Activated charcoal mediated purification of Yellow Sodium Sulphate: a green process to utilize a hazardous by-product of the leather chemical industry

Abstract

Basic Chrome Sulphate (BCS) is an important industrial chemical, which is predominantly used in the leather tanning process. During the production of BCS, huge amounts of non-utilizable, hazardous by-product, Yellow Sodium Sulphate (YSS), are generated which is basically Na₂SO₄ contaminated with ∼1% Na₂Cr₂O₇. Huge accumulation of this YSS has become a matter of concern for BCS manufacturers particularly due to its severe pollution potential. The present study showed that contaminated Cr(VI) could be completely reduced to less toxic Cr(III) when the YSS solution was treated with Activated charcoal at pH ≤ 2. Thereafter, Cr(III) was successfully separated from the solution by precipitating at pH ∼ 12; subsequently 99% pure Na₂SO₄ with 98.5% yield (with respect to YSS) was recovered from the filtrate. A recycling study indicated that the efficiency of activated charcoal for the treatment of Cr(VI) in YSS remained unaffected up to five times of reuse. The purified Na₂SO₄ and the separated Cr(III) residue both may be utilized again for their respective purposes.

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1 Introduction

In the perspective of comfort, durability, fashion and personal hygiene, leather products are seen as being extremely aesthetic throughout the world. This promotes leather processing technology to evolve naturally from traditional practices to a global industrial activity. The tannery industry is one of the oldest and fastest growing industries in South and South-East Asia as this part of the world dominates annual global leather production. There are more than 3000 tanneries in India, mostly located in the four states (West Bengal, Uttar Pradesh, Punjab and Tamilnadu) with a total annual processing capacity of ∼7 million tons of hides and skins.^1,2

The production of leather involves a sequence of complex chemical reactions along with some mechanical processes. Amongst these, tanning is the most fundamental step which converts putrescible hides and skins into leather; a flexible, stable, durable, not easily biodegradable fibrous matrix, which can be used in the manufacturing of a wide range of products of intended use. There are several methods of tanning out of which chrome tanning is predominantly practiced (>90%) throughout the world.³ BCS, the basic chromium sulphate [Cr·(OH)_m·(SO₄)_n·X(H₂O)], is used for this purpose unanimously. Indian leather chemical industries are producing nearly 5 × 10⁵ tons of BCS every year to accomplish the requirement of tanning industries. The manufacturing process of BCS involves thermo alkaline oxidative digestion of chromium ore to Cr(VI) in the form of chromate (CrO₄²⁻) followed by its equilibration to di-chromate (Cr₂O₇²⁻) in acidic condition. BCS manufacturing was done by partial reduction of Cr₂O₇²⁻ using organic material (commonly molasses) or SO₂. The reactions involved in the BCS production are given in Fig. 1, and the associated industrial setup is shown in ESI Fig. 1.† During equilibration of sodium chromate to sodium dichromate in the presence of sulphuric acid, an equivalent mole of sodium sulphate is generated, which is separated from the mixture by filter press application.

Fig. 1 Chemical reactions involved during BCS production.

The generated sodium sulphate contains ∼1–2% of sodium dichromate, and is of intense yellow colour. Due to its strong colour, this Yellow Sodium Sulphate (YSS) is not usable, and over the course of time, huge amounts of YSS have been accumulated in different BCS production units (ESI Fig. 2†). Presently, it has become a matter of concern to the BCS manufacturers and also to the society for its multidimensional pollution potential. Being water soluble, both the components of YSS are easily miscible with the soil and other water bodies. This will affect the soil fertility and also enhance the Total Dissolved Solid (TDS) of water bodies thereby causing harm to aquatic life. Moreover, chromium(VI) is carcinogenic and extremely hazardous having the potential to cause immense multidimensional harms to the society e.g. (i) Cr(VI) compounds may cause irritation of the airways, nose ulcers, impairment of gut, liver and kidneys; affect the blood and may cause lung cancer. (ii) Exposure to Cr(VI) compounds can cause allergic responses (e.g. asthma-like symptoms and allergic dermatitis). Recently, Sharma et al. 2012 in their report have shown that how the quality of soil, water and human lives have been severely affected in Kanpur, the largest heritage city of Uttar Pradesh, India, due to improper, careless dumping of Cr(VI) containing wastes by the BCS manufacturing units and leather industries. The situation is likely to be similar in the other leather clusters in India.⁴ Hence, apart from providing education of safe waste management and good manufacturing practice, there is an urgent necessity to find a simple, economical and sustainable solution from this accumulated hazardous material and utilization of the components in the mixture. This will not only eradicate the pollution but also will enhance the business possibilities as purified sodium sulphate as well as chromium both have various commercial applications. As this YSS material is basically sodium sulphate contaminated with little amount of sodium dichromate, hence separation of Cr(VI) from YSS by some techniques might be the best solution of this problem.

