Metal Doping Silicates as Inorganic Ion Exchange Materials for Environmental Remediation

Environmental remediation of hazardous metals like Cd(II), and Co(II) ions is very necessary due to their toxicity at trace and accumulation levels in the biosystem. In this study, the sorption of Co(II), and Cd(II) ions from aqueous solutions onto titano-silicate (TiSi) ion exchange material and in-situ dopping composites cobalt titano-silicate (Co-TiSi) and cadmium titano-silicate (Cd-TiSi) was achieved. TiSi and in-situ dopping composites were obtained by precipitation technique and characterized using different analytical tools such as FT-IR, XRD, TGA&DTA, and XRF. Results obtained from this study showed that the capacity for Co(II), and Cd(II) ions revealed that Co-TiSi and Cd-TiSi is a higher capacity than those obtained for TiSi by 1.81, and 1.41 times value, respectively. The distribution coefficients for Co-TiSi as a function of HNO3 have high separation factors for Co(II) at different HNO3 concentrations. Langmuir isotherm model is the most representative for discussing the sorption process with a maximum sorption capacity of 16.02, and 10.96 mg/g for Co(II), and Cd(II) ions, respectively. Co-TiSi is suitable for the column technique for the recovery of studied cations. The investigation proved that a Co-TiSi exchanger is suitable for the uptake of the studied ions from liquid solutions and could be considered as potential material for the refining of effluent polluted with these ions. Titano-silicate and in-situ dopped composites were prepared by precipitation technique. The capacity for Co(II), & Cd(II) ions sorbed on Co-TiSi, and Cd-TiSi is higher than obtained for TiSi by 1.81, and 1.41 values, respectively. Co-TiSi is suitable for the column technique for the removal of studied cations. Titano-silicate and in-situ dopped composites were prepared by precipitation technique. The capacity for Co(II), & Cd(II) ions sorbed on Co-TiSi, and Cd-TiSi is higher than obtained for TiSi by 1.81, and 1.41 values, respectively. Co-TiSi is suitable for the column technique for the removal of studied cations.


Introduction
Radwaste is produced through a wide range of operations in power plants, recycling plants, research facilities, and radioisotopes in industry and diagnostic medicine [1]. The stream of waste from nuclear facilities includes low, intermediate, and highly radioactive wastes and may also include emitting radioisotopes. In addition to radioisotopes, these streams could contain various toxic and hazardous materials like heavy metals, organic material from decontamination procedures, etc. These wastes must be treated to decrease the radioisotopes concentrations to levels suitable for discharge to the area [1]. Cobalt is one of these metals that is used in nuclear, medical, enamel, and semiconductor industries, grinding wheels, glass, and porcelain painting, hygrometers, and electricity galvanizing. It is also used as a foam stabilizer in beer, in vitamin manufacture, as a drier for lacquers, varnishes, paints. Cobalt can have various adverse effects on health such as asthma, heart damage, heart failure, damage to the thyroid and liver [2]. Cadmium is common for electroplating, nuclear reactors, certain industrial paints, and batteries of nickel-cadmium [3]. Consistent estimation of Cd(II) ion is very important because of its toxic nature. In addition, Cd(II) ion has many radioisotopes along with 109 Cd: it's t 0.5 of 1.27 years and decays by electron capture to 109 Ag with 88.03 keV emission γ-photons [3] and used as rods for controlling and shielding the absorption of neutrons in reactors with supplementary components [3]. The retention of Co(II) & Cd(II) ions from liquid and radioactive waste solutions was achieved by many researchers [3][4][5][6][7][8][9]. Inorganic ion exchangers now play a significant role in analytical chemistry due to having thermal, chemical, and radiation stability [10]. Synthetic sorbents are used on an extensive range for different applications, extending from environmental therapy, water softening, hydrometallurgy [11], biochemistry [12], catalysis, and selective adsorption [13][14][15] to medical applications [16]. Different inorganic sorbents based on silicates were prepared by Abou-Mesalam et al., magneso-silicate [17,18], and impregnation of silicate group inside polymer layers such as; polyacrylamide acrylic acid magneso-silicate [10,19,20], polyacrylamide acrylonitrile magneso-silicate [21], and polyacrylonitrile-ball clay [22] as well as a combination of natural materials containing silicate group like bentonite dolomite [23] utilized for the elimination of some hazardous metals from industrial, environmental, and hazardous wastes. Environmental remediation for harmful effects of Co(II) and Cd(II) ions was investigated. In this work, sorption behaviour of Co(II) and Cd(II) ions from aqueous solutions was achieved by titanosilicate and insitu-doping composites prepared by precipitation method. The materials obtained were characterized using various analytical tools and exemplified a new sorbent character compared to the original ones. Formulas and applications of obtained compounds for the uptake of Co(II) and Cd(II) ions from the wastewater solution were done.

