Removal of Ni (II) By Sol-Gel Silica Functionalized With E�cient Metal Chelating Agents

In this study a comprehensive investigation of doping of sol-gel silica with e�cient metal chelating agents is provided at room temperature using infrared spectroscopy, elemental analysis, SEM, EDS and XRD. Sol-gel silica was doped with a series of metal chelating agents to investigate the sorption of Ni (II) ions from aqueous media. It is observed that the chelating agents are encapsulated by the pores of the xerogel from where they interact with the metal ions by ion exchange and chelation mechanism. Nickel complexes of the chelating agents were also synthesized. Sol-gel silica was doped with metal complexes for comparison with sol-gel silica having sorbed metal ions. Same amount of Ni (II) was observed in both samples, showing high e�ciency of the method for removal of Ni (II). The batch adsorption experiments were performed at room temperature and neutral pH. The advantage of the method is high sorption capacity at room temperature and neutral pH. The developed method can be applied on large scale removal of toxic heavy metals. The sorption process completed within a minute. 0.1 N HNO 3 resulted in complete desorption of metal ions from the gels. The regenerated sorbents were reused several times with negligible loss of sorption capacity.


Introduction
Water pollution is the major challenge that human beings are facing since long. Naturally, weathering and erosion of rocks is the main source of toxic heavy metals. Along with natural sources there are anthropogenic activities such as mining, welding, metallurgical operations, alloys manufacturing, fertilizers, pesticides and some other industries [1].
Among heavy metals, nickel is highly toxic. Volcanic rocks and soil naturally contain nickel. Nickel is used in aircrafts, automobiles, nickel-cadmium batteries, cosmetics and coins. Mineral leaching by weathering of rocks to water reservoirs is the cause of water pollution [2]. Water soluble nickel salts pollute water systems [3]. Enamel and paint industries also release nickel containing waste matter in water bodies [4].
Nickel is highly carcinogenic. Although it plays a vital role in production of red blood cells but it becomes toxic when its concentration exceeds the permissible limits. Previously, water contaminated by toxic heavy metals has been puri ed by coagulation [5], ion exchange [6], reverse osmosis [7] and adsorption [8]. Adsorption is superior to all other methods.
Morgan in 1920 introduced the term chelate as the groups that coordinate to central metal atom in such a way that a heterocyclic ring is formed [9]. Ethylenediamine (EDA) is a bidentate ligand having two nitrogen donor atoms so it has two donor sites for the attachment of metal ion [10]. Dimethylglyoxime (DMG) is a bidentate ligand. It has been used as selective reagent for the analysis of nickel. Composition of the complexes has been reported as M(DMG) 2 [11]. DMG is a bidentate ligand used for the gravimetric estimation of nickel (II) ions. 1, 10-phenthroline (1,10-Phen) is a bidentate heterocyclic ligand with two nitrogen atoms as donor sites.1,10-phenanthroline form complex with Co (II) [12]. Nickel (II) forms different types of complexes with 1,10-phenanthroline, Tris phenanthroline nickel complex shows light red color while bis phenanthroline nickel shows light blue color [13]. Dithizone (diphenylthiocarbazone) abbreviated as (DZ) has sulfur and nitrogen donor atoms to form chelates with metals. It is a valuable reagent as it forms colored products with metals and can be used in low concentrations. In 1882 dithizone was prepared by Emil Fischer [14]. Dithizone is an e cient metal chelating agent; it separates metals in trace amounts. Mohammad Saraji and coworkers prepared nickel (II) dithizone complex [15].
Sulfanilamide (SNM) ligand has sulfur, oxygen and nitrogen donor atoms to form stable chelates with metals. It is known because of its remarkable bioactivities. It was the rst drug used against bacterial infections.
Organically functionalized sol-gels because of their tunable porosity and surface layer composition have been the area of interest for researchers. Silica is usually selected as inorganic support due to its high surface area and pore size ranges from micro to macroporous. Silica in amorphous state is biogenic. It was the rst material used for bioencapsulation. It shows high thermal and chemical stability. The organic part is an active reagent that is incorporated in silica framework for the removal of metal ions [16]. Alkoxysilanes are usually used to form sol-gel silica because they are stable and easy to handle [17].
In this work we report functionalization of sol-gel with e cient chelating agents as well as their metal complexes by simple doping method at room temperature. High heating costs can be avoided on large scale production and application of these materials. They show good mechanical stability and fast equilibration in metal sorption. E cient chelating agents and their metal complexes have been selected due to their analytical, industrial, biological, catalytic and other applications [29][30][31][32][33]. The synthesized materials can be utilized in biomedical and catalysis.

Apparatus and Instruments
IR Spectra of the materials were recorded on ATR spectrophotometer. Functionalized Gels were analyzed by EDS Oxford 7573 combined with SEM, Atomic absorption spectrophotometry (AAS) was used to determine the amount of nickel (II) in solutions.

Chemicals and reagents:
In this work analytical grade reagents and absolute solvents purchased from Sigma Aldrich and Merck were used. Deionized water was used in all adsorption experiments.

