EFFECT OF HYDROXYL IONS ON RECYCLING OF HEXAVALENT CHROMIUM FROM CHROMIUM HYDROXIDE SLUDGE.
Experiments are conducted to recover chromium from chromium-iron hydroxide sludge. The process comprises separating chromium from the sludge by selectively oxidizing the trivalent chromium precipitate to soluble hexavalent chromium with chlorine gas in alkaline medium. The hexavalent chromium ions enter the solution and are separated from the iron hydroxide precipitate as the sludge is dewatered. Through series of experiments about 73% recovery is achieved. This hexavalent chromium solution can be used in cooling tower make up water and in chrome plating solutions.The method applied is technically less complex and economically viable. It is very useful for the industries having low amount of trivalent chromium in their wastes. The chemicals used in this process are easily available and the process is very efficient.
Leather textile and fertilizer industry extensively use the chromium in both the trivalent and hexavalent forms. Hexavalent chromium is a highly toxic metal even its low
concentrations are toxic to aquatic life. The toxicity of
chromium compounds depends principally on valency as well as physical and chemical properties of the specific compounds. The acute toxicity of chromium compounds increases with water solubility.
Hexavalent chromium is very hazardous to human health. Inhalation may cause acute toxicity irritation and ulceration of the nasal septum and respiratory sensitization (asthma). Skin contact may result in systemic poisoning damage or even severe burns interference with the healing of cuts or scrapes which if not treated promptly may lead to ulceration and severe chronic allergic contact dermatitis. Kidney and liver functions may be affected by ingestion. Eye exposure may cause permanent damage .
Prolonged or repeated exposure to hexavalent chromium may lead to perforation of the nasal septum with an increased risk of respiratory cancer . The Committee within the European Chemicals Bureau responsible for hazard classification has rated the hexavalent chromium products as capable of causing heritable genetic damage and toxic for reproduction .
Waste streams from the process industry power generation facilities and tanneries etc. contain chromium. Chromic acid solutions are used for the electrolytic stripping of copper for chromium plating for passivation of heavy metals. Other industries like metalfinishing magnetic tape manufacturing pigment productionleather tanning chemical manufacturing brassmaking electrical and electronic equipment manufacturing contribute for the presence of hexavalent chromium in waste streams . Waste streams may lead to filtration and transport of the pollutant to ground water sites.
For the removal of heavy toxic metalswidely used treatment processes includes chemical precipitation oxidation- reduction mechanical filtration ion exchange membrane
separation and carbon adsorption. Removal of heavy metals from industrial wastewater using chitosan coated oil palm shell charcoal through biosorption process has been reported and new biosorbent has been prepared . Adsorption of Cr (VI) from aqueous solutions using sawdust coated by polyaniline (SD/PAn) and polyaniline composites with nylon 66 and polyurethane have been reported . Micro- alloyed aluminum composite (MAlC) have also been investigated for the removal of Cr (VI) from aqueous solutions . Chromium removal from waste water has been investigated using the activated carbon derived from coconut fibers coconut shells and acid treated coconut shells . Effects of temperature initial adsorbate concentration adsorbentparticle size and solid-to-liquid ratio were studied. Removal of chromium has also being studied through modified quaternized poly (4-vinylpyridine) PVP coated Silica Gel by synthesizing of a reactive polymer .
Cellulose acetate micro filtration and polysulfone ultra filtration membrane have also being applied for the treatment of electroplating wastewater . In a study applicability of alkaline reagents such as Ca(OH)2 (Lime) and NaOH (Caustic Soda) in removing chromium ions from petrochemical industry cooling water wastes is being studied .
A number of options are available for the elimination of chromium from wastewater stream through biological chemical and physical treatment techniques. In the same way
for the recovery of chromium from wastewater stream techniques like precipitation ionization reverses osmoses are being used. In this paper recovery of chromate is studied from the sludge by selectively oxidizing the trivalent chromium with strong oxidizing agent such as chlorine gas in alkaline medium. This oxidation produces hexavalent ions that are soluble in the solution. The regenerated hexavalent ions can be reused in electroplating process and cooling towers.
2. MATERIALS AND METHODS:
A small portion of cooling water from cooling towers in fertilizer industry is withdrawn having a CrO4-2 concentration of 18-21 ppm as purge stream. Cooling tower blow down is treated to remove chrome before being discharged. Highly soluble hexavalent chromium is converted into insoluble trivalent Chromium through electrochemical cell. The water enters the cell from the bottom and flows upward through the electrodes. The waste stream from the electrochemical cell is pumped to settling pond where precipitated chromium hydroxide and ferrous hydroxide settles down and chromium free clear water overflow. This sludge is taken for the experimentation. Experiments were conducted in a Pyrex glass column with about 10 cm internal diameter and115 cm length. The column bed comprises of PVC (poly vinyl chloride) rings with volume of 5735 cm3.
