficient enough to defeat the band gap energy of such modified semiconductors. Hence, the irradiation of semiconductors involving titanium dioxide, tungsten dioxide and zinc oxide results in the formation of migrating charge carriers like electron-hole pairs (e ^" -h ⁺ ) which are capable of reducing or oxidizing the contaminants. The literature pronounces that maximum reduction process has to be carried at lower pH which results in a Cr-free, but an acidic solution which is again one of the major drawbacks. Hence, in view of this drawback, it is necessary to originate an efficient process where Cr (VI) can be reduced at neutral pH and then filtered off using the same membrane.

Membrane science is now established and is competing with the usual technologies. Membranes hybridized with semiconductors having a synergistic effect of both organic and inorganic materials embedded within and are gaining more importance towards water treatment because of multiple potential applications. Polysulfone is one of the extensively used membrane material for industrial applications because of its mechanical strength and pore size. Regrettably, polysulfone is a hydrophobic material offering lower flux, which limits the commercial use of these membranes. The introduction of charged groups onto polysulfone via sulfonation impressively increases the hydrophilicity of polysulfone. The nonsulfonated hydrophobic chain provides mechanical strength with provisions for incorporation of the hydrophilic segment through sulfonic acid groups ensuring the proton conductivity for modified membranes. Though the properties of polysulfone are sufficiently improved by modifications and composites, use of membranes is still restricted to separation science and fuel cells. The present work uses such modified membranes for photo-reduction of Cr (VI) by forming their composites with photoactive semiconductor nanoparticles. The vast literature survey summarizes the drawbacks of classical methods using Ti0 ₂ and polysulfone membranes for Cr (VI) reduction. The need was hence felt to research and develop an approach for reduction of hazardous Cr (VI) via polymeric composite membranes decorated with hydrophilic nano semiconductors.

FIELD OF THE INVENTION The invention relates to fields of polymer science, membrane technology, water treatment and photo-catalysis. Specifically, this invention relates to chemical modification of polysulfone via electrophilic substitution reaction particularly sulfonation and the preparation of its composite by the addition of nano titania for photo-catalytic reduction of hexavalent chromium, a hazardous contaminant and the invention finally aims at separation of recovered Cr (III). DISCUSSION OF THE PRIOR ART

Indian Patent No.231565 titled "An improved process for removal of chromium from waste water using waste biomass of Rhizopus Arrhizus" discusses an improved process for removal of chromium from wastewater using waste biomass of Rhizopusarrhizus, by treating the chromium-containing solution with fungal waste under constant stirring at 20-40 degree C for 15-240 min followed by removal of the fungal waste from the solution to obtain desired water free from chromium.

US 5380441 A titled "Removal of chromium from solution using mechanically agitated iron particles" discloses metallic iron particles are added to an aqueous solution containing hexavalent chromium and mechanically agitated. The sufficiently available surface of the iron particles which remains precipitate-free substantially reduces all the hexavalent chromium to trivalent chromium. Adjustment of pH allows the formation of insoluble precipitates which may be separated from solution using conventional techniques. The mechanical agitation and adjusting pH are drawbacks of this technique.

US4260491A titled "Chrome removal waste treatment process" disclose the process for the removal of chrome from wastewater on the addition of at least one chelating agent for reduction of hexavalent chromium to trivalent chromium. The chrome-containing wastewater is treated at low pH with both a reducing agent suitable for converting hexavalent chrome to trivalent chrome and with a ferric or aluminium sulfate or chloride salt. Following the reduction step, the pH of the now acidic solution is raised, using an inorganic base, to a pH sufficient to cause the formation of chromic hydroxide. The patented process of this invention is particularly appliedin rapid sedimentation processes, achieved by the use of lamella separators. US 20030136742 Al titled "Wastewater treatment process using 8856 (A&B) to remove metals from wastewater" discloses a process to remove low or high concentrations of common or complexed heavy metals from aqueous solutions as well as oily solutions. The process uses a product (8856A), which is a mixture of 80% phosphoric acid (75-80%) and 20% aluminium sulfate (48%). The percentage of aluminium sulfate in 8856A is very critical in providing optimum solid-liquid separations (in the settling stage) and removing any excess phosphates from the discharge. Heavy metals such as iron, copper, nickel, zinc, chromium and lead are precipitated as metal phosphates. According to the presented invention, metal concentrations in the treated wastewater are decreased to levels below the detection limits.

