Removal Of Chromium From Electroplating Wastewater By Simple Chemical Treatment And Ion Exchange

Wastewater from metal plating works was treated with lime - water suspensions at relatively small concentrations with continuous stirring. Experiments were designed to allow a direct contact of lime suspension with the wastewater constituents for a period of time followed by settling and filtration through a sand filter. The heavy metal content could be precipitated efficiently from the mother liquor by two mechanisms: reaction with calcium ions to yield calcium chromate and the precipitation of the chromium hydroxides in the alkaline medium. Various parameters were studied to reach the optimum conditions for the removal of chromium from the wastewater. It appeared that reasonable removal of chromium (80 – 85%) could be achieved with lime: wastewater ratio of 40 mg/ L and 50 min contact time. After the removal of most of the chromium from wastewater, the level of the pollutant needed to be reduced to the acceptable limit by passing water through ion exchange column. This final treatment gave treated water samples with chromium levels as low as 0.6 - 1.3 mg/L

The wastewater treatment methods vary depending on the pretreatment limits placed on incoming effluent.In most cases the characteristics of the wastewater will be high in metals, cyanides, and pH.In chrome plating, the use of a coagulant or flocculent, such as inorganic salt or polymer will create a solid sludge containing the metals which settles at the bottom of the holding container.The resulting hydroxide sludge will then be dewatered and sent for disposal.The sludge was used to recover (extract) metals to improve the economy of the method.Thus, the metallic waste turns from a hazardous landfill item to useful metal, avoiding the cost of dumping into a hazardous landfill, while supporting the cost of purchasing virgin chromium.
Chromium is reduced chemically prior to treatment or recovery.Many reduction methods are used and it was concluded that, when a method is viable in one case they are marginally acceptable in another [1,2].Jing et al [5] described the preparation of nanoparticles for the adsorption of heavy metals from wastewater utilizing the highly modified surface area of the particles.
Chemical precipitation using lime or magnesia is another attractive method for the removal of chromium from industrial waste-water [6,7].
Huang [8]  As the rinse is passed through the column packing (resin bed) the desired ions are extracted from the flow together with other ions of similar charge, which may also be present in the stream.The flow must eventually be interrupted and the resin bed regenerated.When ion exchange is applied to metal recovery, the metal values obtained from the regeneration process may not be suitable for return directly to the plating bath.
Concentration and pH adjustment are frequently required before reuse.Much of the regeneration rinse waters are too dilute or otherwise unsuitable for reclaim; they will often require treatment in the wastewater system prior to discharge.

Experimental Laboratory Experimental Units:
The experimental unit for the chemical treatment consists of a water bath of (60× 40 × 16 cm); a magnetic stirrer; a 3-neck round bottom flask of 1-L capacity, a thermometer and magnetic stirrer.1.The best of the set was the 200 mg/L, where the chromium content of wastewater could be reduced to 25-30% of the original value.Fig. 1 shows the profile of chromium contents with mixing time at the high lime: wastewater ratios.
After 40 min of mixing the 200 mg/L system attained the chromium level given by the 600 mg/L after 60 min.Thus, the increase of CaO contents lowered the stability of the precipitates.Such an effect is related to the increase of pH of the solution.The lowest (200 mg/L) ratio gave the best chromium removal possibility within this range.The high CaO content is expectedly associated with increase of pH of the solution.Consequently the solubility of the chromium hydroxide increases and hence the removal efficiency of chromium is lowered [2].The mixing time is important in determining the removal efficiency.The chromium concentration decreases slowly down to the lowest value after 50 minutes of mixing and remains almost constant.Thus, it was not necessary to continue the mixing further.Another set of experiments was carried out using low (≤ 100 mg/L ratios).The results are shown in Table 2.With these smaller ratios of lime: wastewater the removal could be highly improved and the chromium contents of wastewater were reduced down to 15-18%.Fig. 2 shows the effect of treatment time on the chromium removal efficiency from WW using the low lime: wastewater ratios.It appeared that the removal efficiencies were much better than those obtained in the high range treatment.
The chromium contents of the treated wastewater decreased down to about 15% of the original value.The best removal efficiency can be obtained at a ratio of 40 mg/L.Thus, higher lime contents do not improve chromium removal.The mechanism of the removal, therefore, is not only a precipitation process.
The pH of the effluent increases slowly with increasing the addition of CaO to 200 mg/L.The PH increased up to a value of 7.35 at 30 o C and a mixing period of 50 min.
Two factors seem to act on the removal mechanism: precipitation as chromium hydroxide and the precipitation as a calcium chromate compound.The chromium hydroxide solubility is affected by the pH of the solution [4].
At the end of mixing (after 80 min.) the chromium removal efficiency was plotted against the lime: wastewater ratio and the results are shown in Fig. 3.It is clear that the best removal efficiency occurs between 30 and 80 mg CaO / L WW and the highest was attained at 40 mg/L.Lime amounts lower than 40 mg/L were not enough to bring about the required stoichiometry of the formation of calcium chromate nor allowed pH to raise to the level required for the precipitation of Cr(OH) 3 .Meanwhile, lime amounts higher than 100 mg/L means that CaO exceeds the stoichiometric value to give the chromate and at the same time raises the pH to much high values at which the Cr(OH) 3 is soluble returning thereby chromium to solution [2].

