Immobilized Thiosalicylic Ligand System Potentials for the Detoxification of Some Heavy Metals from Tannery Wastewater

Polysiloxanes are characterized by silicon and oxygen backbone and are hydrophobic in nature with low moisture uptake widely used for medical applications. Polysiloxane immobilized thiosalicylic acid ligand system has been prepared through sol-gel method, gelation was observed after 40 minutes. The immobilized ligand was characterized using FTIR, the spectrum showed characteristic absorption bands (cm-1) at: 3377 (OH), 2981 (C-H stretch); 2631 (SH); 1587 to 1684 (C=C, C=O); 1032 to 1144 (Si - O) respectively. SEM analysis showed irregular particle sizes of the polysiloxane matrices while EDX elemental composition gave (wt %): 3-CPPS; Si (50.45), O (25.02) and Cl (24.57). F - 3CPPS showed, O (58.68), Si (41.32); thiosalicylic (7.14 of S). The extraction of metal ions (Cr3+, Fe3+, Pb2+, Cu2+ and Zn2+) were studied using Microwave Plasma Atomic Emission Spectrophotometer (Agilent MPAES-4200) at pH 6.0. Thermodynamic range with respect to Cr3+, Fe3+, Pb2+, Cu2+ and Zn2+ yielded negative values forΔGo: Cu2+-(11.483 to 14.842) to Zn2+ -(14.368 to 14.842)KJmol-1; positive values for ΔHo: Fe3+ (0.000) to Pb2+ (105.130)KJmol-1 and ΔSo: Zn2+ (47.421) to Pb2+ (389.328)Jmol1K1 respectively, indicating spontaneous, endothermic reactions and high degree of disorderliness with respect to metal ion binding capacity to the ligand system.


Introduction
Leather industry generates harmful wastes into water bodies [1]. During tanning alone about 300 kg of chemicals are added per ton of hides or skins with large volume of water and sludge generated [2,3]. Not more than 20% of the chemicals are absorbed by leather; the remainder flows out with the effluent [4]. Conventional methods used have limitations such as production of toxic sludge [5] and inability to remove heavy metals at trace level, this prompted the use of polymeric modified surfaces with good thermal, mechanical and chemical stability properties such as polysiloxane immobilized with ligands [6] have been employed as a recyclable extractants for heavy metals. These immobilized ligand system could be synthesized directly by sol gel or by chemical modification of the prepared functionalized polysiloxane [6]. A variety of analytical techniques have been employed such as Fourier Transform Infra-red (FTIR) [7,8], Nuclear Magnetic Resonance (NMR), Scanning Electron Microscopy (SEM) [9,10] and Energy Dispersive X-ray Analysis (EDX) [9,10]. This study described the Immobilization of thiosalicylic acid ligand system and its potential in the purification of tannery wastewater.

Location of the Study Area
Kano state covers an area extending between latitudes 120° 40' and 100° 30' and longitudes 70° 40' and 90° 30'. Climate is tropical wet and dry with mean annual rainfall of 850 mm, and a population of 9.3 million, it is a flat city drained by the Jakara river and several streams (Niger river watershed) and River Challawa (Lake Chad watershed) all are severely polluted by urban and industrial effluents. Kumbotso local government is the area of study and it lies between latitudes 11°50'S to 12°N and longitude 8°24'W to 8°40'E. It falls within the Kano State settlement zone bordering the south and west by Madobi Local Government Area, in the Northern west; Rimin-Gado, in the North by Gwale and East by Tarauni local government areas respectively [11,12].

Digestion of Tannery Effluent
Tannery wastewater sample of 1000 cm 3 was transferred into a conical flask and evaporated till dried. The dried sample was digested in 10:1 HNO 3 :HClO 4 (v/v). White crystals were found in the digested samples and were dissolved in 150 ml double distilled water. The supernatant were filtered using Whatman No.41 filter paper and was read directly with Agilent MPAES-4200 [15]. The total metal contents were determined as described by Parven et al., [16]. No specific speciation was conducted to ascertain the various valencies of the species at the pH values of 2, 4, 6, 7 and 9 as used in this study.

