Description:FIELD OF THE INVENTION
The invention relates to the field of photocatalysis. More specifically, the present invention relates to a composite material comprising molybdenum based quantum dots decorated molybdenum oxysulfide nanosheet and a process for the preparation of the composite material. The invention also relates to a process for the removal of toxic hexavalent chromium from wastewater using the composite material under the treatment of sunlight.

BACKGROUND OF THE INVENTION
Chromium exists in trivalent [Cr (III)] and hexavalent state [Cr (VI)] in the environment. Hexavalent chromium is highly toxic, mutagenic, and carcinogenic while Cr(III) is an essential dietary element to humans and mammals. Hexavalent chromium persist long in environment has serious effects on living organisms. Cr (VI) pollution has become one of the world’s most serious environmental concerns. Hexavalent chromium inters into the water bodies from effluents of various industries such as paints, electroplating, metal finishing, and leather tanning. Hexavalent chromium is carcinogenic and long term exposure causes nausea, ulceration, skin sensitization, kidney damage and lung cancer. Hexavalent chromium should be removed before discharging the chromium containing wastewater into the river. Therefore, considerable attention has been paid for the effective removal of hexavalent chromium at low concentrations from contaminated water. The development of low-cost, highly efficient techniques is required for the removal of hexavalent chromium from wastewater. Photocatalytic reduction of hexavalent chromium under sunlight is cost effective and highly efficient technique. This technique has no sludge formation and no need for large amount of chemical uses. Generally, TiO2, graphitic carbon nitride, transition metal based sulphide, quantum dots, and metal organic frameworks (MOF) are used for photocatalytic reduction of hexavalent chromium. CN110937653A disclose a method of preparation of indium oxide-indium sulfide-titanium dioxide compound for photocatalytic reduction of hexavalent chromium containing wastewater. About 70% of hexavalent chromium reduces in 120 minutes by adding indium oxide-indium sulfide-titanium dioxide compound. US10807072B2 discloses a method for the reduction of hexavalent chromium by using carbon fiber cloth supported SnS2/SnO2 composite catalyst. 120 mg of catalyst is required to reduced 10 ppm of Cr(VI) in 60 min. CN110918099A discloses a method of treating hexavalent chromium-containing wastewater by photocatalytic reduction of nickel oxide-nickel cobaltate-black titanium dioxide compound. The reduction rate is about 70% using 3-10 mg of catalyst for 30 ml of 3-7 mg/L of Cr(VI) solution. Wang et. al shows improved photocatalytic reduction of Cr (VI) by 2-mercaptoethanol modified molybdenum disulfide conjugated with polyvinyl alcohol. The reduction efficiency is about 96 % under sunlight for 105 min [Chemical Engineering Journal 359 (2019) 1205-1214]. However, still there exists a requirement of efficient catalyst for the removal of hexavalent chromium for wastewater.
OBJECTIVES OF THE INVENTION
The main objective of the present invention is to provide a molybdenum based composite material for the reduction and removal of toxic hexavalent chromium from wastewater.

Another objective of the present invention is to provide a process for the preparation of the molybdenum based composite material.

Another objective of the present invention is to provide a molybdenum based composite material as catalyst for the reduction and removal of toxic hexavalent chromium from wastewater in the presence of sunlight.

Another objective of the present invention is to provide a process for the reduction and removal of toxic hexavalent chromium from wastewater using the molybdenum based composite material.

SUMMARY OF THE INVENTION
This summary is provided to introduce a selection of concepts, in a simplified format, that are further described in the detailed description of the invention. This summary is neither intended to identify key or essential inventive concepts of the invention and nor is it intended to determine the scope of the invention.

The present invention provides a molybdenum based composite material for the reduction and removal of toxic hexavalent chromium from wastewater, the molybdenum based composite material comprises:
i. a molybdenum oxysulfide nanosheet; and
ii. molybdenum based quantum dots,
wherein the molybdenum based quantum dots are uniformly distributed over the molybdenum oxysulfide nanosheet; the molybdenum based quantum dots have a particle size in a range of 6 to 10 nm.

The present invention provides a process for the preparation of the molybdenum based composite material as defined above, the process comprises:
i. contacting a molybdenum precursor solution to a sulfur precursor to obtain a solution, wherein the solution has a pH in a range of 1 to 3;
ii. heating the solution to obtain a black product;
iii. washing the black product with a solvent; and
iv. drying the black product to obtain the molybdenum based composite material.