Materials
All chemicals and reagents in this work are analytical grade and used without further purification. Double distilled water (DDW) was used in all analytical measurements.

Preparation
TiSi composite was fabricated by the dropwise addition of Na 2 SiO 3 to TiCl 4 equimolar solutions (0.05 M) with a volumetric ratio of Ti/Si equal 1.5 at 25 ± 1 °C and constant stirring as a replacement reaction (Scheme 1). Diluted NH 3 (10%) was applied after complete addition before precipitation occurred and then a reaction mixture remained standing overnight. A precipitate was decanted and washed by DDW. In an electric drying oven, the precipitate was dried at 60 °C, grained and sieved for various mesh sizes, and held at 25 ± 1 °C.
Doping of some toxic elements like Co(II) & Cd(II) ions with TiSi composite was synthesized by addition of equimolar solutions (0.05 M) of Na 2 SiO 3 to TiCl 4 and metal chloride, MCl 2 with volumetric ratios M:Ti:Si = 1:1.5:1 (where M are Co and/or Cd) with agitation at 25 ± 1 °C as substitution reaction (Scheme 2). With the same method as TiSi, the mixed solutions were immediately hydrolyzed with DDW.

Characterization
The stoichiometry of the constituents in TiSi and metal in-situ doping composites was detected by Philips sequential x-ray spectrometer-2400. The percent of Ti, Si, Co and Cd were measured according to Super-Q quantitative application program. All studied powders were analyzed Scheme 1. The formation mechanism of TiSi composite Scheme 2. The formation mechanism of in-situ doping TiSi composites with IR spectrophotometer (Alpha II Bruker, Germany) at 4000-400 cm −1 . XRD patterns of prepared powders were take placed using a Shimadzu XD-D1, X-ray diffractometer with Cu-Kα radiation tube source (λ = 1.5406 A°), and graphite monochromator working at 30 kV and 30 mA. Prepared powders were analyzed for DTA & TGA with a sample holder made of Pt in the N 2 atmosphere using a Shimadzu DTG-60 H.

Influence of contact time
All the measurement of equilibrium was done by shaking 0.05 g of TiSi sorbent with 2.5 mL of Co(II), and/or Cd(II) ions (50 mg/L) in a shaker thermostat at 25 ± 1 °C with V/m = 50 mL/g. After an interval in time, the shaker is stopped and the solution is isolated at once from the solid. Hence, an atomic absorption spectrometer (AAS) was utilized to detect the concentration of metal ions found in the filtrate. The percent uptake can be computed by equation [15]; where: C i & C f , initial & final concentration (mg/L) of Co(II), and/or Cd(II) ions in solution.

Influence of batch factor
A batch factor was optimized to obtain varying V/m ratios (V/m = 25, 50, 100, 200, and 400 mL/g) by shaking various weights of composite with different volumes of studied cations. The solutions were separated after equilibrium and the filtrate was taken to test for the metal ion.