US 7105087 B2 titled "Hexa-valent chromium removal from aqueous media using ferrous-form zeolite materials" discloses both the system and the method invented for the removal and disposal of chromium from an aqueous medium. It describes the removal of chromium from a source by contact with either natural or synthetic zeolite that has been modified with ferrous ion like substance. The spent zeolite is required to be disposed and replaced each time with freshly modified zeolite and the systems operate under de-oxidizing conditions.

The above-mentioned techniques have their own merits and demerits. Herewith, are disclosed both a novel product and a novel process to reduce chromium (VI).

SUMMARY OF THE INVENTION

The present invention offers a reliable, cost effective, environmental friendly, simple and user-friendly method to reduce hexavalent chromium (VI). Most of the patents disclose such reduction process to occur at low pH/acidic pH, and this method prevents the use of any such acids. The material also is photoactive with good resistance to sunlight and is capable of reducing Cr (VI) without the use of any other reagents or chemicals. In the present embodiment, the photo active material is nano structured anatase Titania (Ti0 ₂ ) producing (e ^~ - h ⁺ ) pair on light irradiation of sufficient energy which can defeat the band gap energy of Ti0 ₂ . The electron-hole pairs initiate the reduction process by scavenging the hole by proton donation from the sulfonic group, this avoids electron-hole pair recombination and allows electrons acceptance by Cr(VI) to form Cr(III). Ti0 ₂ is immobilized on sulfonated polysulfone membrane matrix and this, in addition, can help the repeated use of the product. 100 % reduction can be observed without the usage of any other added materials/compounds.

This invention discloses, A bi-functional product comprising a proton exchange membrane and its composite with the inorganic nano Ti0 ₂ which can reduce the toxic hexavalent Cr and which can recover the expensive trivalent Cr from the water.

An environmental friendly and acid-free reduction process for the reduction of toxic Cr (VI) which is a major contaminant in industrial effluents.

A method of making a proton exchange membrane and its composite with the inorganic nano Ti0 ₂ semiconductor particles being dispersed in the polymer matrix.

Completely reusable product toreduce hexavalent chromium so as to meet the growing demands for decentralized and point of use water treatment plants.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates the sulfonation reaction of polysulfone. Figure2 illustrates H ¹ NMR spectrum of the sPSf polymer.

Figure 3 illustrates Attenuated Total Reflection Infrared (ATR-IR) spectra of PSf and sPSf.

Figure 4 illustrates Thermo Gravimetric Analysis (TGA) curves for PSf and sPSf.

Figure 5 illustrates Differential Scanning Calorimetry (DSC) curves for PSf and sPSf. Figure 6 illustrates X-ray diffraction patterns of Ti0 ₂ nanoparticles, bare sPSf/PSf membrane and sPSf/PSf/Ti0 ₂ composite membranes.

Figure 7 illustrates the surface scanning electron microscope (SEM) images of membranes a) S; b) S I; c) S2; c) S3. Figure 8 illustrates the cross-sectional SEM images of membranes a) S; b) S I; c) S2; c) S3.

Figure 9 illustrates the process of chromium reduction in sunlight.

Figure 10 illustrates the normalized concentration depletion curve for photo- catalytic reduction of Cr (VI) using composite membranes. Figure 11 illustrates the schematic representation of electron-hole charger in composite membranes.

Figure 12 illustrates the percentage removal of Cr as a function of applied pressure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described more specifically with reference to the above embodiment. Itis to be noted that the following descriptions of preferred embodiments of this invention are in attendance for the purpose of illustration and description only; it is not intended tobe exhaustive or to be limited to the precise form disclosed. The following example is illustrative of the best mode now reflected for carrying out the invention.

I. Materials

A number of important chemicals of the present invention will be listed up as follows. Polysulfone (PSf) (30000 Da) will be procured from Sigma- Aldrich chemical company. Titanium tetrachloride, chloro sulfonic acid, N-Methyl Pyrollidone (NMP), ethylene dichloride, potassium dichromate, silver nitrate, ammonia, sulfuric acid, ethanol, and acetone will be purchased from Merck Company, India. II. Preparation of compositemembranes

1. Synthesis of sulfonated polysulfone

100 to 120 niL of ethylenedichloride (EDC) was used as a solvent with 10 to 15 niL of chloro sulfonic acid as sulfonating agent. 0.5 to 1 g of PSf/ 100 to 120 niL of EDC on dissolutionwere added to 1 to 1.5 ml of chlorosulfonic acid taken in 20 ml of EDC. The reaction mixture was stirred for 6 h at 80°C and the product was precipitated out using water and dried in an oven to get a modified polymer named as "sPSf ' or "S".