The present invention also provides a process for the reduction and removal of hexavalent chromium from wastewater in the presence of sunlight using the molybdenum based composite material as defined above, the process comprises:
i. adding the molybdenum based composite material in water, followed by sonicating to form a dispersion;
ii. adding the dispersion to a solution comprising hexavalent chromium, followed by stirring the solution in dark; and
iii. stirring the solution in the presence of sunlight.

BRIEF DESCRIPTION OF THE DRAWINGS:
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:

Figure 1 depicts (a), (b) TEM images of molybdenum based quantum dots decorated molybdenum oxysulfide nanosheet (MQD/MOS) composite material, (c) TEM images of pure MOS nanosheet (d) SEM images of MQD/MOS composite material.
Figure 2 depicts (a) Raman spectra of MQD/MOS composite material, (b) XPS survey spectra of MQD/MOS composite material, (c) De-convoluted spectrum of Mo 3d of MQD/MOS composite material.
Figure 3 depicts (a) UV-VIS spectra of MQD/MOS composite material, (b) Tauc’s plot of MQD/MOS composite material.
Figure 4 depicts (a) UV-VIS spectra of Cr (VI) solution in presence of 0.02 mg/mL of catalyst at different time interval in acidic medium (b) UV-VIS spectra of Cr (VI) solution in presence of 0.05 mg/mL of catalyst at different time interval in acidic medium, (c) Effect of catalyst loading on photo reduction of 150 ppm Cr(VI) (d) effect of pH on the photo reduction of 150 ppm Cr(VI) in presence of 0.05 mg/mL of catalyst.
Figure 5 depicts (a) EDX spectra of MQD/MOS composite material, (b) EDX spectra of MQD/MOS composite material after photo reduction of Cr (VI).
Figure 6 depicts efficiency of catalyst for the photo reduction of Cr(VI) after different cycle.

DETAILED DESCRIPTION OF THE INVENTION
For the purpose of promoting an understanding of the principles of the invention, reference will now be made to the embodiments in the specific language to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated process, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skilled in the art to which this invention belongs. The composition, methods, and examples provided herein are illustrative only and not intended to be limiting.

The articles “a”, “an” and “the” are used to refer to one or to more than one (i.e., to at least one) of the grammatical object of the article.

The term “some” as used herein is defined as “none, or one, or more than one, or all”. Accordingly, the terms “none”, “one”, “more than one”, “more than one, but not all” or “all” would all fall under the definition of “some”. The term “some embodiments” may refer to no embodiments or to one embodiment or to several embodiments or to all embodiments. Accordingly, the term “some embodiments” is defined as meaning “no embodiment, or one embodiment, or more than one embodiment, or all embodiments”.

More specifically, any terms used herein such as but not limited to “includes”, “comprises”, “has”, “consists” and grammatical variants thereof is inclusive or open-ended and does not exclude additional, unrecited elements or method steps. The specification will be understood to also include embodiments which have the transitional phrase “consisting of” or “consisting essentially of” in place of the transitional phrase “comprising”. The transitional phrase “consisting of” excludes any element, step, or ingredient not specified in the claim, except for impurities associated therewith. The transitional phrase “consisting essentially of” limits the scope of a claim to the specified materials or steps “and those that do not materially affect the basic and novel characteristic(s)” of the claimed invention.

Whether or not a certain feature or element was limited to being used only once, either way it may still be referred to as “one or more features” or “one or more elements” or “at least one feature” or “at least one element”. Furthermore, the use of the terms “one or more” or “at least one” feature or element do NOT preclude there being none of that feature or element, unless otherwise specified by limiting language such as “there NEEDS to be one or more” or “one or more element is REQUIRED”.

Use of the phrases and/or terms such as but not limited to “a first embodiment”, “a further embodiment”, “an alternate embodiment”, “one embodiment”, “an embodiment”, “multiple embodiments”, “some embodiments”, “other embodiments”, “further embodiment”, “furthermore embodiment”, “additional embodiment” or variants thereof do NOT necessarily refer to the same embodiments. Unless otherwise specified, one or more particular features and/or elements described in connection with one or more embodiments may be found in one embodiment, or may be found in more than one embodiment, or may be found in all embodiments, or may be found in no embodiments. Although one or more features and/or elements may be described herein in the context of only a single embodiment, or alternatively in the context of more than one embodiment, or further alternatively in the context of all embodiments, the features and/or elements may instead be provided separately or in any appropriate combination or not at all. Conversely, any features and/or elements described in the context of separate embodiments may alternatively be realized as existing together in the context of a single embodiment.

