Environ. Sci. Technol. 2000, 34, 4128-4132
Simultaneous Recovery of Metals and Destruction of Organic Species: Cobalt and Phthalic Acid SUSAN M. GRIMES,* JOHN D. DONALDSON, ABDUL J. CHAUDHARY, AND MUKHTAR-UL HASSAN Centre for Environmental Research, Department of Materials Engineering, Brunel University, Uxbridge, Middlesex, UB8 3PH, U.K.
In mixed industrial effluent, the presence of metal ions can retard the destruction of organic contaminants, and the efficiency of recovery of the metal is reduced by the presence of organic species. Results are presented for cobalt-phthalic acid in which both those effects occur. An electrochemical cell alone can be used to recover cobalt at pH 4.5 but is not capable of achieving complete mineralization of phthalic acid by anodic oxidation. A photolytic cell alone can achieve the destruction of phthalic acid at an optimum pH of 2.5 but leaves the metal ions in solution. A combined photolytic-electrochemical system using an activated carbon concentrator cathode is described that achieves the rapid simultaneous pHindependent destruction of phthalic acid and recovery of cobalt.
Introduction There are many situations where industry is producing effluent streams containing both heavy metal ions and organic pollutants. Successful treatment of effluents of this type to achieve legislative compliance will depend on whether the heavy metals affect the process of degradation of the organic species and whether the presence of organic molecules hinders the process of removal of the heavy metal. Although there are many possible methods of metal removal, including chemical precipitation (1), solvent extraction (2), ion exchange (3), cementation (4), membrane separation (5), liquid membrane separation (6), reverse osmosis (7), and fluidized bed electrolysis (8), there are practical limitations arising from the presence of organic pollutants in using most of the methods. Similarly, different methods have been used to destroy and reduce the levels of organic pollutants including treatment with activated sludge (9), chemical oxidation (10), biological oxidation (11), thermal degradation (12), ozonization (13), and photooxidation with ultraviolet radiation (14), but the efficiency of many of these methods is reduced in the presence of heavy metal ions because of complex formation between the metal and the organic species (15, 16). We now report on the effects of cobalt on the photolytic degradation of phthalic acid (PA) and on the effects of PA on the electrolytic recovery of cobalt and describe a combined photolytic-electrolytic cell system for the simultaneous removal of cobalt and degradation of PA to achieve total effluent cleanup. This mixture models * Corresponding author telephone: (01895)-256502; fax: (01895)203350; e-mail:
[email protected]. 4128
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the effluent treatment required in terephthalic acid (TPA) production (17).
Experimental Section The photolytic cell system used (section A of Figure 1) consists of UV probe (C) surrounded by a reaction chamber (D) of 3.5 dm3 capacity through which the fluid to be treated is pumped from a reservoir (E) via an inlet (F) and back to the reservoir via an outlet (G). Compressed air is used as the oxidant in the photolysis and is supplied through an inlet (H) and exits through an outlet (I). The temperature in the system can be measured by a digital thermocouple probe (J). The temperature in the reaction chamber is maintained at 25-30 °C by passing water through a cooling jacket (K) surrounding the UV probe. The effects of pH, UV source, cobalt ions, hydrogen peroxide, and TiO2 on the degradation of PA were studied. The effect of the UV source on the degradation of PA was studied by carrying out duplicate experiments under identical conditions using 150- and 400-W UV probes. Only 25% degradation is achieved after 10 h with the 150-W UV probe, but the corresponding value with the 400-W UV probe is 99.9%. All subsequent studies on the effects of cobalt ions, using hydrogen peroxide as an oxidant and TiO2 as a catalyst were therefore carried out using 400-W UV probe. The photocatalyst, used as supplied by BDH, was TiO2 (Degussa P-25), which is predominantly anatase as shown by X-ray diffraction with average particle size of 30 nm. The BET surface area of the TiO2, determined from nitrogen adsorption at -196 °C (ASAP 2000 Micromeritics) was 