The commonly used procedures for removing Cr(VI) from effluents include ion exchange, reverse osmosis, solvent extraction, chemical reduction followed by precipitation, emulsification and adsorption, etc.^5–7 Among these, adsorption is an efficient, economical and sustainable technique. There are various adsorbents of diverse origin which have already been tested for the removal of Cr(VI) from aqueous solutions and also from wastewater.^8–11 Yet, till date activated carbon is the most established and effective adsorbent not only for the treatment of Cr(VI) bearing water but also for the removal of all sorts of pollutant from water.^12–16 However, it has been observed that apart from adsorption, Cr(VI) sometimes gets reduced to Cr(III) when treated with carbon material particularly at low pH.^17–23 Nevertheless, till now there is no report of the removal of Cr(VI) from a solution where Na₂SO₄ exists at a very large excess.

In present work, we have studied the efficiency of Activated Charcoal (AC) to separate Cr(VI) from the solution of YSS to recover highly pure sodium sulphate.

2 Experimental

2.1. Materials

Yellow Sodium Sulphate (YSS) was collected from Lords Chemicals and IndoTan, Kolkata, India and all other chemicals (AR grade) including Activated Charcoal (AC) were purchased from the local suppliers of E. Merck.

2.2. Estimation of Cr(VI) in YSS

Estimation of Cr(VI) during the entire study was performed using UV-Vis Spectrophotometer with SICAN software by standard Di-Phenyl Carbazide (DPC) method.²⁴

2.3. Pre-treatment of activated charcoal

Prior to use, charcoal was heated at 100 °C in a hot air oven for 30 min. After that the material was kept in a desiccator to bring the temperature down to room temp. (RT) (28 ± 2 °C).

2.4. Treatment of YSS by activated charcoal

To observe the effect of AC on the YSS solution, different amounts of AC (0.25 g, 0.5 g, 0.75 g, 1.0 g) were taken to 20 mL of YSS solution (10 g L⁻¹) at different pH range (2, 4, 7, 10, 12) under shaking (200 rpm) condition at room temp. After desired incubation period (12 h), the solid charcoal material was filtered and the Cr(VI) content in the filtrate was estimated.

To optimize the treatment dose, 1 g of AC was employed to various concentration of YSS solution (10 g L⁻¹ to 100 g L⁻¹). During investigation, pH of the YSS solution was maintained at pH ≤ 2, all other experimental conditions were maintained same as mentioned above.

Effect of temperature over the process was investigated similarly as mentioned above at three different temps (25, 30 and 35 °C) by keeping the concentration of YSS solution fixed at 70 g L⁻¹.

Kinetic of the reaction was investigated up to 12 h by measuring the Cr(VI) concentration in the reaction mixture (YSS solution: 70 g L⁻¹; AC: 1 g; maintained at pH ≤ 2 at RT with shaking speed 200 rpm) in the various predetermined time intervals.

2.5. Separation of Cr(III) from solution and recovery of sodium sulphate

Soluble Cr(III) was precipitated out from the solution in alkaline condition. After the AC treatment, pH of the resulting mixture was elevated to pH ∼ 12.0 by the addition of dilute NaOH and vigorous stirring for 15 min. Resulted greyish green precipitate of Cr(OH)₃ was separated using Whatman No. 1 filter paper and dried. The crystalline sodium sulphate was recovered from the filtrate by solvent evaporation followed by vacuum drying technique after neutralizing the filtrate with dilute sulphuric acid solution.

2.6. Recycling of charcoal

To evaluate the efficiency of the charcoal material, the used charcoal material was collected by filtration and after washing it with deionized water, the material was vigorously shaken for 30 min with water of pH 1.0. Afterwards, the charcoal material was collected by filtration, washed with deionized water till neutral. The material was dried in vacuum oven till constant weight and then it was treated by following the process described in section 2.3.