The terminology and structure employed herein is for describing, teaching, and illuminating some embodiments and their specific features and elements and does not limit, restrict, or reduce the spirit and scope of the invention.

The present invention discloses a molybdenum based quantum dots decorated molybdenum oxysulfide nanosheet composite material (MQD/MOS) as a catalyst for simultaneous reduction and adsorption of toxic hexavalent chromium from wastewater under sunlight exposure. The catalyst shows improved efficiency for hexavalent chromium reduction.

The present invention provides a molybdenum based composite material for the reduction and removal of toxic hexavalent chromium from wastewater, the molybdenum based composite material comprises:
i. a molybdenum oxysulfide nanosheet; and
ii. molybdenum based quantum dots,
wherein the molybdenum based quantum dots are uniformly distributed over the molybdenum oxysulfide nanosheet; the molybdenum based quantum dots have a particle size in a range of 6 to 10 nm.

The molybdenum oxysulfide nanosheet has a size in a range of 200 to 500 nm, and a thickness in a range of 8 to 15 nm.

The molybdenum based composite material has a bandgap of 2.96 eV.

The present invention provides a process for the preparation of the molybdenum based composite material as defined above, the process comprises:
i. contacting a molybdenum precursor solution to a sulfur precursor to obtain a solution, wherein the solution has a pH in a range of 1 to 3;
ii. heating the solution to obtain a black product;
iii. washing the black product with a solvent; and
iv. drying the black product to obtain the molybdenum based composite material.

In an embodiment of the present invention, the molybdenum precursor solution comprises a molybdenum precursor selected from a group comprising sodium molybdate dihydrate, molybdic acid, ammonium molybdate, and ammonium heptamolybdate tetrahydrate; wherein the molybdenum precursor solution is prepared by adding 2.0 to 2.4 weight % of molybdenum precursor in hydrochloric acid having a concentration in a range of 0.1 to 0.5 M.

In an embodiment of the present invention, the sulfur precursor is selected from a group comprising thiourea thioacetamide, and cysteine.

The solution containing the molybdenum precursor and the sulfur precursor is stirred for 2 to 3 h.

The solution is heated at a temperature in a range of 160 to 180 ? for 12 to 16 h. In an embodiment, the solution is heated in a Teflon lined autoclave in a hot air oven.

The solvent for washing the black product is selected from water and ethanol.

In an embodiment of the present invention, the molybdenum precursor solution comprises a dispersion of a molybdenum oxysulfide nanosheet; wherein the dispersion is prepared by adding the molybdenum oxysulfide nanosheet in water; and sonicating the dispersion for 1 to 2 h.

In an embodiment of the present invention, the molybdenum oxysulfide nanosheet is prepared by a process comprises:
i. grinding a molybdenum precursor with sulfur powder to obtain a mixture;
ii. heating the mixture to obtain a black product;
iii. adding a solvent to the black product to form a dispersion;
iv. sonicating the dispersion to obtain a product;
v. washing the product, followed by drying the product to obtain the molybdenum oxysulfide nanosheet.

The molybdenum precursor is selected from a group comprising ammonium heptamolybdate, molybdic acid, molybdenum oxide, and sodium molybdate dihydrate.

The mixture is heated at 400 to 500 °C for 3 to 5 h in nitrogen atmosphere.

The dispersion containing the black product is sonicated for 2 to 4 h.

The present invention also provides a process for the reduction and removal of hexavalent chromium from wastewater in the presence of sunlight using the molybdenum based composite material as defined above, the process comprises:
i. adding the molybdenum based composite material in water, followed by sonicating to form a dispersion;
ii. adding the dispersion to a solution comprising hexavalent chromium, followed by stirring the solution in dark; and
iii. stirring the solution in the presence of sunlight.

The molybdenum based composite material is added in a concentration of 0.02 to 0.05 mg/mL; the solution has hexavalent chromium in a concentration of 100 to 150 ppm.

The solution containing the hexavalent chromium has a pH in a range of 2 to 3.

In an embodiment, the solution is stirred for 30 minutes in dark to establish the equilibrium.

The concentration of hexavalent chromium reduces upto 97.5% in 30 min using the molybdenum based composite material in the presence of sunlight.

The molybdenum based composite material can be regenerated upto 5 time after the completion of the process. The molybdenum based composite material is regenerated by washing with a dilute acid. The dilute acid is selected from hydrochloric acid, and nitric acid.