Effect of Adsorbent
A volume of 60 cm 3 solution of the tannery wastewater adjusted at pH 6 (optimum) was transferred into 150 cm 3 conical flask and 10 mg of the polysiloxane immobilized thiosalicylic ligand was added and adjusted in a thermostatic multi-shaker (Gallenkamp Model) at 100 rpm for 2 h at 30°C. The resultant solutions were filtered using Whatman No.41 and the residual metal concentrations analysed (Cr 3+ , Fe 3+ , Pb 2+ , Cu 2+ and Zn 2+ ) using Agilent MPAES-4200 [17,18,19] This procedure was repeated for 20 and 30 mg of polysiloxane immobilized thiosalicylic ligand with mesh size of (125-150 µm) respectively.

Thermodynamic Studies
A volume of 60 cm 3 solution of the tannery wastewater adjusted at pH 6 (optimum) was transferred into 150 cm 3 conical flask and 20 mg of the polysiloxane immobilized thiosalicylic ligand was added and adjusted in a thermostatic multi-shaker (Gallenkamp Model) at 100 rpm for 2 h at 30°C. The resultant solutions were filtered using Whatman No.41 and the residual metal concentrations analysed (Cr 3+ , Fe 3+ , Pb 2+ , Cu 2+ and Zn 2+ ) using Agilent MPAES-4200. [17,19,20]. This procedure was repeated for temperatures of 35 and 40°C respectively.

Results and Discussion
PITSLS was employed to extract heavy metals in tannery wastewater due to the availability of reactive sites in the polysiloxane matrix (Figure 1.), the protonation of COOH to COOby triethylamine, SH to S -, and the presence of oxy ions contributes to the removal of these heavy metals.

SEM/EDX Analysisof PITSLS
SEM (EVO/LS10-ZEISS) showed irregular particle sizes of the following polysiloxane matrices at various magnifications (µm): 3-chloropropylpolysiloxane (500); functionalized 3-chloropropylpolysiloxane (500) and thiosalicylic immobilized ligand (200) in Figure 3, with the EDX (EVO/LS10-ZEISS). The morphology shows a rough surface with a pore volume of 52.9940 nm 3 in Table 1, which provides a better adsorption environment. EDX data of functionalized 3-CPP with immobilized thiosalicylic acid ligand gives 7.14 wt% of S (Table 2). This is in agreement with the suggested structure of the targeted system and is an indication of successful polymerization by nucleophilic displacement of a halide anion by Brad B [21], which the sulphur atom was originally not on the polymer framework with 7.14 wt %, within the range of 6.1-10.4 wt % reported by Issa M. E. [7]; 8.0 wt %, [6]; 4.30-11.30 wt % El-Nahhal I. M [22]; 3.90-6.80 wt % [23] for similar synthesis. The value of 7.14 wt% was obtained because of the availability of reactive sites in Figure 3, which assisted in the immobilization process, with particle size of 3.4397 ± 1.5659 nm. This agreed with the nano-particle sizes of silica in the range of 2-5 nm with an extraordinary surface-to-volume ratio El-Nahhal I. M [22]. This particle size plays a vital role in adsorption of heavy metals in the tannery wastewater. This is in agreement with the suggested structure of the targeted system and is an indication of successful immobilization of the ligand in the polymer matrix as originally there was no sulphur atom on the polymer framework. The 7.14 wt % was. The presence of thiol in the matrix agrees with the FTIR results presented in Figure 1, as follows: thiol (SH, 2631cm -1 ); silicone (Si-O-Si, 1032 cm -1 ) Figure 2.

Effect of pH
The leather industry generates wastewater without proper treatment, thereby contaminating the eco-system [24,25]. As a result of that PITSMCBLS was employed to extract heavy metals (Cr 3+ , Fe 3+ , Pb 2+ , Cu 2+ and Zn 2+ ) available in the wastewater. This was made possible due to the availability of reactive sites in the polysiloxane matrix in Figure 1. The mechanism could be surface adsorption or chemisorption. The deprotonation of COOH to COOby triethylamine, SH, to S -, and the presence of oxy-ions contributed to their extractions. The results for the effect of pH on the purification of the metals are presented in Table 3. There was a gradual increase in the extraction from pH 2 to a maximum at pH 6 mostly, which was within the limit of 5.5-7.0 [25] recorded using aminopolysiloxane for similar adsorption studies [26]. Above pH 6, the removal efficiency decreased as the pH increased to pH 9, which promoted the precipitation of metal hydroxide in solution.

