Removal of Toxic Metal Ions in Water by Photocatalytic Method

Aims: To study the photocatalytic method for decreasing the concentration of Cu(II), Cd(II), Cr(VI), and Pb(II) in the solutions. Study Design: The photocatalytical process was proceeded for single solutions containing respective Cu(II), Cd(II), Cr(VI), and Pb(II) ions. Reaction time, photocatalyst dose, and solution pH were optimized. The influence of Cr(VI), Cd(II) and Pb(II) ions on the Cu(II) removal was also evaluated. Place and Duration of Study: Laboratory of Analytical Chemistry, Chemistry Department, Gadjah Mada University, Indonesia. June 2013 – January 2014. Methodology: Photocatalytic process was carried out by irradiating 100 ml of 4 single solutions containing 10 mg/L of Cu(II), Cd(II), Cr(VI) and Pb(II) added with TiO2 with UV lamp having 290390 nm of the wavelength, for certain time. The same procedure was also taken place with variation of photocatalyst dose (10, 25, 50, 75, and 100 mg for 100 ml solution), irradiation time (1, Original Research Article Wahyuni et al.; ACSj, 5(2): 194-201, 2015; Article no.ACSj.2015.019 195 3, 5, 10, 25, 50, and 75 h), and solution pH (1, 3, 5, 7, 9, and 13), and for 3 solutions containing Cu(II), that was added with Cd(II), Cr(VI), and Pb(II) respectively. The concentrations of Cu(II), Cd(II) and Pb(II) ions in the solutions were measured by AA Spectrophotometer and by spectrophotometer UV-Visible for Cr(VI) ion. Results: The maximum effectiveness of the photocatalytic process can be obtained by using 50 mg of TiO2, in 24 h and at pH 5, that can take Cu(II), Cr(VI), Cd(II) and Pb(II) out from the solutions as high as 45.56%, 77.72%, 15.23%, and 40.32%, by photocatalytic reduction, adsorption on TiO2 and photocatalytic oxidation, respectively. The presence of Cr(VI) and Cd(II) can decrease Cu(II) photo reduction from 45.36% into 15.24% and into 40.53%, meanwhile Pb(II) improves the photo reduction from 45.56 into 58.76% for low Pb(II) concentration. Conclusion: The concentration of the toxic Cu(II) and Cr(VI), Cd(II), and Pb(II) ions can be successfully decreased by photocatalytic process through different mechanisms. The removal of the metal ions was controlled by photocatalyst dose, irradiation time, and solution pH. The presence of Cr(VI) and Cd(II) can inhibit Cu(II) photoreduction due to the competition, meanwhile Pb(II) with low concentration promotes Cu(II) photoreduction by synergic effect.


INTRODUCTION
Several heavy metal ions such as Cd(II), Cr(VI), Cu (II) and Pb(II) can be distributed in the environment from wastewater disposal of some industries including electroplating activities, paint, electric equipment, nylon and plastic factories. Since all metal ions are hazard for human and environment [1], efforts for removal the metal ions from wastewater have attrached much attention.
Removal of Cd(II), Cr(VI), Cu (II) and Pb(II) ions has been intensively studied by conventional adsorption method, i.e by using zeolite [2], activated carbon [3], and biomass [4]. This method is normally simple and effective, but it is unable detoxify the hazardous ions, except the method only transfers the ions from the solution into the solid adsorbent. In addition, when the adsorbent is saturated with the metal ions, it becomes a solid waste that lead to a further environmental problem. Hence, a treatment method which is able to remove as well as detoxify the hazardous ions is required. Such method may be satisfied by photocatalytic reduction-oxidation.
Photocatalytic reduction and photocatalytic oxidation are reduction and oxidation induced by photon or UV light and sensitized by photo catalyst such as TiO 2 . This oxide can be functioned as a photo catalyst due to its semiconductor structure, that is a structure characterized by electron filled valence band and empty conduction band, separated by a gap called as band gap energy as much as 2-3.5 eV [5]. With such structure, when a semiconductor is irradiated by UV or visible light, one electron in the valence band can be transferred into conduction band by leaving a hole symbolized as h + which is actually a positive radical. When the hole contacts with water and TiOH photocatalyst surface, OH radical is formed. The photo generation of electron and hole, and formation of OH radical are written as reactions (1), (2) The OH radical can act as a strong oxidizing agent that has been tested for removing pesticides [6] and phenolic pollutants [7] through photodegradation, as well as Pb(II) [8] from the solutions through photooxidation mechanism. Meanwhile, the electron is widely used for reducing some metal ions, including Hg(II) [9,10] and Cr(VI) [11,12], as well as for Ag recovering from photography wastewater [13]. It was reported that hazardous Hg(II) and Cr(VI) could be successfully reduced into less or non toxic Hg(0) and Cr(III), respectively. Meanwhile, Ag(I) could be reduced into Ag(0) deposited on the photocatalyst surface, enabling it to be easily taken out as pure and valuable silver metal. Moreover, the study on photocatalytic reduction of Cu(II) in the presence of TiO 2 for removing the ion from its solution has also been reported [14,15]. However, a comparison study on the removal of Cu(II), Cr(VI), Cd(II) and Pb(II) ions by photocatalytic technology has not been explored yet. In the present paper, the removal of Cu(II), Cr(VI), Cd(II) and Pb(II) ions by photocatalytic technology and the study on the influence Cr(VI), Cd(II) and Pb(II) with varying concentration and solution pH on the photoreduction of Cu (II) ion are reported.

