Cadmium Removal by Flotation from Diluted Aqueous Solutions. Determination of the Optimum Separation Conditions

The paper presents the removal of cadmium by flotation from dilute aqueous solutions, at a laboratory scale, using an anionic collector (oleic acid). The optimum values of the main parameters influencing this process were determined: pH of Cd(II) solutions, molar ratio collector:Cd(II), air flow rate, flotation time and initial concentration of Cd(II). Using these optimal conditions, a very high removal degree of cadmium (over 99%) was obtained.

Heavy metals are considered hazardous pollutants due to their toxicity and non-biodegradable properties.They are defined as metals that have specific gravity greater than about 4 or 5, but more often, the term heavy metals is simply used to denote metals that are very toxic, even at low concentration (Hg, Cd, As, Cr, Tl, Pb, Cu, Zn, Ni) [1].
Cadmium is one of the most toxic heavy metal elements for animals and humans.In humans, cadmium is accumulated in the kidneys which will begin to malfunction at overdoses, spilling proteins in the urine and disrupting potassium metabolism [2].It is well known that chronic cadmium toxicity has been the cause of Japanese Itai-Itai disease [3].Cadmium may damage the testicle (male reproductive glands) and may affect the female reproductive cycle [4].Exposure to cadmium may increase the risk of lung, prostate and kidney cancer in humans [5,6].Cadmium has a long biological half-life in the human body, ranging from 10-33 years [7,8].
Cadmium enters into the aquatic system through mining and metallurgical processes, industrial effluents from cadmium-bearing alloys, soldering materials, electroplating, and Ni-Cd batteries [9,10].It is, therefore, necessary to remove cadmium from aqueous industrial wastes before discharging it into the natural water stream.
The recovery and/or removal of heavy metals from wastewater or aqueous solutions may be achieved by applying a variety of methods: chemical precipitation, solvent extraction, cementation, reverse osmosis, adsorption, flotation, ion exchange and evaporation, among others.Of these, flotation presents some advantages: simplicity, flexibility and effectiveness of operation, low space requirements, the production of small and concentrate volume of sludge [11].
Precipitate flotation is a subcategory of the flotation technique.It involves a preliminary step where the ionic species of metal is precipitated and the precipitate is then floated by using an appropriate collector to render the minute solid particles hydrophobic.Precipitation is usually achieved by pH regulation, since most metal ion hydroxides are insoluble at some range of pH values.
The aim of the present study is to establish the optimum flotation conditions in term of: pH of Cd(II) solutions, molar ratio collector:Cd(II), air flow rate, flotation time and initial concentration of Cd(II) in solution.
-bench-scale equipment for dispersed-air flotation technique.A schematic representation of the flotation bench-scale equipment is shown in figure 1.

Effect of solution pH
The pH of the solution is one of the most important factors which influences the ion separation by flotation, as it determines the magnitude and sign of the charge on the ions and also the dissociation degree of the ionic groups of the surfactant molecules [12].Preliminary experiments, shown in figure 2, were conducted in order to determine the pH effect on the cadmium removal efficiency, (R%) and on the cadmium concentration after flotation, (Cd(II) rez.[mg/dm 3 ]).at molar ratio collector:Cd(II) value = 0.5, so much less than that stoichiometric.This is because for pH = 10 cadmium is almost all precipitated and flotation is a precipitate flotation process in which in order to obtain a hydrophobic surface of precipitate particles is sufficient a monomolecular layer of collector on their surface.

Effect of air flow rate
In the separation processes by adsorptive bubbles a determining role have the hydrodynamic factors and mass transfer.The mass transfer is promoted by large liquid-gas interface which increases by increasing the air flow rate and by decreasing of bubbles size.The bubbles size is about 5 -10 times higher than that of frit pores and they must have enough energy for raise to the liquid surface.Small bubbles are suitable because they realize a greater contact surface and an efficient adsorption which finally determine high removal efficiency [12].The effect of air flow rate on Cd(II) removal efficiency is shown in figure 4. It can be seen that the flotation of cadmium ions, with oleic acid as collector, has significantly values of removal efficiency at pH > 9, but a good separation efficiency (≥ 99%) is obtained at pH values ≥10.The calculated pH interval in which cadmium ions precipitates as hydroxide, at the working concentrations, is 8.69 -10.17 and so the separation process, at pH >10, is a precipitate flotation process.

Effect of molar ratio oleic acid:Cd(II)
The second studied factor was the effect of the molar ratio oleic acid:Cd(II) (Cc:Cco).As surfactant was used 1 M oleic acid solution in ethanol.Addition of ethanol as frother had the further advantage that the sizes of bubbles are smaller, because of the lower surface tension of the solution.The results are shown in figure 3. The increase of the molar ratio oleic acid:Cd(II) determines a fast increase of removal efficiency until Cc:Cco = 1 and a slowly decrease at Cc:Cco values greater than 1.25.
The cadmium separation by flotation, with oleic acid as collector, runs with high removal efficiency (over 97%) even Good removal efficiency (> 98%) was obtained at low air flow rates.The increase of air flow rate over 15 dm 3 /h determines a slow decrease of removal efficiency, on the studied interval, but not under 95%.

Effect of flotation time
The effect of flotation time on Cd(II) removal efficiency is shown in figure 5.It can be seen that the separation process is very fast.In only 3 min is achieved a removal efficiency greater than 95%, and increasing the flotation time over 9 min the efficiency is greater than 99%.

Effect of initial concentration of Cd(II) in solution
Another factor which influences the flotation process is the initial concentration of Cd(II) in solution.The results are shown in figure 6.By decreasing the Cd(II) concentration from 100 to 20 mg Cd(II)/dm 3 it can be observed a decrease of removal efficiency until 91%.

Conclusions
Using a precipitate flotation process and using oleic acid as anionic collector, we obtained high removal efficiency

Fig. 1 .
Fig.1.The flotation bench-scale equipment: 1-compressor; 2-cock for air flow rate adjustment; 3-rotameter for air flow rate determination; 4-3.3 cm inner diameter glass flotation column (60 cm in height); 5-porous glass frit (porosity G 4 ); 6-air inlet; 7-treated wastewater outletWorking procedure From stock solution with 2 g Cd(II)/dm 3 were prepared solutions with 100 mg Cd(II)/dm 3 , the pH was adjusted to the desired value by adding NaOH or HNO 3 solutions and

Table 1
OPTIMUM CONDITIONS FOR Cr(II) REMOVAL FROM SYNTHETIC SOLUTIONS BY PRECIPITATE FLOTATION WITH ANIONIC COLLECTOR (> 99%) of cadmium from aqueous solutions, in a very short time.A summary of the optimum conditions for the recovery of cadmium from aqueous solutions is presented in table 1.