56.8 m2 g-1. The effect of TiO2 was investigated by using the semiconductor particles suspended in solution. The effect of an oxidant on the photodegradation of PA was investigated by using hydrogen peroxide solution (30-31%) supplied by BDH. The electrochemical cell system used (section B of Figure 1) consists of an electrolytic chamber (L) of 1.5 dm3 capacity through which the fluid to be treated is pumped from a reservoir (E) via an inlet (M) and back to the reservoir via an outlet (N). The cell contains two mixed metal oxide-coated titanium mesh anodes (O) and a single stainless steel plate cathode (P). The effects of the presence of PA on the deposition of cobalt were studied. The combined photolytic-electrochemical cell system used for the simultaneous destruction of PA and the removal and recovery of cobalt is shown in Figure 1. The electrochemical process was carried out at a constant current of 1.00 A. Samples were collected periodically from the reservoir tank to determine the levels of metal ions and of PA present. Model solutions (10 dm3) containing known concentrations of cobalt and PA were prepared by dissolving reagentgrade CoSO4‚5H2O and PA in distilled water. Reagent-grade sulfuric acid and sodium hydroxide were used to adjust the pH of the solution when required. The analysis of cobalt was carried out by atomic absorption spectroscopy (AAS), and the degradation of PA was followed by optical spectroscopy, total organic carbon analysis (TOC), and HPLC. The degradation was followed by a HPLC system using 25 cm × 3 mm i.d. separation column packed with Pinnacle ODS 5-µm particles, eluting the analyte from the column in 50:50 acetonitrile:buffer pH 3 eluent containing 1 g dm-3 centramide and using a UV detector to monitor the PA absorbance at 240 nm. The percentage recovery of cobalt was calculated from the weight of the cobalt recovered and analysis by AAS of the cobalt remaining in solution. The percentage degra10.1021/es990784k CCC: $19.00
2000 American Chemical Society Published on Web 09/01/2000
FIGURE 1. Schematic diagram for the simultaneous recovery of cobalt and destruction of PA.
TABLE 1. Effect of pH on the Photolytic Degradation of Phthalic Acid degradation of PA (%) TOC data
HPLC data
time (h)
pH 2.5
pH 4.5
pH 2.5
pH 4.5
2 4 6 8 10
29.7 47.2 65.5 78.8 97.4
25.7 41.9 58.7 72.8 87.9
30.3 48.8 66.3 79.4 98.6
26.4 42.3 59.4 73.1 88.2
dation of PA was calculated from the HPLC peak height data and by monitoring changes in total organic carbon with time.
Results and Discussion Photolytic Degradation of PA. An aqueous solution containing 50 mg dm-3 of PA has a natural pH of 2.5. A set of experiments was carried out to investigate the effect of pH (1.5-12.5) on the photolytic degradation rate of PA. The results show that the natural pH of the solution is the optimum value for total degradation of the acid and that almost complete mineralization at pH 2.5 is achieved after 10 h while TOC analysis shows that the concentration of organic carbon reduces to less than 1.5 mg dm-3 after 10 h of reaction time, corresponding to 97% degradation (Table 1). Increasing or decreasing the pH from the optimum value of 2.5 results in a decrease in the degradation rate of PA, and longer reaction time is required to achieve complete degradation. To permit the simultaneous recovery of cobalt and the destruction of PA, it is necessary to operate the combined system at pH where cobalt is electrodeposited efficiently. Since the optimum pH for recovery of cobalt is 4.5, the efficiency of the destruction of PA has tested over a range and particularly at this pH. Figure 2 shows the changes in the UV absorbance of PA and its intermediates with time at pH 2.5 and pH 4.5. At pH 2.5, the 8-h peak has disappeared completely, which corresponds to a complete degradation of PA, whereas at pH 4.5 the 8-h peak has reduced to an
FIGURE 2. UV absorbance of PA and its intermediates with time using a photolytic cell system at (a) pH 2.5 and (b) pH 4.5. absorbance value of 0.1 and a further 2 h is required to achieve complete destruction mineralization. The data do, however, confirm that the destruction of PA can be achieved at pH 4.5. These results were also confirmed by HPLC and TOC analyses. The effects of the presence of cobalt ions on the percentage degradation of PA at pH 2.5 and pH 4.5 and with cobalt concentrations of 500 and 100 mg dm-3 are presented in Figure 3. pH 4.5 was chosen for study because it is within the VOL. 34, NO. 19, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Effect of Cobalt, H2O2, and TiO2 on the Degradation of Phthalic Acid Using a Photolytic Cell System at pH 2.5 degradation of phthalic acid (%) absence of oxidant or catalyst