Materials and Instruments
TiO 2 powder, Cu(NO 3 ) 2 .3H 2 O, K 2 Cr 2 O 7 , CdCl 2 , Pb(NO 3 ) 2 , HCl and NaOH pellet purchased from E.Merck, with analytical grade were used without further purification. The instruments used were a photoreaction apparatus having dimension of 50x150x 50 cm 3 , installed with UV Lamp and magnetic stirring plate as illustrated by Fig. 1, Perkin-Elmer atomic absorption spectrophotometer (AAS) for analyzing Cu, Cd, and Pb in the solutions and GBC Cintra 100 UV/Visible spectrophotometer to determine Cr(VI) ions in the solution.

Photocatalytic Experiment
Removal of Cu(II) in the solution was carried out in a closed reactor equipped with a 40 watt UV lamp having wave length in the range of 290-390 nm. For that purpose, 100 ml solution containing 10 mg/L, that is equal to 0.16 mmole of Cu(II) with alteration pH as designed, was mixed with 50 mg TiO 2 , then the suspension was irradiated by UV light in the photoreaction apparatus for 24 h. The solution obtained by filtering the suspension, was analyzed by Perkin-Elmer AAS to determine the concentration of unreduced Cu(II) ions. By subtracting the initial and unreduced concentrations of the ions, the degree of Cu(II) photoreduction could be calculated. The removal of Cr(VI), Cd(II) and Pb(II) as a single solute in the solution was performed by using the same procedure as carried out for Cu(II) removal. The influence of respective Cr(VI), Cd(II) and Pb(II) on the photoreduction catalytic of Cu(II) was performed by irradiating 3 serial suspension of Cu(II) solutions suspended with TiO 2 that was added by Cr(VI), Cd(II) and Pb(II) respectively. The next step is same as above.

Removal of the Metal Ions by Photocatalytic Process Using TiO 2 Photocatalyst
The results of the photocatalytic process is illustrated by Fig. 2, indicating that the concentration of Cu(II), Cr(VI), Cd(II), and Pb(II) ions in the solution can be decreased. The most effective decreasing is found by Cr(VI), following by Cu(II), Pb(II), and Cd(II). The decrease of the concentration of Cr(VI) [11,12] and Cu(II) is caused by photocatalytic reduction [14,15]. The photoreduction of Cr(VI) and Cu(II) is generated by capturing the electrons released via photolysis of water and TiO 2 photocatalyst [3], as shown by reaction (1), (4), and (5):

Fig. 2. Removal of Cu(II), Cr(VI), Cd(II) and Pb(II) ions in the various conditions of process
The effectiveness of the photocatalytic reduction of Cr(VI) is higher than that of Cu(II) which is consistence with their reduction potential values,

Metal ions removal 1) Cu(II), 2)Cr(VI), 3)Cd(II), 4)Pb(II)
Metal ion removed (%mol) light+TiO2 TiO2, no light Light, no TiO2 as written as reaction (4) and (5). The higher potential reduction represents the easiness of the ions to be reduced. Meanwhile in the dark process, decreasing concentration of Cr(VI) is lower than that of Cu(II) ion. In the dark condition, the removal is induced by adsorption on TiO 2 surface that provide negative charge as active sites and in such condition, a photoreduction is unlikely to be occurred. It is implied that the adsorption of Cu(II) as cationic on the surface of TiO 2 is favorable than an anionic CrO 4 2of Cr(VI) ion. This fact can be understood, as known well that the negative sites prefer to attach the positive ion or cationic.
The lowest removal of Cd(II) from the solution by photocatalytic process is supposed to be caused by adsorption on the surface of TiO 2 . Because Cd(II) cannot be reduced, as indicated by negative standard reduction potential (E°), as seen in reaction (6). The adsorption of Cd 2+ takes place on the surface of TiO 2 that has more electrons.
The adsorption of Cd 2+ is also supported by data obtained in the dark (no light) condition in the presence of TiO 2 , showing that in such condition the concentration of Cd(II) goes down as much as in the lighted process. It is strongly proved that photoreduction or photooxidation of Cd(II) ions does not occur.
Meanwhile, by photocatalytic process, the declining concentration of Pb(II) may be due to photooxidation induced by positive radicals of h + provided by TiO 2 photocatalyst. In the solution, Pb(II) ion exists as Pb 2+ that is ulkiley to be reduced but is possible to be oxidized into Pb(IV) formed as PbO 2 [8]. The photooxidation of Pb(II) is written as reaction (7) and (8), that also shows negative reduction potential value, indicating that the Pb(II) is not reducible ion, but it is oxydizable one. In addition, the low value of Pb(II) removal indicates that oxidation of Pb(II) ion takes place slowly, explaining the low decreasing Pb(II) concentration.