EXAMPLES:
The present disclosure with reference to the accompanying examples describes the present invention. However, those of skill in the art should, in light of the present disclosure, appreciate that many changes can be made in the specific embodiments which are disclosed and still obtain a like or similar result without departing from the spirit and scope of the invention. It is understood that the examples are provided for the purpose of illustrating the invention only and are not intended to limit the scope of the invention in any way.

Example 1: In-situ synthesis of molybdenum based composite material
In this method, molybdenum based quantum dots decorated molybdenum oxysulfide nanosheet (MQD/MOS) was synthesized in-situ by one step method. 1.2 g of sodium molybdate dehydrate was dissolved in 50 mL of 0.1 M hydrochloric acid to form a solution. After 30 min, 0.45 g of thiourea was added to the solution and stirred for 3 h. After complete mixing, the solution was transferred into a 100 mL Teflon lined autoclave and heated at 180 °C for 14 h in a hot air oven. After the completion of the reaction, the black product was washed with water and ethanol and freeze-dried.

Example 2: Ex-situ synthesis of molybdenum based composite material
In this method, first molybdenum oxy sulfide nanosheet was synthesized by pyrolysis method after that molybdenum oxysulfide quantum dots (MQD/MOS) was grown on the surface of nanosheet. 1.0 g of ammonium heptamolybdate was mixed with 0.65 g of sulfur powder and grinded into a powder. Next the white powder was kept in alumina boat and heated at 400 °C for 5 h in a tubular furnace in presence of nitrogen atmosphere. After the completion of reaction, the black product was ground and dispersed in ethanol and sonicated for 3 h. After the sonication the product was washed with water and ethanol and dried. 0.25 g of as-synthesized molybdenum oxysulfide nanosheet was dispersed in 50 mL water and sonicated for 1 h to form a mixture. 150 mg of sodium molybdate dehydrate and 50 mg of thiourea was added to the mixture and adjusted at pH 1 by using dilute acid to form a solution. The solution was then stirred for another 2 h. After stirring the solution was transferred into a 100 mL Teflon lined autoclave and heated at 160 °C for 8 h in a hot air oven. After the completion of reaction, the black product was washed with water and ethanol and freeze-dried.

Characterization: TEM and FESEM analysis: The molybdenum based quantum dot decorated molybdenum oxysulfide nanosheet was characterized by FESEM and TEM analysis. In TEM images quantum dots were clearly visible and they were decorated almost uniformly on the surface of molybdenum oxysulfide nanosheet. The sizes of quantum dot were less than 10 nm. Figure 1a and Figure 1b shows the TEM image of molybdenum based quantum dots decorated molybdenum oxysulfide composite. Figure 1c shows the TEM images of pure molybdenum oxysulfide nanosheet. From this images it is clearly seen that the thin sheet of oxysulfide was formed following above synthesis methods. In FESEM images fine aggregated nanosheet were found as shown in Figure 1d. The quantum dots on the surface of oxysulfide were not visible in SEM because the sizes of quantum dot are very small.

RAMAN Analysis: The molybdenum based quantum dots decorated molybdenum oxysulfide sheet was characterized by RAMAN analysis. In this spectra the peaks appeared at 152, 284,378, 405, 663, 819, 992 cm-1 due to wagging mode of terminal oxygen atom, Mo-S in plane and out of plane vibration, asymmetric and symmetric stretching of terminal oxygen. Figure 2a shows the RAMAN spectra of composite sample.

XPS analysis: The XPS spectra of composite material consists of five peaks appearing at 284 eV, 531 eV, 415 eV, 235 eV and 164 eV which corresponds to C 1s, O 1s, Mo 3p, Mo 3d and S 2p respectively. C 1s peak appeared because the material was coated on carbon coated substrate. From the deconvoluted spectra of Mo 3d, two peaks fitted at 232.6 and 234.9 eV corresponds to Mo+6 3d5/2 and Mo+6 3d3/2 respectively. Molybdenum was present in composite material in +6 oxidation state. This confirmed the formation of molybdenum oxysulfide. Figure 2b and Figure 2c shows the survey spectra and deconvoluted spectra of Mo 3d of the composite material.

UV-VIS spectra analysis: In UV-VIS spectra, the composite shows a small peak at 240 nm and very small hump in the range of 320 nm to 400 nm. The corresponding bandgap was 2.96 eV which was calculated using Tauc equation. Figure 3a shows the UV-VIS spectra of the composite material and Figure 3b shows the corresponding Tauc plot of the composite material.