Capacity measurements
For capacity assessment, regular batch balancing of metal chloride solutions Co(II), and/or Cd(II) ions with the solid materials in the 50 mL/g V/m ratios was performed. A solution was blended at 25 ± 1 °C in such a blender thermostat. The solid was isolated after standing overnight and concentration for ions was determined. From the next equation, a capacity was calculated [23]: where V & m are volume of solution, (L) & weight of composite (g).

Sorption studies
Batch equilibration calculated distribution coefficient (K d ) of Co(II), and/or Cd(II) ions on the CoTiSi sorbent. A shaker thermostat fixed at 25 ± 1 °C for 30 min was put in the mixture. The solutions were isolated after equilibrium by centrifugation. K d & separation factor were computed using the following equations [17]: are distribution coefficients for the two conflicting species a & b in the system.

Sorption isotherm
Sorption isotherms of studied cations on CoTiSi sorbent were determined over the entire concentration range (100-1000 mg/L) at a constant V/m ratio of 50 mL/g. The testing was achieved in a shaker thermostat at 25 ± 1 • C. An equilibrium concentration (C eq ) and amount uptake (q e ) were calculated in mg/g as follows [17]; A plot of logC eq against logq e and/or C eq against C eq /q e was achieved to reach the applicable isotherm model.

Column investigations
Chromatographic columns breakthrough investigations were conducted as follows, 0.5 g of CoTiSi with particle size 0.380 mm were packed in a glass column (0.6 cm diameter and 5 cm heights) to give a bed height of 1.1 cm 3 volume. 1000 mL of the desired solutions (pH = 3) containing 50 mL/g of metal chloride of studied cations were passed through the column beds at a flow rate of four drops/min, equal portions were collected and the concentrations were continuously measured. The values of breakthrough capacity were calculated using the formula [15]; where; V (50%) is a volume for effluent at 50 percent breakthrough (L).
Distilled water and different NaNO 3 concentrations (0.01-0.5 M) were used to elute Co(II), and/or Cd(II) ions loaded CoTiSi composite at the flow rate of 4 drops/min, the eluent was collected every 20 min and the concentrations were measured.

Characterization
IR spectra (Fig. 1A), reveal that five characteristic bands were observed in regions 2800-3500, 1625, 1335, 1000, and 800-480 cm −1 for each TiSi, CoTiSi, and CdTiSi composite. The absorption band found at 2820-3500 cm −1 for these substances can be due to the free water and OH groups absorbed by the composites in stretching mode [15,23]. In-situ doping of Co(II), and/or Cd(II) ions with TiSi composites reduces the strength of this band. This decrease may be related to the reduction of the H 2 O content of metals by in-situ doping. At 1625 cm −1 , a strong band attributed to a bending vibration for water molecules absorbed on composites prepared [18,19]. At 1000 cm −1 due to Ti-OH deformation vibration [24], or overlapping of the Si-O and Si-OH [17], and Ti-O bonds in the structure, the band appeared at 780 cm −1 [25].
XRD of prepared composites (Fig. 1B), reveals that TiSi and in-situ doping composites have a crystalline structure. A crystallinity of TiSi was increased with in-situ Cd(II) ion and reduced with in-situ Co(II) ion with TiSi. This variation may be related to the role of a radius of in-situ ions, where a radius of Co(II) ion is smaller than Cd(II) ion. This means that the ions hold with the H 2 O in the surface of the composite according to unhydrated order Co(II) > Cd(II) and leads to an increase in the H 2 O content and increasing of amorphous nature. These results were agreeing with data obtained from the XRD of composites materials treated at different heating temperatures [26].
Differential thermal & thermogravimetric analyses (DTA&TGA) of studied composites (Fig. 2). In the DTA curve for TiSi, CoTiSi, and CdTiSi one endothermic peaks appear at (108 °C, 104 °C, and 94.9 °C) respectively, which may be due to loss in free water [17,27]. One exothermic peak is appeared at (478 °C & 561 °C) for TiSi & CoTiSi respectively, related to the phase change to metal oxide [17,23]. Due to a loss of chemical bond water, an endothermic peak occurs at 320 °C for Cd-TiSi [17]. The chemical formula of prepared composites is based on XRF and DTA&TGA whose mass loss allowed the expression to measure a percent of moisture with matrix [17].
where X is the percent mass loss of the water, n is the mole number of water and M is the molar weight of a compound without water molecules. An X value was found 15