Filtration
Water emerging from the sedimentation basin is routed to the top of the filtration sand bed.
The filtration unit traps those particles that did not settle in the sedimentation basin (because they were too small) or did not have sufficient time to settle and were carried along with the effluent out of the precipitation basin.
Precipitates of faint yellow sludge containing calcium chromate and chromium hydroxide settle and stick on top of the sand bed.Water passes through the bed free of the metal ions load and hence became colorless.The first appearance of the filtered water reflects how successful the treatment was after the designed time.Filtration completes the metal treatment process.The pH of the water discharged from the sand filter was higher than the acceptable value for disposal and thus it has to be adjusted with calculated amount of mineral acid to be reused.
As filtration progresses and more metal hydroxides and other solids clog the filter material, pressure drop through the filter rises and some solids may pass through the filter.
When either of these two situations occurs, the filter must be backwashed by reversing the flow of water through the filter.This backwash water is sent back to the rapid mix tank for mixing with the incoming water since it contains a significant concentration of solids from the dislodging that has occurred.The filtrate, therefore, showed some chromium contents which is still higher than the permissible limits.The final chromium removal was aided by ion exchange purification.After passing through the ion exchange columns, the atomic absorption analysis of the samples showed chromium levels of 0.6 -1.3 mg/L.However, only minor differences could be noticed in the results of analysis of the wastewater samples treated by the various amounts of lime-water suspension.

Sludge Treatment
The solids produced in the sedimentation stage (and possibly solids from filtration) are denoted as a sludge and periodically removed.In diatomaceous earth and fiber filters, the entire filter media (diatomaceous earth, filter cartridge) is dumped with the captured metal hydroxide solids.This sludge may be sent to a dewatering stage to remove excess water and leave only solids.The water from the dewatering stage may not be completely free of metals and should be piped to the rapid mix tank.
The sludge now contains the precipitated metal hydroxide solids, made up of identifiable quantities of heavy metals, which are regulated according to state and federal guidelines.The solids produced from heavy metal wastewater treatment must then be disposed of as a hazardous waste.Incomplete removal of chromium is not a drawback of the method but a result of the presence of substances in the wastewater.
Compounds such as cyanide or ammonia can inhibit precipitation of metals, and limit their removal to the point where discharge limits can be exceeded [12].However, the present, rather simple, treatment compares well with the results of Rao [13], who reported an efficient recycling of unused chromium as well as water with electro-dialysis technique using Selemion AMV and CMV membranes and a prototype electro-dialysis cell.Besides, it resulted in improved sludge properties in comparison with the system adopted by the European countries [14] which uses soda ash or some other alkali is added to promote the combination of the tanning agent with skin substance.

Table 2 : Variation of chromium concentration in the treated wastewater at different lime concentrations (LOW) with treatment time
Chromium removal profiles for the low Lime : WW range.
Figure 3: Chromium removal efficiency at various Lime: WW ratios.