H2O2 (1.5 g dm-3)
TiO2 (1 g dm-3)
time (h)
no Co
Co (500 mg dm-3)
no Co
Co (500 mg dm-3)
no Co
Co (500 mg dm-3)
2 4 6 8 10
30.0 48.0 66.0 79.0 98.0
6.2 38.0 48.0 68.0 70.0
42.6 75.5 89.9 99.3 99.9
29.9 45.8 80.5 96.1 99.9
55.9 85.6 97.3 99.9 99.9
39.9 65.8 85.7 96.9 99.9
TABLE 3. Effect of pH on Electrooxidation of Phthalic Acid and Recovery of Cobalt Using an Electrochemical Cell System degradation of phthalic acid and recovery of cobalt (%) pH 2.5
FIGURE 3. Effect of cobalt ions on the photolytic degradation of 50 mg dm-3 of PA at (a) pH 2.5 and (b) pH 4.5: ([, absence of cobalt; 2, presence of 100 mg dm-3 of cobalt; f, presence of 500 mg dm-3 of cobalt). optimum for the electrochemical removal of cobalt from solution. Complete mineralization is not achieved even after 10 h at either pH. This decrease in PA degradation is probably due to complex formation between cobalt and the acid, and the results show that a photolytic process alone cannot be used to destroy the organic species in the presence of the metal ions and in the absence of a catalyst. The cobalt complex with phthalic acid, [Co(C8H4O4)]H2O, is known to be stable (18), and any complex formation must alter the susceptibility of the organic molecule to photolytic degradation, for example in Mn-EDTA complexes (19). Both optical and infrared spectroscopic data show that there is an interaction between cobalt and PA; we interpret the changes in the efficiency of the destruction of the organic molecules in the presence of cobalt to the effects of these interactions on the bond systems responsible for the photolytic absorption. The reduction in the extent of degradation of PA in the presence of cobalt can be largely overcome by the addition of hydrogen peroxide or of titanium dioxide as a heterogeneous catalyst. The effects of hydrogen peroxide were determined by adding different volumes (1.5-4.5 g dm-3) of 30% hydrogen peroxide to a solution containing the same initial concentration of PA. The results (Table 2) show that the percentage degradation is increased in the presence of 1.5 g dm-3 of hydrogen peroxide, presumably because of the ease of free radical formation. The addition of more concentrated hydrogen peroxide solutions did not increase the percentage degradation presuamably because the 1.5 g dm-3 solution was sufficient to sustain the necessary free radical reaction. Similarly the addition of TiO2 (1 g dm-3) increases the percentage degradation of PA by catalyzing the photolytic reaction. Photodegradation was conducted in the presence of TiO2 and or H2O2, using batch reactors with an immersed lamp. 4130
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pH 4.5
time (h)
PA degradation
Co recovery
PA degradation
Co recovery
2 4 6 8 10
1.0 1.0 1.0 2.0 3.0
2.0 5.0 9.0 11.0 15.0
1.0 2.0 2.0 2.0 4.0
16.5 40.2 52.3 61.6 70.4
The advantages of photodegradation lie in the complete mineralization of the organic substrate with the absence of dangerous residual intermediate compounds. The degradation of organic contaminants takes place by reaction with hydroxyl radicals. These hydroxyl free radicals are produced by hole capture of either water molecules or OH- anions adsorbed on the semiconductor surface or due to the homolytic dissociation of H2O2 in the presence of UV light. The OH radical is one of the most powerful oxidant species and is capable of oxidizing a wide variety of organic contaminants. In the presence of a suitable semiconductor material, such as TiO2 as a heterogeneous catalyst, the hydroxyl radical (•OH), the primary oxidant in the photocatalytic oxidation process, can be generated via two main routes: by the reaction of valence band holes (h+) with either adsorbed water or with surface hydroxyl groups:
semiconductor + hν f h+ + eh+ + H2O(ads) f •OH + H+ h+ + OH-(surface) f •OH The addition of H2O2 to a reaction solution has the effect of increasing the degradation rate. An important step in the formation of a radical species is the cleavage of H2O2 in the presence of UV light. The addition of H2O2 increases the rate of radical formation (both •OH and HO2•) and ultimately the degradation rate:
H2O2 + hν f HO2•- + H+ H2O2 + hν f 2•OH Electrochemical Oxidation and Removal of Cobalt. The anodic oxidation of various organic species, in the absence of metal ions, has been investigated using different types of electrode materials (Pt, Ti/IrO2, Ti/SnO2). It was found that some selective oxidation can be achieved by using different types of mixed metal oxide anodes. Anodic oxidation occurs
TABLE 4. Effect of H2O2 and TiO2 on Degradation of Phthalic Acid and Recovery of Cobalt Using an Electrochemical Cell System at pH 4.5 degradation of phthalic acid and recovery of cobalt (%) H2O2 (1.5 g dm-3)
absence of oxidant or catalyst
TiO2 (1 g dm-3)
time (h)
PA degradation
Co recovery
PA degradation
Co recovery
PA degradation
Co recovery
2 4 6 8 10
1.0 2.0 2.0 2.0 4.0
16.5 40.2 52.3 61.6 70.4
4.0 6.0 6.0 8.0 10.0
37.5 58.2 64.4 74.3 85.1
1.0 2.0 2.0 5.0 8.0
18.5 38.9 42.8 54.5 64.9
difference in the transport of hydrated cobalt and the complexed cobalt ions under the influence of the electric field. A 15% decrease in the electrolytic recovery of cobalt, in the presence of PA, is a clear indication that there is a difference in the transport of hydrated cobalt and complexed cobalt ions toward the cathode. Combined Photolytic-Electrolytic Cell System. The results from the photolytic cell and electrochemical cell systems confirm that a combined photolytic-electrolytic cell system is required to achieve the simultaneous destruction of organic species and recovery of heavy metals. Since cobalt cannot be recovered from solutions at pH 2.5, the pH chosen for the combined system was 4.5. The results from the combined system show that a complete mineralization of PA in the absence of metal ions can be achieved in 6 h, as compared to a photolytic system alone, which requires 10 h to achieve the same level of degradation because of the combined effects of photolytic and anodic oxidation reactions in the system. In the presence of cobalt ions, 90% degradation of PA and 82% recovery of cobalt can be achieved in 10 h in the combined system at pH 4.5. Addition of hydrogen peroxide (1.5 g dm-3) further increases the rate of degradation of PA and slightly increases the percentage recovery of cobalt giving complete PA degradation and 88% cobalt removal after 10 h. This increase in PA degradation is attributed to the continuous cobalt removal at the cathode, which ultimately reduces its effect on the photolytic oxidation. Little advantage can be gained by the use of TiO2 as a heterogeneous catalyst, but it leads to a decrease in the percentage recovery of cobalt. Complete PA degradation in the presence of TiO2 can be achieved, but only 65% of cobalt is recovered in the same time. The data for the simultaneous destruction of PA and recovery of cobalt are given in Table 6. Activated Carbon Concentrator Cathode System. The simultaneous destruction of PA and recovery of cobalt can be further improved in the combined system by replacing the cathode with an activated carbon concentrator cathode (21). The results (Table 7) show that the combined system with activated carbon concentrator cathode system is capable of destroying PA and at the same time recovering cobalt from mixed effluent streams at pH 2.5 and pH 4.5. The
TABLE 5. Effect of Phthalic Acid on Recovery of Cobalt Using an Electrochemical Cell System at pH 4.5 recovery of cobalt (%) absence of phthalic acid
presence of phthalic acid
time (h)
Co (100 mg dm-3)
Co (500 mg dm-3)
Co (100 mg dm-3)
Co (500 mg dm-3)
2 4 6 8 10
32.6 58.7 79.3 92.7 99.9
17.5 42.5 62.5 79.9 85.3
20.6 57.2 71.2 86.4 96.5
16.5 40.2 52.3 61.6 70.4
at electrodes at the surface of which OH free radical can be accumulated (20). An electrochemical system can be used for both metal recovery and electrooxidation of organic contaminants. The electrodeposition of metal ions from an aqueous electrolyte is a pH-dependent process. In some cases, the pH of the solution must be kept constant to achieve the highest percentage removal of metal ions, and a pH in the range 4-5 has been found to be the optimum value for the removal of cobalt (8). A set of experiments was carried out using the electrochemical cell system to deposit cobalt ions on the cathode surface and to oxidize PA at the anode surface. The solution used contained 500 mg dm-3 of cobalt and 50 mg dm-3 of PA, and experiments were carried out at pH 4.5 and pH 2.5, the optimum