Influence of shaking time
Retention of Co(II), and/or Cd(II) ions onto TiSi with shaking time (Fig. 3A) was done. It is appreciated that % uptake improves with the increasing of agitating time and maximum removal was detected at 25 min for Cd(II) ion and 30 min for Co(II) ion on TiSi. Therefore, we can consider these times are sufficient to achieve equilibrium for studied cations and used for all further experiments.

Batch factor impact (V/m)
The influence of V/m on % uptake of studied cations onto the TiSi (Fig. 3B) was investigated using V/m ratios from 25 to 400 mL/g. The data perfect that % uptake of studied cations on the TiSi decrease with increasing the V/m ratio and the best ratio at V/m 50 mL/g and this ratio was used for further experiments.

Capacity measurements
The capacity of TiSi for Co(II), and/or Cd(II) ions was determined at 25 ± 1 °C (Table 2). Table 2, indicated that the affinity order for all cations is Co(II)˃Cd(II), this sequence is in agreement with the unhydrated radii of the exchanging ions. Ions with smaller unhydrated radii simply enter cavities of an exchanger, resulting in higher adsorption [10,15]. A high TiSi capacity for Co(II) ions also can be due to the greater complexing capacity of cobalt with more than one oxidation state presence [10].
Also, data in These data imply that by holding the hole for exchangeable ions in a coordination bonding system, a composite can be constructed. A product will display high selectivity for exchangeable ions just as to be kept in ion memory after replacing a particular ion with another ion to maintain fundamentally original structures by impregnation of these cations with TiSi. And the capacity of these composites for Co(II), and/or Cd(II) ions have the sequence order; CoTiSi > CdTiSi > TiSi, so CoTiSi was used for further experiments. Comparable findings were obtained for MgAlSi and MgSi by Abou-Mesalam et al. [26]. Comparing these findings with the potential indicated by MgSi and metal in-situ (Co & Cd) for these metals, TiSi has a greater capacity than MgSi [17]. A capacities of MgSi, P(AM-AA), and {P(AM-AA)-MgSi} for Ni(II), Cd(II),

Influence of [H + ] ion on K d
A separation potentiality of CoTiSi for Co(II), and/or Cd(II) ions can be explored by studying K d & a b at range (10 -3 -1 M) HNO 3 medium were calculated and tabulated in Table 3. Table 3, indicated that distribution coefficients have an affinity sequence; Co(II)˃Cd(II), this sequence supports the sorption of ions in a dehydrated state. a b for studied cations were calculated and revealed that Co(II) has a greater separation factor. Table 3, displays the contrariwise proportionality of the removal percentage with the [H + ]. Ion mobility of studied cations is decreased by increasing [H + ]. A reduction of ion mobilities may be clarified by a growth of the frictional forces exerted in ions because of a change of the nature of hydrogen bonds in water [28]. A subsequent water cluster ions are produced when a proton concentration rises, H 3 O + , H 5 O 2 + , H 7 O 3 + , H 9 O 4 + changing water structure, and thus an ionwater interaction [29]. Also, a sorbent takes H + from a solution, hence, a surface converts positively charged, which eventually limits the uptake of Co(II), and/or Cd(II) ions [28].