The Influence of Solution pH
The degree of metal ions removal at pH alteration is shown by Fig. 3. It is seen in the figure that in the solution with increasing pH from 1-3, the removal degrees of all metals raise slightly, and sharp improvement is observed when the pH is increased up to 5. Increasing solution pH from 5 to 7, causes the degree of Cr(VI) removal sharply declined, while that of Cu(II), Cd(II) and Pb(II) enlarge drastically. Further increasing pH up to 14, makes the removal degree of Cr(VI) further decreased, meanwhile the removal degrees of Cu(II), Cd(II), and Pb(II) remain constant.

Fig. 3. The influence of the solution pH on the toxic metal ions removal by photocatalytic process
In the solution having pH 1-5, all of Cu(II) exists as Cu 2+ as seen in Fig. 4 of Copper Pourbaix diagram [16], that can be reduced into Cu(s). Based on this speciation, the degree of the removal is supposed to be same, but the degree is improved. The improvement may be induced by the speciation of TiO 2 surface. At pH 1-4, the surface of TiO 2 in the aqueous solution is found as a mixture of TiOH 2 + and TiOH, in which TiOH. is more difficult to release electrons, than TiOH 2 + . Increasing pH from 1 to 4, gives an increase in the fraction of TiOH, and TiOH is entirely found at pH 4-8 [5]. The increase of TiOH fraction can provide more electrons in the solution, explaining the rise of Cu(II) photo reduction degree. The sharp improvement of the removal degree as the pH increases from 3 to 5, is promoted by a lot of electrons provided by TiO 2 presence entirely as TiOH. The further sharp increasing in the Cu(II) removal degree when the pH was increased from 5 to 7, is caused by solid or precipitate of Cu(OH) 2 formation as seen in Fig. 4. The precipitate may cover the surface of TiO 2 solid, that is not found in the solution and undetected during the analysis by AAS. This explanation is supported by the fact that at pH 7-14, where Cu(II) ion is completely formed as precipitate of Cu(OH) 2

Solution pH
Metal ion removed (% mol)