Example 3: Photocatalytic reduction and removal of hexavalent chromium from water using molybdenum based composite material
15.0 mg of molybdenum based quantum dots decorated molybdenum oxysulfide nanosheet was used as a catalyst in 50 mL of DI water and sonicated for 30 min to for a dispersion. Next, the dispersion was added into 250 mL of hexavalent chromium solution in which the concentration of chromium was 150 ppm. After that the mixture was stirred in dark for 30 min to achieve the equilibrium. After 30 min, the mixture was kept under sunlight with low stirring condition (200 rpm) and after certain time of interval (every 10 min) 50 mL of mixture was taken out and centrifuge to remove the catalyst then recorded the UV-VIS spectra of supernatant solution. The experimental data has been shown in Figure 4a. The percentage of reduction of Cr(VI) was calculated by using the following equation:

% R = [(C0-C) x 100]/C0

Where, C0 is the initial concentration of Cr (VI) in mg/L, C is the concentration of Cr(VI) in medium after certain time. The same experiment was carried out without adding catalyst and with using different amount of catalyst loading from 0.02 to 0.05 mg/mL. In absence of catalyst the reduction of Cr(VI) does not happen. But it was seen that in presence of catalyst and with increasing the amount of catalyst the rate of the reduction of Cr (VI) was increased. The 97.5% hexavalent chromium was reduced in 30 min containing 0.05 mg/mL of catalyst. Figure 4b and Figure 4c shows the experimental data.

The similar experiment was carried out using 0.05 mg/mL catalyst for 150 ppm hexavalent chromium solution at three different pH medium (acidic, neutral and basic medium) e.g. pH 2-3, pH 6-7, and pH 8-9 respectively. It was seen that the reduction percentage of chromium was highest at acidic medium at pH-2 and the reduction efficiency was decreased with increasing the pH of medium. The percentage of reduction of hexavalent chromium decreased from 97.5 % to 12 % from pH 2 to pH 9 as shown in Figure 4d.

During the reduction of chromium, toxic chromium (VI) was converted to non-toxic Cr (III) and that chromium was adsorbed on the molybdenum oxysulphide nanosheet. The adsorption of chromium by the catalyst was studied by EDX and ICP-OES analysis. In the EDX technique, after the complete reduction of hexavalent chromium in presence of catalyst, the catalyst was separated through centrifuge and washed with ethanol. The collected catalyst was dried and analysed by EDX. Similarly, EDX analysis was done for unused pure catalyst. It was seen that for pure catalyst shows only the presence of C, O, Mo and S. Whereas catalyst collected after Cr (VI) reduction, shows the elements C, O, Mo, S, and Cr respectively. Therefore, it was clear that besides the reduction of hexavalent chromium, chromium was also adsorbed on the catalyst. The carbon appeared in EDX because sample was coated on carbon tape. The experimental data has been shown in Figure 5.

50 ppm of aqueous solution of hexavalent chromium was taken before addition of catalyst and the chromium content in that solution was analysed by ICP-OES technique. Then catalyst was added into the Cr(VI) solution and exposed to sun light. After certain time interval the mixture was taken out and the catalyst was removed by centrifugation method. The catalyst free supernatant solution was analysed by ICP-OES to estimate the percentage of chromium present in mixture. The ICP-OES data was shown in Table-1, it was seen that after addition of catalyst the concentration of chromium decreased compared to pure Cr (VI) solution. This is because of the adsorption of chromium on the catalyst.

Reusability of MQD/MOS catalyst for the reduction of Cr (VI):
The method disclosed the reusability of the catalyst. In this method, the similar experiment was carried out using 100 ppm of hexavalent chromium solution containing 0.05 mg/mL composite catalyst. The UV-VIS spectra was taken before addition of catalyst and after the complete reduction of Cr (VI) in presence of catalyst. After complete reduction, the catalyst was separated from mixture by centrifugation and washed with dilute hydrochloric acid to remove the adsorbed chromium from catalyst. After the acid wash, the catalyst was washed with distilled water followed by ethanol and dried in air oven. The collected catalyst was reused for same experiment and studied using the UV-VIS spectra. The process was repeated by 5 times and the percentage reduction of hexavalent chromium was calculated from UV-VIS spectra.

Figure 6 shows the plot of percentage reduction of Cr (VI) with respect to number of cycle. After five cycles the efficiency of catalyst was reduced very less. And even after five cycles, the efficiency of the catalyst for hexavalent chromium reduction was more than 90 % after 60 min in acidic medium.