2.7. Optimization of treatment scale

To evaluate the flexibility and validity of the process, treatment of AC on YSS (70 g L⁻¹) was performed in various scales starting from 50 mL YSS : 0.5 g AC to 1000 mL YSS : 10 g AC. During this step all other experimental conditions were maintained at the optimum condition.

2.8. Estimation of purity of sodium sulphate

The purity of sodium sulphate was measured by barium chloride method.²⁵ 0.5 M Na₂SO₄ solution was acidified (with HCl), warmed at 80 °C and mixed with equal volume of 0.6 M BaCl₂ solution. The mixture was stirred vigorously for 1 h at 80 °C and then the resulting barium sulphate was filtered using Whatman no. 41 filter paper after cooling it down to RT. The purity of the sodium sulphate was estimated from the weight of barium sulphate.

2.9. Material characterization procedure

Surface character and functionalities of activated charcoals were analysed by SEM-EDX, X-ray photoelectron spectroscopy and FTIR techniques. Composition analysis of mineral samples was done by XRF analysis. Details of these procedures are given in the ESI.†

3 Results and discussion

3.1. Estimation of Cr(VI) in YSS

Cr(VI) content was measured from a standard curve prepared using Na₂Cr₂O₇ by DPC method. Results indicated that YSS material contained 0.4–0.5% Cr(VI) of its weight.

3.2. Purification of YSS by activated charcoal

The treatment of YSS by AC on YSS solution is reported in Fig. 2. It shows that even 0.25 g of AC was enough to remove the entire Cr(VI) from 20 mL of YSS solution (10 g L⁻¹) at pH ≤ 2. However, with increase in pH, efficacy of Cr(VI) removal by the AC decreased gradually, and the process became inoperative in alkaline pH.

Fig. 2 Results of YSS purification by activated charcoal.

Optimization of AC dose over the range of concentration of YSS solution indicated that 1 g of AC could treat 100 mL YSS solution of maximum 70 g L⁻¹ concentration efficiently (Fig. 3). However, it was found that effect of temperature on this process over the range of 25–35 °C is insignificant (data not shown). Kinetics of experimentation indicated that (Fig. 4) in optimum reaction condition, 8 h time was required for the complete removal of Cr(VI) from the YSS solution (ESI Fig. 3†).

Fig. 3 Optimization of the dose of activated charcoal.

Fig. 4 Kinetics of Cr(VI) removal from the YSS solution.

It was assumed that adsorption of Cr(VI) by the AC might be the prime mechanism for Cr(VI) removal process till this investigation point. However, during recovery of sodium sulphate it was observed that instead of expected white crystalline solid, a slightly greenish crystalline material was obtained. On oxidation using mixed acid (H₂SO₄ : HClO₄ ≡ 1 : 2), it developed an intense yellow colour solution indicating the presence of Cr(VI), which was further confirmed by DPC method. It was also found that before and after the treatment with AC, the amount of total chromium present in the solution remained the same. This concluded that during the experimentation, Cr(VI) of YSS was converted into Cr(III) i.e. why DPC method indicated the non-existence of Cr(VI) in YSS solution. Facts were also confirmed by the XRF analysis of YSS sample and the greenish solid obtained after initial treatment of YSS by AC (ESI Fig. 4–6†). Adsorption possibility of Chromium by the AC during the process was invalidated by the nonexistence of chromium in the SEM-EDX analysis of the surface of AC recovered after the YSS treatment (ESI Fig. 7†). In an attempt to ascertain the reducing role of AC, sets of experiment were carried out at the optimum process conditions (pH ≤ 2, RT, 8 h, 200 rpm) and the findings are summarized in Table 1.

Table 1 Treatment of different Na₂SO₄/Cr(VI) mixture by activated charcoal^a

Sample solution (70 g L^−1; 100 mL)	Activated charcoal used	Colour of the solution		Detection of Cr(VI) by DPC method
		Initial	Final	Initial	Final
a Y: yellow; LG: light green; d: detected; nd: not detected.
YSS	Nil	Y	Y	d	d
YSS	1 g	Y	LG	d	nd
Na2SO4 with Cr(VI) Na2Cr2O7/K2Cr2O7	Nil	Y	Y	d	d
Na2SO4 with Cr(VI) Na2Cr2O7/K2Cr2O7	1 g	Y	LG	d	nd