Cu(II) Cr(VI) Cd(II) Pb(II)
that the high removal at high pH is not caused by photoreduction, but due to the precipitation.
Species of Cr(VI) in the solution with pH 1 found as HCrO 4 which is in equilibrium with Cr 2 O 7 2as illustrated by Fig.4 b and 4 c. The species of Cr 2 O 7 2in solution with low pH can be easily reduced as indicated by high standard reduction potential as presented by reaction (5). The improvement of the removal degree as the increasing pH from 1 to 5, is resulted by increasing number of TiOH that can provide more electrons, as discussed above. degree of Cr(VI) removal from the solution with pH higher than 5 can be caused by the formations of CrO 4 2-, TiOof the TiOH surface, and Cr(OH) 3 precipitate. The species of formed at pH higher than 6, as shown by Fig. 4b, that is lesser reducible than HCrO 4 as indicated by the lower standard reduction potential as seen in reaction (9). As a consequency, the lower photoreduction is obtained.
Cr(OH) E 0 = +0,13 volt (9) In addition, at pH higher than 5, resulted from Cr(VI) photoreduction precipitated to form Cr(OH) 3 . The precipitate can inhibite the penetration of UV light into the solution, leading to the less effective photoreduction.
Based on the species of TiO 2 surface, at pH 5 TiOH is formed that can provide more electrons. This condition should give high photoreduction degree. It seems that in this condi photocatalyst does not play significant role. At pH higher than 8, TiO 2 surface is found as TiO less electrons, that can lead photoreduction decreased. In the such range pH, it seems that the species of TiO 2 contributes on the decrease of the Cr(VI) removal degree.
In the case of Cd(II), Cd 2+ is completely formed in the solution with pH from 1-8,as illustrated by Fig. 5 b. The cation is easy to be adsorbed the surface of TiO 2 . Based on this speciation, the degree of the removal is supposed to be same. The change of the removal degree with the change of pH must be controlled by the species of TiO 2 surface. At low pH, the surface of TiO 2 existing as TiOH + that may inhibit adsorption of Cd 2+ and as TiOH that prefers to ACSj, 5(2): 194-201, 2015;Article no. 198 igh removal at high pH is not caused by photoreduction, but due to the precipitation. lution with pH 1-5, is which is in equilibrium with as illustrated by Fig.4 b and 4 c. The in solution with low pH can be easily reduced as indicated by high standard reduction potential as presented by reaction (5). The improvement of the removal degree as the increasing pH from 1 to 5, is resulted by increasing number of TiOH that can provide more electrons, as discussed above. Decreasing degree of Cr(VI) removal from the solution with can be caused by the the TiOH surface, . The species of CrO 4 2is 6, as shown by Fig. 4b,   4 and Cr 2 O 7 2-, as indicated by the lower standard reduction potential as seen in reaction (9). As a consequency, the lower photoreduction is Cr(OH) 3 (s) + 5 OH -= +0,13 volt (9) In addition, at pH higher than 5, Cr(III) ion resulted from Cr(VI) photoreduction is . The precipitate can UV light into the solution, leading to the less effective surface, at pH 5-8, TiOH is formed that can provide more electrons. This condition should give high photoreduction degree. It seems that in this condition, the photocatalyst does not play significant role. At pH surface is found as TiO giving less electrons, that can lead to the photoreduction decreased. In the such range pH, 2 surface also s on the decrease of the Cr(VI) is completely formed 8,as illustrated by easy to be adsorbed by ased on this speciation, of the removal is supposed to be . The change of the removal degree with the change of pH must be controlled by the surface. At low pH, the surface of that may inhibit the and as TiOH that prefers to adsorb the cation. As the pH increases, the fraction of TiOH + decreases followed by the increase of TiOH fraction, promoting higher adsorption. At pH higher than 8, all of Cd (II) has precipitated as Cd(OH) 2 , and deposited on t surface of TiO 2 that cannot be measured in the solution. By AAS. It is obvious that the high degree of Cd (II) removal at high pH is through adsorption, but by precipitation. As the pH increases, the decreases followed by the increase of TiOH fraction, promoting higher adsorption. At pH higher than 8, all of Cd (II) has , and deposited on the that cannot be measured in the AAS. It is obvious that the high Cd (II) removal at high pH is not , but by precipitation. that can be oxidized by OH radicals from water and photocatalyst [5] and the same degree of removal should be resulted. Thus, the enhance of the removal degree, as explained previously, is promoted by the increase of TiOH fraction in the solution. It takes the note that TiOH is easier to release electron as well as OH radical. In the solution with further increasing pH from 8 to 14, the species of Pb(II) is found as Pb(OH) 2 precipitate, that may be stacked on the surface of TiO 2 , that is undetected by AAS during analysis. It is obvious that the high degree of Pb(II) removal at high pH is promoted by precipitation as happen in Cu(OH) 2 and Cd(OH) 2 .

The Influence of Other Metal Ions on Cu(II) Photocatalytic Reduction
In wastewater, Cu(II) ions can be along together with some heavy metal ions such as Cr(VI), Cd(II), and Pb(II  further increased. In the photocatalytic reaction, Pb 2+ ion is oxidized simultaneously with the reduction of Cu 2+ , promoting in the synergic effect. Such effect is implied by the increase of Cu 2+ photoreduction. On the other hand the oxidation of Pb 2+ forming PbO 2 solid can inhibite the light penetration, that can reduce the production of OH radical for oxidation. The higher Pb(II) concentration can result in more PbO 2 giving more inhibition. It is clear that in the low Pb(II) concentration the synergic effect is prominent, but in the higher Pb(II) concentration, the inhibition effect plays stronger role.

CONCLUSION
It is concluded that the removal of Cd(II), Cr(VI) as well Cu(II) and Pb(II) ions from the water has been succesfully carried out by respective adsorption, photoreduction and photooxidation. The removal of the studied metal ions is controlled by pH solution, and the most effective removal is achieved at pH 5. The presence of Cr(VI) and Cd(II) with all range given concentrations lead to a decrease in the removal of Cu(II) by competition in photoreduction and adsorption respectively, meanwhile synergic effect, that enhances the Cu(II) photoreduction, is shown by Pb(II) ion in the low concentration but contrary effect is observed for high Pb(II) concentration. However, the mutual effect of Cd(II), Cu(II), and Pb(II) on the photoreduction of Cr(VI) has not been evaluated, so it is suggested to be further assessed.