pH for photolytic degradation of PA. The results obtained show that little cobalt removal or PA degradation was achieved at pH 2.5 but that 70% removal of cobalt ions along with 4% degradation of PA was achieved at pH 4.5 (Table 3). Addition of hydrogen peroxide as an oxidant or of TiO2 as a heterogeneous catalyst is found to have very little effect on the anodic oxidation of PA. The recovery of cobalt is, however, increased in the presence of hydrogen peroxide, but the addition of titanium dioxide decreases metal recovery (Table 4). The effects of the presence of 50 mg dm-3 of PA on the percentage recovery of cobalt at pH 4.5 and with cobalt concentration of 500 and 100 mg dm-3 are shown in Table 5. The results show that the recovery of cobalt decreases in the presence of PA and the decrease is attributed to the
TABLE 6. Effect of H2O2 and TiO2 on Degradation of Phthalic Acid and Recovery of Cobalt Using a Combined Photolytic-Electrochemical Cell System at pH 4.5 degradation of phthalic acid and recovery of cobalt (%) absence of oxidant or catalyst
H2O2 (1.5 g dm-3)
TiO2 (1 g dm-3)
time (h)
PA degradation
Co recovery
PA degradation
Co recovery
PA degradation
Co recovery
2 4 6 8 10
23.9 40.2 65.6 88.9 90.2
20.9 38.9 58.9 79.5 82.5
30.0 60.0 80.0 95.0 99.9
25.5 37.5 62.4 75.4 87.9
35.0 75.0 90.0 99.9 99.9
25.3 40.3 55.3 64.5 65.1
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TABLE 7. Effect of pH on Degradation of Phthalic Acid and Recovery of Cobalt Using an Activated Carbon Concentrator System and a Combined Photolytic-Activated Carbon Concentrator System degradation of phthalic acid and recovery of cobalt (%) activated carbon concentrator cell system pH 2.5
combined photolytic-activated carbon concentrator system
pH 4.5
pH 2.5
pH 4.5
time (h)
PA degradation
Co recovery
PA degradation
Co recovery
PA degradation
Co recovery
PA degradation
Co recovery
2 4 6 8 10
27.9 63.5 77.6 89.9 99.9
24.0 56.0 65.0 78.0 89.0
23.8 43.2 62.6 71.6 97.1
40.2 55.5 76.5 95.2 99.9
47.5 69.9 81.5 93.7 99.9
29.0 48.0 68.0 82.0 91.0
32.2 45.2 69.1 97.4 99.9
35.5 66.3 82.7 97.3 99.9
The change in the concentration of PA with time was followed, and the results in Figure 4 show that a combined photolytic-electrolytic system can be used for the degradation of organic pollutants and the recovery of metal ions and that the extent of degradation is increased by combining the photolytic and activated carbon concentrator cell system.
Acknowledgments We thank EPSRC/Environmental Technology Best Practice Programme (ETBPP) and Fluid Dynamics International Ltd. for a grant under the Link (WMR 3) programme. The Government of Pakistan, Ministry of Education, is thanked for financial support to M.-U.H.
Literature Cited
FIGURE 4. Comparison of the change in concentration of PA with time using different systems: (9, combined photolytic-electrolytic system; b, activated carbon concentrator system; 1, combined photolytic-activated carbon concentrator system). purpose of the concentrator cathode is to increase the concentration of cobalt ions near the electrode surface, which leads to increase the efficiency in metal recovery. The concentrator cell has three further advantages: (i) in-situ regeneration of activated carbon occurs in the system due to the production of H3O+ ions at the anode surface; (ii) the effectiveness of activated carbon concentrator cathode is independent of pH of the solution, and cobalt can be recovered at pH 2.5; and (iii) complete effluent cleanup can be achieved rapidly without the use of additional oxidants or catalysts. The results of this work show that an electrochemical cell system alone can be used to recover cobalt ions at pH 4.5 but is not capable of achieving the anodic degradation of PA. On the other hand, use of a photolytic cell system alone can achieve PA degradation but leaves cobalt ions in solution. The use of a combined photolytic-electrochemical cell can however lead to the simultaneous destruction of PA and the recovery of cobalt only at pH 4.5, but replacement of the standard cathode in the electrochemical cell with an activated carbon concentrator cathode gives an efficient system for the rapid simultaneous destruction of PA and recovery of cobalt that is independent of pH. 4132
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Received for review July 14, 1999. Revised manuscript received May 30, 2000. Accepted June 9, 2000. ES990784K