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It is evident from the results that only in the presence of AC, Cr(VI) got reduced to Cr(III) in both YSS and Na₂SO₄/di-chromate mixture (simulated YSS). Hence, it can be concluded that AC exclusively acted as a reducing agent in the experimental condition. Reducing behaviour of AC at low pH has already been reported for the treatment of aqueous solutions of chloramine,^26,27 azo dyes²⁸ and chromium(VI).^{16–18,23,29–41} Cr(VI) in aqueous solution can exist in different ionic forms such as HCrO₄⁻, Cr₂O₇²⁻, CrO₄²⁻, dominance of each species in solution is dependent on the concentration of Cr(VI) and pH of the aqueous solution. In acidic condition (pH 1–6), HCrO₄⁻ predominantly exists together with little amount of Cr₂O₇²⁻; whereas Cr₂O₇²⁻ and CrO₄²⁻ commonly found at higher pH (pH > 6).^16,17,30 Removal of Cr(VI) by AC at low pH generally explained by adsorption along with the partial reduction of Cr(VI) to Cr(III).^{16–18,23,29–41} However, at very low pH (pH ≤ 2) priority of reduction becomes exclusive.^29,31–41 To get an insight of the role of AC in Cr(VI) reduction process, the surface character of AC was analyzed using SEM-EDX, XPS and FTIR spectroscopy at different stages of the investigation (ESI Fig. 7–11†). SEM EDX analysis of the procured AC samples indicated that surface of AC contains fair amount of oxygenated functionalities. It is already reported that AC surface may possess surface oxides (CxO, CxO₂, where Cx represents carbon),^42,43 hydroxyl, phenolic, carbonyl, carboxylic acid/ester, lactone groups.^29,35 FTIR analysis of the procured AC showed distinct bands at 3400 cm⁻¹, 1575 cm⁻¹, 1205 cm⁻¹ and 1115 cm⁻¹. IR band at 3400 cm⁻¹, indicated the presence of hydroxyl functionalities. Intense band at 1575 cm⁻¹ may be attributed to the overlapping band of conjugated C=C and carbonyl groups. Overlapping band of C–O stretching and –OH bending of phenolic group appeared at 1205 cm⁻¹. The band at 1115 cm⁻¹ may be associated with C–O stretching of RO–C=O allied groups.^29,35 In the core C1s and core O1s XPS spectra of the AC sample existing C–H, C–C and C=C bonds of AC surface were appeared at 284.3 eV. C–OH/C–O functionalities appeared at 286.6 eV (C1s) and 533.2 eV (O1s). C=O functionalities appeared at 288.2 eV (C1s) and 531.4 eV (O1s). Presence of O–C=O indicated at the 290.5 eV of C1s spectra.^44–47 During investigation prior to its use for YSS treatment, AC was heated at 100 °C. Practically identical SEM-EDX, FTIR and XPS spectra (ESI Fig. 8–10†) of the AC before and after heating indicated that this pretreatment had no impact on the oxygenated functionalities on the surface of AC. However, significance alteration of the FTIR pattern of AC surface has been noted after its interaction with acidic YSS solution. The FTIR spectra of the AC collected after half treatment of YSS indicated mild broadening of the hydroxyl band (3400 cm⁻¹) and enhancement of C–O band intensity at 1205 cm⁻¹ (ESI Fig. 9†). This might have been possible due to the interaction of AC with aqueous acidic YSS, few hydroxyl groups are generated by hydrolysis of surface oxide which were added up with the existing hydroxyl groups in AC. However, FTIR spectra of the AC recovered after the completion of the YSS treatment, showed slight shriveling of hydroxyl band and noticeable decrease in intensity of the C–O band at 1205 cm⁻¹. As the change in C=O band is not much distinct in the FTIR spectra, hence to identify the fate of decreasing C–O functionalities, O1s XPS spectra of the recovered AC sample has been carried out and the deconvoluted spectra (ESI Fig. 11†) clearly indicated the enhanced percentage of C=O group in AC surface than C–O containing group after interaction of AC with YSS; before this interaction the ratio of these groups was almost 1 : 1 (ESI Fig. 10†). This fact indicated that during interaction of AC with Cr(VI) of YSS, some of the C–O groups were oxidized into C=O groups through subsequent reduction of Cr(VI) to Cr(III). Hence, from the surface analysis of AC at the different stages of this investigation, it can be concluded that the reduction of Cr(VI) to Cr(III) took place via the oxidation of some of the C–O functionalities to C=O functionalities of the AC surface. Similar observation has also been reported by previous researchers.^29,35,37 Therefore, overall YSS treatment process by AC could be explained as; in aqueous acidic conditions AC surface becomes highly +ve due to protonation of oxygenated functionalities. This facilitates the binding of the neighboring Cr(VI) anions onto its surface. During this process Cr(VI) interacts with hydroxylated functionalities (CxOH) which is an active reducing substrate.^{18,19,22,29,30} Interaction of Cr(VI) with these groups resulted in its reduction to Cr(III) by simultaneous oxidation of C–O to C=O functionalities as shown schematically in Fig. 5. Furthermore, at lower pH, Cr(VI) reduction was assisted by higher reduction potential of Cr(VI)/Cr(III) system and higher thermodynamic stability of Cr(III) in acidic solution.^17,29,30 YSS solution contained sodium sulphate almost exclusively, therefore, the ionic strength of the solution was very high which probably impeded any possibility of the adsorption process.^17,48,49 As the surface of the charcoal was highly positive, the chance of adsorption of generated cationic Cr(III) onto the charcoal surface was almost nil due to strong electrostatic repulsion.^37,49,50 Any probability of Cr(III) adsorption was further diminished in experimental condition due to the high ionic strength of the YSS solution. The Cr(III) which was present in the solution was precipitated out in the form of hydroxide at alkaline pH, and ∼95% of total chromium in YSS was separated. Subsequently, white crystalline sodium sulphate of 99% purity (product quality was further evaluated by XRF analysis; ESI Fig. 12†) with 98.5% yield was recovered from the neutralized supernatant by solvent evaporation followed by vacuum drying technique. The overall purification process is schematically presented in Fig. 6.