Sorption isotherm
The nature of adsorption processes for studied cations on CoTiSi was investigated by a steady rise of the sorbate concentration and determining the quantity sorbed at each equilibrium concentration. Two types of isothermal equations were conventional for the sorption data (Freundlich & Langmuir). Freundlich isotherm used mathematical model extensively, given a heterogeneous surface expression observed and a probability function of functional groups and their energies measured in the next equation [17]: where K F &1/n are Freundlich constants integrating all aspects disturbing adsorption procedure such as adsorption intensity and capacity of adsorption and computed from (9) logq e = logk f + 1 n logC eq  both slope & intercept of a linear plot of logq e & logC eq , as exposed in Fig. 4A. Higher K F values show greater Co(II), & Cd(II) ions affinity. Adsorption intensity or heterogeneity for a surface is calculated through slope 1/n [17]. K F , R 2 & n −1 are existing in Table 4. As exposed in Table 4, n −1 ˂1 values show promising ion exchangers [30]. The adsorption model Langmuir adsorbs molecules in a fixed set of well-defined locations, each of which has a single molecule and no adsorbate movement at the surface level. These positions are therefore energy equal and distant, so there are no interactions between adsorbed molecules in their neighboring locations. By the next equation [30], a linear form of Langmuir is determined: In which q m & K L are Langmuir constants involving adsorption capacity & energy of adsorption and computed from slope & intercept respectively for plots of C eq /q e & C eq . The linearized Langmuir adsorption of Co(II), & Cd(II) ions at (Fig. 4B). Constants (q m & K L ) assessed from isotherms and their R 2 are existing in Table 4, and this data, reveals that Freundlich isotherm provides fit to equilibrium adsorption data, giving R 2 of Co(II), & Cd(II) ions (0.986, and 0.949), while Langmuir isotherm provides a brilliant fit to equilibrium adsorption data, giving R 2 of Co(II), & Cd(II) ions (0.988, and 0.9995). From these data, the Langmuir is more relevant than the Freundlich model. Figure 5A shows the breakthrough curves for studied cations (50 mg/L for each) onto the CoTiSi column, which depicts % concentrations of a respective metal ion within an effluent to a feed solution (C/C o %) Vs. effluent volume (mL), a corresponding uptake of Co(II), & Cd(II) ions per gram for solid is calculated from Fig. 5A using Eq. (7). A breakthrough capacity for all ions studied is calculated from Fig. 4B and found to be 26.0, and 17.0 mg/g for Co(II), & Cd(II) ions respectively. From these results, it is found that selectivity followed order: Co(II) > Cd(II). This sequence supported that the sorption process was taking place in dehydrated ionic radii, ions with smaller dehydrated ionic radii easily (10) C eq q = C eq q m + 1 K L q m enter cavities of an exchanger resulting in a higher uptake [10]. And a column technique breakthrough capacity is smaller than the capacity measured from a batch technique. These results reflect a great competition of ions sorbed. From these data, it can be assumed that: CoTiSi sorbent is suitable for the sorption of Co(II), & Cd(II) ions from aqueous elutions at pH = 3.

Elution studies
The elution profile for Co(II), & Cd(II) ions is illustrated in Fig. 5B. The elution of the investigated ions is studied using DDW and different NaNO 3 concentrations (0.01, 0.05, 0.1, and 0.5 M). Cd(II) ions were released using 0.01 M NaNO 3 as eluent. However, the separation of Co(II) ions was released using 0.05, and 0.1 M NaNO 3 as /n is adsorption capacity, K F is adsorption intensity, q m is maximum adsorption capacity (mg/g), and K L is the energy of adsorption (L/g)

Fig. 5 (A)
Breakthrough capacity of Co(II), and Cd(II) ions on the CoTiSi at 25 ± 1 °C and pH 3 (B) Chromatographic separation of Co(II), and Cd(II) ions using different eluents eluent. At 0.5 M NaNO 3 , the column packed with CoTiSi becomes free from any sorbed metal ions and can be reused again for chromatographic separation. From these data Cd(II) ion can be separated from Co(II) ion using 0.01 M NaNO 3 as eluent.

Conclusion
Titano-silicate and in-situ dopant composites were synthesized, characterized, and employed for batch sorption of cobalt & cadmium ions from an aqueous medium in this study.