Effect of PITSLS Dose on the Adsorption of Heavy Metals
The results in Table 4, shows that, increase in adsorbent dose from 20-30 mg/60 cm 3 increased the adsorption of metal ion in the solution due to large availability of the surface area at higher concentration of the adsorbent in thiosalicylic with respect to Cr 3+ . Increase in the adsorbent dose from 10-30 mg/60 cm 3 had no significant effect on the adsorption of Fe 3+ and Zn 2+ metal ion in the solution because any further addition of the adsorbent beyond this point did not cause any significant change in the adsorption due to overlapping adsorption sites of the adsorbent particles [27,28], discrepancies were recorded with respect to Pb 2+ , which showed significant adsorption at 10 mg/60 cm 3 and decreased from 20-30 mg/cm 3 due to complete overlapping of adsorption sites.

Thermodynamic Study of PITSLS
The distribution coefficients (K D ) for the extraction and percentage adsorption (% ADS) of Cr 3+ , Fe 3+ , Pb 2+ , Cu 2+ and Zn 2+ metal ions from solutions of tannery wastewater by Polysiloxane immobilized thiosalicylic ligand system was studied at different temperatures of 303, 308 and 313 K in Table 5. The results for (K D ) and (% ADS) ranged from: Cr 3+ 287.838 to 296.384 and Pb 2+ 193.788 to 300.000 and % ADS for Cr 3+ 96 to 99; Pb 2+ 65 to 100 respectively. This showed that, the distribution coefficients K D and % ADS increased with increase in temperature while Fe 3+ , Cu 2+ and Zn 2+ showed no significant changes with increase in temperature. In order to determine the thermodynamic feasibility and the thermal effects of sorption, the thermodynamic parameters were evaluated using ∆G o = -RT InK D and ∆G o =∆H o -T∆S o , where ∆G o , ∆H o , ∆S o and T are Gibbs free energy, enthalpy, entropy and absolute temperature respectively [6,29]. R is the gas constant (8.314 Jmol -1 K -1 ) and K D is the equilibrium constant. Plots of InK D against 1/T gave the numerical values of ∆H o and ∆S o from slope and intercept respectively [30]. 47.4214J mol 1 K 1 ) is characterized by irregular increase in the randomness at the composite material solution interface during adsorption procedure of the system [31]. The results above are characterized by chemisorption process, favoured at higher temperatures. The thermodynamic parameters considered are in agreement with the work of El-Ashgar et al., [32].

Conclusion
The PITSLS has been prepared and subjected to instrumental methods of analysis such as: FTIR, SEM and EDX the PITSLS showed high potential for the extraction of Cr 3+ , Fe 3+ , Pb 2+ , Cu 2+ and Zn 2+ at an optimum pH of 6.0 in the tannery wastewater. This achievement could be employed in the treatment of tannery wastewaters in the industry. Extraction of metal ions increased with the increase in the adsorbent dose and temperature respectively. The thermodynamic parameters suggest a spontaneous and an endothermic affinity of the chelating ligand. The complex formation process is favoured at higher temperatures. The distribution coefficient values increased with increasing temperature indicating that the complex formation process of metal ions Cr 3+ , Fe 3+ , Pb 2+ , Cu 2+ and Zn 2+ with polysiloxane immobilized thiosalicylic ligand System was accompanied by an absorption of heat. Shekarri (TNBS), S. T. Dahiru (STD), Paul O. Nsude (POS) Emmanuel Agboeze (EA), Ike Christian Ozoemena (ICO) Olajide J. Igbehinadun (OJI), participated in the laboratory work. The final version was written by BH, E C E and Simon Moses, critically reviewed the manuscript for its intellectual content. All the authors gave final approval of the revised version for publication.

Funding
This research did not receive any specific grants from funding agencies in the public, commercial, or not-for-profit sectors.

Conflicts of Interest
The authors declare they do not have a conflict of interest.