Fig. 5 Schematic presentation of Cr(VI) reduction by activated charcoal.

Fig. 6 Schematic presentation of total purification process of YSS.

3.3. Recycling of charcoal and optimization of scale of treatment

Purification of sodium sulphate from YSS was a two-step process. The overall efficiency of the system depended on the first step i.e. AC mediated reduction of Cr(VI) to Cr(III). Results of the recycling experiment indicated that there was no deterioration of the activity of the charcoal up to five cycles (Fig. 7). Further investigation also suggested that, process was also highly effective in wide range of application scale (Table 2).

Fig. 7 Recycling of activated charcoal for the conversion of Cr(VI) to Cr(III).

Table 2 Maximization of scale of operation

Experiment	S-1	S-2	S-3	S-4	S-5	S-6	S-7
YSS (70 g L^−1) in mL	50	100	150	200	250	500	1000
AC (g)	0.5	1.0	1.5	2.0	2.5	5.0	10
Removal (%)	100	100	100	100	100	100	100

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4 Conclusions

Activated charcoal mediated green and sustainable recovery of pure sodium sulphate from Yellow Sodium Sulphate, a hazardous by-product of leather chemical industries has been established. The overall purification process is a two-step process and very inexpensive in terms of consumables and experimental setup required. Reduction of Cr(VI) to Cr(III) via the oxidation of some of the C–O to C=O functionalities of AC surface at low pH is the first step and separation of Cr(III) from the mixture by forming insoluble Cr(OH)₃ is the second step. In this process, ∼99% pure Sodium Sulphate can be recovered from the YSS with 98.5% Yield. The performance of AC for Cr(VI) reduction remained unaffected up to five recycling steps, and the process is well operative in a wide scale of applications. The generated sodium sulphate as well as Cr(OH)₃/Cr-residue both are reusable for their respective purposes. Therefore, this process is an absolutely green and sustainable remedy of the YSS problem. Furthermore, this procedure has been successfully verified with the YSS samples obtained from other local leather chemical industries.

Acknowledgements

This work was supported by CSIR-CLRI. The authors are grateful to industrialist Mr J. C. Juneja (IndoTan) and Late Mr M. N. Kini (Lords Chemical Ltd.) for providing the Yellow Sodium Sulphate samples for investigation. Authors are also grateful to Dr V. K. Balla, CSIR-Central Glass and Ceramic Research Institute, Dr M. Sudarshan, Dr R. J. Chaudhary and Ms Ritu Rawat of UGC-DAE consortium for scientific research (Kolkata and Indore center respectively) for providing instrumental support for this investigation (CSIR-CLRI Communication No. 1214).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra01271j

‡ Authors contributed equally in this work.

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