Synthesis of Functionalized Xerogels
(i) Synthesis of Sol-Gels Functionalized with Chelating Agents 5 mL tetraethoxysilane and 5 mL distilled water were added to 10 mL ethanol solution of 0.005 M chelating agent containing appropriate amount of ammonium uoride as catalyst in a 100 mL beaker. A transparent gel was formed. It was dried at room temperature for two days and then placed in oven at 45 ºC till constant weight. Then the gel was crushed, ground and sieved. Undoped chelating agent was removed by soaking the gel in distilled water. After removing water, the gel was dried again. Blank xerogel was also prepared by the same procedure without adding the chelating agent.
(ii) Synthesis of Sol-Gels Functionalized with Metal Complexes 1 mL tetraethoxysilane and 1mL distilled water were added to 5 mL ethanol solution of 0.002 M complex containing 0.002 M ammonium uoride as catalyst in a 100mL beaker. Gel was formed in few seconds. It was dried at room temperature for two days to evaporate all the solvents, then placed in oven at 45 ºC till constant weight. The dried gel was crushed, ground and sieved. Undoped complex was removed by soaking the gel in distilled water. After removing water, the gel was dried in oven.

Sorption of Ni (II) ions
Sorption experiments were performed by the batch method at room temperature by letting undoped/doped sol-gel (50 mg) in contact with 20 mL of Ni (II) solutions of neutral pH in labelled test bottles. To study sorption isotherms, Ni (II) solutions of 15-50 mg/L were prepared and 20 mL of each solution was taken in the test bottles having pre-weighed sorbents. The suspensions were shaken, centrifuged and analyzed for nal metal concentrations on atomic absorption spectrophotometer. The amount of metal sorbed (q e in mg/g) was determined using the following equation: Where C and C e are metal concentrations (mg/L) before and after sorption respectively, m is the amount of sorbent in grams and V is the suspension volume in liters.
The above experiments were repeated using blank gel as well.

Physical Properties
Chelating agent's metal complexes are brightly coloured. Their colours and melting points are different from respective chelating agents. Blank gel is white in color. Doped gels of chelating agents and their metal complexes are coloured having the same colour as that of chelating agent/metal complex showing no chemical change in the process of doping.

IR Studies
IR spectra of blank and doped gels presented in Fig. 2 [27]. C = S absorption is observed at 1212 cm − 1 in dithizone. On complexation this band shifts to 1214 cm − 1 . In DMG a band observed at 3196cm − 1 for OH groups is absent in its complex showing hydrogen bonding in the complex. C = N absorption in the range of 1625 − 1500 cm − 1 is absent in ligand due to intramolecular hydrogen bonding and it has been observed in metal complex at 1570 cm − 1 [28].

Scanning Electron Microscopy (SEM)
SEM of blank and doped gels showed porous structure. Gels functionalized with chelating agent showed partly stick together particles of roughly spheric form. Gels functionalized with metal complex showed tubular structure. Functionalized gels showed shrinkage of pores as compared to blank gel. After sorption of metal ions complete shrinkage of pores of xerogels is observed. SEM images of blank and functionalized gels are presented in Fig. 3. Sol-gel silica was also functionalized with metal complexes for comparison with chelating agent functionalized gels with sorbed Ni (II) ions. Same amount of Ni (II) was observed in both, showing high e ciency of the process.

Energy Dispersive X-Ray Spectroscopy (EDS)
EDS of blank gel, sol-gel functionalized with Ni-DMG complex and DMG functionalized sol-gel with adsorbed Ni are presented in Fig. 4. EDS of sol-gel functionalized with Ni-DMG complex and DMG functionalized sol-gel with adsorbed Nickel are taken for comparison. Both samples show the same amount of nickel.

XRD Studies
XRD spectra of blank and modi ed gels showed pure state of amorphous structure. No disorder or crystallinity was observed due to undoped chelating agent as evidenced in Fig. 5. The main diffraction peak after 2θ; 20º is observed for amorphous silica. Silica in amorphous state is biogenic. It has been used in bioencapsulation. Crystalline silica is highly toxic. The synthesized amorphous silica sol-gel can be used in biomedical applications as well.

Sorption Studies
Sorption of Ni (II) by blank (BG) and functionalized xerogels has been provided in Fig. 6. Blank gel shows negligible sorption. Functionalized gels show high sorption capacity. Sorption data shows successful incorporation of chelating agents. DMGSG shows the highest sorption capacity for Ni (II) ions. 10 mg Ni (II) was adsorbed by functionalized silica having 20 mg DMG, showing Ni/DMG ratio of 1:2.

Desorption of Ni (II) from functionalized sol-gels
A fast quantitative desorption of metal ions is essential for a high e ciency of adsorbent in metal removal mode. It was found that 0.1 molar HNO 3 was fairly effective for complete recovery of Ni(II) ions from the gels. The sol-gels loaded with known amounts of metal ions were washed with deionized water. After washing and drying they were shaken with 20 mL of 0.1 molar HNO 3 solution for a minute. The concentration of released metal was determined using atomic absorption spectrophotometer. After the complete recovery of the gels, they were washed with enough deionized water for repeated use. 100 % elution of metal ions were achieved for the functionalized gels under study. They exhibited chemical stability and e ciency for several sorption-desorption-regeneration cycles for each batch of sorbent.

Conclusions
The chelating agents and their metal complexes did not show any chemical change after entrapment in the solid cages of the xerogels. The chelating agents retained their activity for the target metal ions. The entrapped chelating agents played major role in the sorption of metal ions by the functionalized gels. Thus, the simple preparation of doped gels, fast sorption of Ni (II), desorption and recyclability increase their commercial scope. All the sorption experiments were performed at room temperature and neutral pH. Thus, it is possible to use chelating agents entrapped into sol-gels for analytical and industrial applications. XRD showed amorphous nature of blank and functionalized gels. Amorphous silica is biogenic and can be utilized in biomedical. Sorption of Ni (II) mg/g (from 15 to 50 mg/L Ni (II) solution)