2. Preparation of Ti0 ₂ composite membranes

300 to 350 mg ( was the optimum and called "S I", 400mg, and 500 mg were also tested and are named as "S2" and "S3") of Ti0 ₂ nanoparticles was stirred in 16 ml of N-Methyl Pyrollidone (NMP) for 4 hours and sonicated for 30 mins for complete dispersion of nanoparticles in the solution. The desired amount of 1:3 ratio of "sPSf ' and "PSf were added to the beaker and stirred for dissolution at 60°C for 24 hours to get a light brown viscous solution. The obtained viscous solution was cast on a glass plate and dipped into a coagulation bath of distilled water to get a membrane with good mechanical strength. The membrane is further used for reduction of Cr (VI).

III. Characterization of Modified Polymer (sPSf)

Figure 2 shows H ¹ NMR spectrum of "sPSf '. Tetramethylsilane (TMS) was used as the reference solvent. The peak for methyl groups of "sPSf ' can be observed at δ 1.627 ppm. The solvent Dimethyl sulfoxide (DMSO) peak was observed at δ 2.501 ppm. Multiplets from δ 7 to 8 ppm were assigned to aromatic protons. According to literature, the H ¹ NMR peaks for aromatic protons of bare "PSf are observed at ~ 6.99, 7.03 and 7.72 ppm. Upon sulfonation, the peaks are observed to shift to 7.044, 7.265 and 7.878 ppm. In particular, the proton resonance peak at δ 7.878 ppm is attributed to the proton adjacent to new -SO ₃ H pendent groups and hence confirms the sulfonation of "PSf. ATR-IR spectra of "PSf ' and "sPSf ' are presented in Figure 3. On analyzing the transmittance curves as a function of wave number, common peaks for both "PSf and "sPSf ' are observed at ~ 1584 cm ^_1 for C=C stretching vibrations, peaks at -1148 and 1238 cm ^"1 for S0 ₂ groups and characteristic vibrations at -1110 and 1350 cm ^_1 for C-0 single bond. Upon sulfonation, the broad peak at - 2900 cm ^"1 was assigned to OH stretching of sulfonic groups and peaks at -1027 and 1098 cm ^"1 were accounted for symmetric and asymmetric stretching of 0=S=0 group of sulfonic acid respectively. A new peak at -1104 cm ^"1 is accounted for para-in-plane aromatic C-H bend vibration which confirms the sulfonation to "PSf.

The thermal properties of "sPSf were studied by TGA and DSC curves. Figure 4 and Figure 5 respectively shows the TGA and DSC curves for bare "PSf and "sPSf . The thermal properties of "sPSf were comparatively complex when co-related to "PSf. The TGA curve of bare "PSf shows decomposition at ~500°C and is attributed to the decomposition of a polymer chain. Whereas, the curve for "sPSf shows totally three weight losses which are also supported by endothermic peaks of DSC curves. Among which, the weight loss around 100°C is attributed to desorption of water bonded to hydrophilic sulfonic groups and the evaporation of residual solvent. According to the reports, the upper limit for the stability of these materials is around 220°C to 250°C. Correspondingly, the next weight loss observed around 300°C refers to the loss of sulfonic groups and the last one around 440°C is due to main chain decomposition. From DSC curves, it is clear that the glass transition temperature (T _g ) for "sPSf has been increased from 190°C ("PSf) to about 230°C. The increased T _g is due to lesser internal rotation of high molecular chain segments caused by intramolecular interactions of pendent sulfonic acid groups and hydrogen bonding in "sPSf . Even though good practical results are observed, a negligible variation in thermal properties, in comparison to other reports, may be associated with the polymer structure and/or nature of binding/presence of newly introduced pendent groups, degree of modification and environmental conditions at which the thermal properties are measured. Morphology Studies