Table 1: Estimation of chromium in water after photo reduction of Cr (VI) by ICP-OES analysis
Time (min) Concentration of chromium in solution (ppm) estimated by ICP-OES Percentage of adsorption of chromium (%)
0 48.26 --
20 30.03 37.77
40 26.95 44.16
60 21.33 55.80 , Claims:1. A molybdenum based composite material for the reduction and removal of toxic hexavalent chromium from wastewater, the molybdenum based composite material comprises:
i. a molybdenum oxysulfide nanosheet; and
ii. molybdenum based quantum dots,
wherein the molybdenum based quantum dots are uniformly distributed over the molybdenum oxysulfide nanosheet; the molybdenum based quantum dots have a particle size in a range of 6 to 10 nm.

2. The molybdenum based composite material as claimed in claim 1, wherein the molybdenum oxysulfide nanosheet has a size in a range of 200 to 500 nm, and a thickness in a range of 8 to 15 nm.

3. The molybdenum based composite material as claimed in claim 1, wherein the composite material has a bandgap of 2.96 eV.

4. A process for the preparation of the molybdenum based composite material as claimed in claims 1-3, the process comprises:
i. contacting a molybdenum precursor solution to a sulfur precursor to obtain a solution, wherein the solution has a pH in a range of 1 to 3;
ii. heating the solution to obtain a black product;
iii. washing the black product with a solvent; and
iv. drying the black product to obtain the molybdenum based composite material.

5. The process as claimed in claim 4, wherein the molybdenum precursor solution comprises a molybdenum precursor selected from a group comprising sodium molybdate dihydrate, molybdic acid, ammonium molybdate, and ammonium heptamolybdate tetrahydrate; wherein the molybdenum precursor solution is prepared by adding 2.0 to 2.4 weight % of molybdenum precursor in hydrochloric acid having a concentration in a range of 0.1 to 0.5 M.

6. The process as claimed in claim 4, wherein the sulfur precursor is selected from a group comprising thiourea, thioacetamide, and cysteine.

7. The process as claimed in claim 4, wherein the solution is stirred for 2 to 3 h; the solution is heated at a temperature in a range of 160 to 180 ? for 12 to 16 h; wherein the solvent is selected from water and ethanol.

8. The process as claimed in claim 4, wherein the molybdenum precursor solution comprises a dispersion of a molybdenum oxysulfide nanosheet; wherein the dispersion is prepared by adding the molybdenum oxysulfide nanosheet in water; and sonicating the dispersion for 1 to 2 h.

9. The process as claimed in claim 8, wherein the molybdenum oxysulfide nanosheet is prepared by a process comprises:
i. grinding a molybdenum precursor with sulfur powder to obtain a mixture;
ii. heating the mixture to obtain a black product;
iii. adding a solvent to the black product to form a dispersion;
iv. sonicating the dispersion to obtain a product;
v. washing the product with the solvent, followed by drying the product to obtain the molybdenum oxysulfide nanosheet.

10. The process as claimed in claim 9, wherein the molybdenum precursor is selected from a group comprising ammonium heptamolybdate, molybdic acid, molybdenum oxide, and sodium molybdate dihydrate.

11. The process as claimed in claim 9, wherein the mixture is heated at 400 to 500 °C for 3 to 5 h in nitrogen atmosphere; the solvent is selected from water and ethanol; and the dispersion is sonicated for 2 to 4 h.

12. A process for the reduction and removal of hexavalent chromium from wastewater in the presence of sunlight using the molybdenum based composite material as claimed in claims 1-3, the process comprises:
i. adding the molybdenum based composite material in water, followed by sonication to form a dispersion;
ii. adding the dispersion to a solution comprising hexavalent chromium, followed by stirring the solution in dark; and
iii. stirring the solution in the presence of sunlight.

13. The process as claimed in claims 12, wherein the molybdenum based composite material is added in a concentration of 0.02 to 0.05 mg/mL; the solution has hexavalent chromium in a concentration of 100 to150 ppm.

14. The process as claimed in claims 12, wherein the solution has a pH in a range of 2 to 3.

15. The process as claimed in claims 12, wherein the concentration of hexavalent chromium reduces upto 97.5% in 30 min.

16. The process as claimed in claims 12, wherein the molybdenum based composite material is regenerated upto 5 time after the completion of the process; the molybdenum based composite material is regenerated by washing with a dilute acid; wherein the dilute acid is selected from hydrochloric acid, and nitric acid.