Chromium iron hydroxide sludge from the settling pond of a fertilizer industry is taken. It is then dried completely and grinded to make its powder. This powder is washed with plenty of distilled water in order to ensure that all the soluble salts it contains have been rinsed. In this sludge proportion of chromium and iron was 24.18 % and 70.25 % respectively.
Total circulation volume is fixed (6 liters) according to the system capacity. Sludge is taken to make its solution in distilled water. This solution is taken to thecirculation tank and NaOH is added into the tank to maintain the pH around
8.5. Solution is passed through the bed from top to bottom. Constant volume of chlorine gas is passed through bottom of the bed and flow counter current to the solution. Chlorine
gas is absorbed into the solution to oxidize the trivalent chromium. As the reaction proceed solution color changes to bright yellow indicating the presence of hexavalent chromium in solution. Samples were taken after appropriate time and analyzed to check results.
The instant process utilizes chlorine gas as strong oxidizing agent to selectively cause the chromium constituent to enter solution for separation. In dilute alkaline solution chlorine
gas will react as follows to form hypochlorite ion:
The hypochlorite ion then reacts in turn with trivalent
chromium as follows:
The overall reaction for the oxidation of trivalent chromium to hexavalent chromium is as follows:
The presence of ferric hydroxide precipitate does not substantially interfere with the above reaction and the reaction proceeds very rapidly producing a bright yellow color solution as the hexavalent chromium ion is formed. Reaction is conducted at ambient temperature. At elevated temperatures hypochlorite ions will disproportionate and form chlorate ion.
Concentration of hexavalent chromium was measured colorimetrically. A UV-visible spectrophotometer (UV- 1750) was used to measure the color intensity of the chromophore complex at its wavelength of 540 nm. This
chromophore complex is resulted from the reaction of hexavalent chromium with15-diphenylcarbazide with pH in the range of 3-6.5. Standard solutions of potassium dichromate were used to draw the calibration curve. 1N sulfuric acid solution was used to adjust the pH.Concentration of hexavalent chromium was determined from the calibration curve using the absorbance value. Repeatability of this analytical procedure is better than 1 mg/L .
3. RESULTS AND DISCUSSION:
Various experimental conditions were used in this study to
find the maximum conversion of trivalent chromium into
It was observed that pH of the solution decrease with the time as shown in fig. 2. Selective oxidation was carried out as a batch system. At the start of the circulation pH of the
solution was maintained 8.5. During the circulation Chlorine gas get absorbed into the solution and reaction proceed as per equation 3 hexavalent chromium was formed which is acidic in nature. As its concentration increases the pH of the solution decreased.
As pH of the solution decrease less OH- would be available for chlorine to make the hypochlorite ion that reacts in turn with trivalent chromium to form hexavalent chromium that resulted in low conversion to hexavalent chromium.
In next series of experiments pH of the solution was maintained so that more OH- were available for chlorine to react and shift the reaction in the forward direction which resulted in more conversion as shown in fig.3 and fig.4 for initial concentrations of 2.17 and 9.03 ppm respectively.
When pH was maintained more conversion to hexavalent chromium was observed.
The effect of circulation time on the %age conversion of trivalent Chromium to hexavalant Chromium for different initial concentrations of sludge is shown in figures 5 and 6.
In fig. 5 only the initial pH was maintained around 8.5 while in fig. 6 pH of the circulating solution was continuously maintained with solution of caustic soda. It is clear from these figures that as the amount of sludge dissolved in a fixed volume is increased conversion decreases. It is obvious that for lesser amount of sludge with all other parameters constant more chlorine will be available which results in more conversion. Also the conversion is greater as pH of the solution is maintained around 8 ~8.5. It is due to the fact that OH ions decrease with the circulation time as the reaction proceeds. With maintaining pH of the solution more OH-- ions will be available for reactions and shift the reaction in forward direction. Hexavalent chromium can
exist in several stable forms such as Cr2O7 HCrO4 etc.
Under the prescribed conditions of experiment max recovery of trivalent to hexavalent chromium was achieved up to 73 %. Results show that the regeneration process has good conversion and technically less complex which make its base for the utilization on industrial scale for reusing chromium (trivalent).
For regeneration of hexavalent chromium chlorination technique was applied in laboratory batch experiments. Results were very satisfactory as about 73% chromium was regenerated. The method adopted is economically viable and technically less complexfor the conversion of trivalent chromium to hexavalent chromium for reuse as corrosion inhibitor in cooling towers or in plating solutions. Use of sodium hypochlorite is also recommended due to the safety issues associated with chlorine.
Authors acknowledge the financial support from Department of Chemical Engineering UET Lahore for this project.
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