X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) are among the powerful tools to confirm the incorporation of nanoparticles (NPs) into the polymer matrix. Figure 6 demonstrates the XRD patterns of Ti0 ₂ , and composites with different doses of Ti0 ₂ . From the figure, it is clear that the prepared Ti0 ₂ is of anatase phase with 2Θ = 25.3°. But it changes to rutile, once incorporated into the polymer matrix and can be observed as apeak at 20=27.4°. The presence of intentional or unintentional impurities, the particle size of NPs, surface stress, pH, temperature and loading pressure are the main parameters which influence the anatase to rutile transformation. Here, the main reason for phase transformation would be an excess of internal strain caused by intramolecular interactions of pendent sulfonic groups in the polymeric material that exists while preparing the casting solution. As reported by Hu et.al [2], pH will also play a great role in phase transfer process. They confirmed that in lower pH, the growth of grains would be more during diffusion. As "sPSf" is used in membrane preparation which is acidic in nature, phase transformation tends to be more feasible.

The surface and cross- sectional morphology of prepared membranes were investigated by means of SEM surface and cross-sectional images. The surface structure of the membranes is displayed in Figure 7 and their cross-sectional images in Figure8. Figure 7a illustrates the surface image of control membrane and images in Figures7b, 7c, and 7d illustrate the presence of NPs on composite membranes with Figure 8 illustrating their corresponding cross-sectional images. Surface images clearly depict the nature of dispersion of Ti0 ₂ NPs on membrane surface as per the dosage. The cross-sectional images present fine information on the incorporation of NPs embedded in the polymer matrix layers as well as productivity of the membranes.

Figure 8a showed no Ti0 ₂ in the membrane, while all other images had NPs (Figures 8b, 8c, 8d). Also, it was observed that the structure of membrane layers had changed accordingly with Ti0 ₂ content. For membrane, "Sl"can observe fine finger-like projections where as for membranes "S2" and "S3" the structure appears completely distorted. As mentioned earlier, with an increase in Ti0 ₂ content, the viscosity of the casting solution increases, disturbing the penetration of NPs deeper into the polymer matrix during phase inversion process. The fingerlike projection in the sub-layer of the membranes becomes fainted and changed to spongy like projections with the increase in dosage of the nano-fillers. The increased NP content intensifies the formation of cross-linking structure between NPs and polymeric chains which inhibits the growth of finger- like projections. The liquid-liquid demising offers a condition for the formation of macro voids. Since Ti0 ₂ has more affinity towards water than polymeric chains, the penetration velocity of Ti0 ₂ is more towards water.

Photocatalytic reduction of Cr (VI)

Figure 9 illustrates the process using a sunlight focused on the water to be depolluted from Cr(VI) 5, placed in a reactor vessel 4. The composite membrane 2 is attached to the bottom of the reactor vessel 4, which also contains the magnetic pellet 3 for uniform stirring. The whole set up of the reactor vessel 4 is placed on a magnetic stirrer 1 and exposed to Sun 6.

Accordingly, the present invention provides a process of Cr(VI) reduction using a composite membrane which comprises of charges and light active semiconductor particles. The membrane inducts charge into a conduction band of the semiconductor, which enhances charge separation and allows electron transport for reduction of Cr (VI).

The entire photo-catalytic experiments were carried out on sunny days between 12.00 pm to 3.00 pm in Bangalore city.

The reaction vessel of the area around 544 cm containing the invented composite membrane and contaminant solution of Cr (VI) was exposed directly to sunlight. The intensity of sunlight is between 700 to 900 W/m . However, no steps were taken to maintain the intensity of sunlight during subsequent reactions. The residual concentration of Cr (VI) in the reaction vessel at regular intervals of time was estimated from standard calibration curves of absorbance versus concentration at a particular value of _max = 350 nm by a Shimadzu 1800PC UV- Visible Spectrophotometer. The extent of reduction C/Co was calculated using concentration of the initial and treated sample (C = concentration of treated sample; Co = concentration of feed) with respect to time. Recovery of Chromium

All the rejectionexperiments were carried out using self-constructed dead end cell unit. 10 ppm chromium solution was used as feed solution. The membrane with an area 19.6 cm was fixed at the bottom of the separating unit, and the permeate was tested for chromium at varied pressures. The residual concentrations of chromium in all the samples were analyzed by Atomic Absorption Spectrometry. The residual solution was treated with an acid such as nitric acid and precipitating agent like solutions of lime, sodium hydroxide 15% and magnesium oxide 10% to obtain chromium.

Findings

The results of photo-catalytic experiments conducted in triplicates are described in Figure 10. A near complete reduction of Cr (VI) is achieved using NPs embedded inthe polymer matrix. As Jafarzadeh and his co-workers [1] have observed and reported, a membrane casting solution with nanofillers when quenched in a water bath, facilitate nanofillers to act as crystal nuclei leading to crystallized polymer chains around them as shown in the cross-sectional views. Hence, the concentration of NPs plays an important role to modify the structure as well as the performance of the membranes. The initial reduction is credited to the acidic nature of the membranes, were indirect chemical reduction occurs due to acidified polymer. Further, reduction in Cr(VI) requires photocatalytic action. Chemical modification performed in the present study will induce an acidic nature to the polysulfone with the pendent groups to its backbone. When an acidic polymer/Ti0 ₂ system is illuminated with sunlight, both Ti0 ₂ and polymer absorb the photons at their interface, followed by the charge separation at the interface. This is because the conduction band of Ti0 ₂ and the lowest unoccupied molecular orbital level of the polymer are well matched for the charge transfer. Titania has a band gap which matches with the UV light and the UV portion of the sunlight causes electron-hole pair on irradiation. The electrons generated by the polymerinjects into the conduction band of Ti0 ₂ , which enhances the charge separation and consequently promote the photocatalytic activity. Simultaneously, a positively charged hole might beformed by electrons migrating from Ti0 ₂ valence band to the polymer. The schematic representationof the electron-hole charge transfer system of immobilized NPs on the conductive polymer is represented in Figure 11. Removal of Cr (III) from water

The removal experiments of Cr (III), were carried out using the established dead end cell filtration equipment as a batch process. Each experiment is repeated three times to check the reproducibility, and the results are presented as error bar graphs in Figure 12. The rejection was around 80% for composite membranes. In the present work, chemical modification of PSf and the incorporation of Ti0 ₂ NPs into membranes provide a combined positive effect on the separation process. It has been proved by many reports that the separation can be influenced by the charge on the membrane, solute concentration, charge density of inorganic solute, pressure and also by the interaction of charge (on the membrane) and inorganic ion.

The initial rejection of Cr (III) ions by control membrane is credited to the charged group (pendent sulfonic ions) on the membrane surface. But, to achieve higher rejection, the role of Ti0 ₂ comes into an action. At lower pH, the surface charge of Ti0 ₂ acts as a fixed charge on the membrane and since the Cr exists in its anionic form; there exists a strong cationic and anionic interaction between Ti0 ₂ and Cr ions resulting in maximum rejection. Diffusion of metal ions is strongly influenced by its ionic size and the pore size of membranes through which it diffuses. It is presumed that S0 ₃ H groups on the membrane capture Cr(III) ions by electrostatic interaction. Thisfavourable package of groups surrounding the interface of pores may eventually retain Cr(III) ions during the separation process. Charge interaction between-S0 ₃ H and Cr(III) ions may mask the pores, resulting in ahigher removal.

Comparatively, the method of preparing modified polymer and composite membrane and the mechanism of obtaining simultaneous flux and rejection in the present paper is observed to be significant in terms of better time consumption, good selectivity, and productivity.

Main advantages of the present invention are the use of easily available, renewable, eco-friendly and abundant source of energy such as the sunlight for reduction of chromium. Further, the present composite membrane avoids the addition of acids to reduce hexavalent chromium (VI). The method is simple and can be carried out at large capacity even by a common man with no much knowledge of technical know-how, and itdoes not involve corrosive chemicals. The application of the method can be to landfill leachates and industrial effluents mainly electroplating and leather industry effluents. Complete reduction of Chromium (VI) to chromium (III) and recovery of Chromium (III) without any by-products. The use of decentralized and distributed, recyclable composition results in an economical feasibility.

REFERENCES

1. Y. Jafarzadeh, R. Yegani, M. Sedaghat, Preparation, characterization and fouling analysis of ZnO/polyethylene hybrid membranes for collagen separation, chemical Engineering Research and Design 94 (2015) 417-427.

2. Y. Hu, H.L. Tsai, C.L. Huang, Phase transformation of precipitated Ti0 ₂ nanoparticles, Material Science Engineering A 344 (2003) 209-214.