Removal of Lead from Water Using Calcium Alginate Beads Doped with Hydrazine Sulphate-Activated Red Mud as Adsorbent

Calcium alginate beads doped with hydrazine sulphate-treated red mud are investigated as adsorbent for extracting lead ions from water using batch methods of extraction. Different extraction conditions are optimised for maximum lead extraction. Substantial amount of lead is removed, and the adsorption ability is found to be 138.6 mg/g. Surface characterization using FTIR, EDX, and FESEM confirms that lead is “onto” the surface of the adsorbent. Thermodynamic parameters, adsorption isotherms, and kinetics of adsorption are analysed. Adsorption is “physisorption” in nature and spontaneous. The adsorbent developed can be regenerated using 0.1 M HCl. Thus regenerated adsorbent can be used as the adsorbent for further removal of lead at least 10 times, and this enables the complete removal of lead from water by repetitive use of the regenerated adsorbent. The beads facilitate the easy filtration. The methodology developed is successfully applied for removing lead from industrial waste waters.


Introduction
e consumption of water containing lead ions is hazardous, and it is reported that it causes various ailments such as kidney and neurological problems, anaemia, brain hemorrhage, and even death [1,2]. e maximum permissible limit is 10 ppb (WHO), and presently, zero lead concentration of waters is preferred [3]. Further, the lead is not biodegradable, and hence, the problems are ampli ed.
Intensive investigations are being carried out throughout the globe to remove lead from the waters and they have one or other disadvantage, and a globally acceptable eco-friendly, economical, and e ective method is still eluding the researcher. In this contest, the unconventional methods are attracting the researchers. Our research group has developed some methodologies based on adsorbents derived from biomaterials for removing various polluting ions such as chromium (VI) [4,5], zinc [6], aluminium (III) [7], uoride [8,9], nitrite [10], ammonia [11], phosphate [12], and dyes [13] from water.
Red mud is a waste product from aluminium industries, and its adsorption nature towards various pollutants is being investigated [14][15][16][17][18]. e uses of treated red mud in the removal of lead are reported in the literature. HCl-activated red mud is found to have an adsorption capacity of 6.207 mg/g at pH 4 and other optimum conditions of extraction [19]. Hydrogen peroxide-activated red mud is found to have an adsorption capacity of 64.79 mg/g at pH 6 [20]. Heat-activated red mud at 700°C is found to have an adsorption capacity of 38.2 mg/g at pH 4 [21]. Red mud coagulant is used to remove lead ions at pH 7, and its sorption ability is reported as 98.695 mg/g [22]. Zirconium-treated ne red mud impregnated in Zn-alginate beads (ZRMAB) is investigated for their adsorption nature towards phosphate from water, and its sorption ability for phosphates is found to be 13.64 mg/g of the adsorbent [23]. Organically modi ed magnesium silicate nanocomposites are also used for the removal of lead ions from water [24].
In this investigation, an adsorbent is prepared by doping hydrazine sulphate-treated red mud in calcium alginate beads, and its adsorption nature towards lead ions is studied. e present developed adsorbent is found to have 138.6 mg/g adsorption capacity towards Pb 2+ ions, and further, the entrapping of the adsorbent in the beads facilitates the easy ltration.

Chemicals.
e AR grade chemicals, namely, lead nitrate, sodium alginate, calcium chloride, nitric acid, hydrochloric acid, and sodium hydroxide, were purchased from SD Fine Chemicals Pvt. Ltd. and Merck & Co. Double distilled water was used for preparation of solutions.

Adsorbent.
e red mud was obtained from Vedanta Aluminium Ltd., Utkal Alumina, Lanjigarh Re nery, Rayagada, Orissa, and the chemical composition of the raw red mud was presented in Table 1.

Treatment.
e collected raw red mud sample was repeatedly washed with distilled water till the washings were neutral to pH and dried for two hours at 378 K. en, the red mud was grounded and sieved to 75 µ. us obtained ne red mud was mixed with 1% hydrazine sulphate solution in 1 : 2 (w/w) ratio, and the resulting solution was boiled for 2 hrs. en, the red mud was ltered, repeatedly washed with double distilled water, dried in hot-air oven at 378 K for two hours, and then stored in airtight coloured bottle for further work.

Immobilization of Treated Fine Red Mud in Calcium
Alginate Beads. A 100 mL of double distilled water was taken in a 250 mL beaker and to it, 3.0 g of sodium alginate powder was slowly added by continuous stirring at 363 K temperature and maintaining 1000 rpm until the solution was homogeneous and clear. en, 0.1 g of hydrazine sulphatetreated red mud was slowly added to it by constant stirring, and the resulting mixture was cooled to room temperature. en, this solution was dropwisely added to a supercooled (−2°C) 2% calcium chloride solution by maintaining uniform size of beads. e beads thus formed were ltered, repeatedly washed with distilled water for removing any remaining CaCl 2 on beads, dried at 343 K, and stored in a coloured bottle ( Figure 1).

Surface
Characterization. FESEM images of the adsorbent (HRMCAB) were taken by using HITACHI S-3700N SEM instrument manufactured by HITACHI High-Technologies Ltd., India. e magni cations from 500 to 15,000x and accelerating voltage of 15,000 V were maintained while taking the SEM images, and the images are presented in Figure 2.
FTIR spectra of HRMCAB (before and after adsorption) were recorded using Shimadzu (8400S) FTIR Spectrophotometer. e spectrum was recorded by adopting the KBr pellet method in the range 4000 to 500 cm −1 at room temperature and at optical resolution of 4 (1/cm). e observed spectrum is presented in Figure 3.

Journal of Analytical Methods in Chemistry
EDX spectrum of the adsorbent before and after adsorption of lead was recorded by using Hitachi (S-3700N) EDX detector and is presented in Figure 4.
By adopting the pH equilibrium method [25,26], pH ZPC of the adsorbent (HRMCAB) was determined by using Hanna pH meter, model HI2211-02, and the obtained plot is presented in Figure 5.
2.6. Method. Batch modes of extractions were adopted [27][28][29]. 100 mL of 100 mg/L lead solution was taken in 250 mL conical asks and to it, di erent quantities of adsorbent (0.5 to 3 g) were added. en, using 0.1 M HCl and 0.1 M NaOH solutions, the pHs were adjusted to 2 to 12, and the conical asks were shaken using the orbital shaker at 300 rpm for 240 min at 303 K. After the completion of the required time, the conical asks were removed, and their contents were ltered. e remaining Pb 2+ ions in the solution were analysed by atomic adsorption spectroscopy (AA 500) at di erent parameters such as measure method: ame absorption, wavelength: 217.00 nm, slit: 0.4 nm, high voltage: 416.25 V, lamp current: 5.0 mA, and fuel ow rate: 1200 mL/min. e percentage removal of lead and adsorbed amount of lead was calculated by using the following equations. Adsorbed amount: percentage removal: where C i is the initial Pb 2+ concentration (mg/L), C e is the Pb 2+ concentration at equilibrium (mg/L), V is the volume of Pb 2+ solution (simulated) in litres, and m is the mass of the adsorbent in grams. e same procedure was adopted in nding the e ect of the various physicochemical parameters on the extraction of Pb 2+ "onto" the surface of HRMCAB. e results are presented in Figures 1-9 and Tables 2-4.

Results and Discussion
e raw red mud composition is presented in Table 1. It mainly consists of alumina, iron oxide, SiO 2, Na 2 O, CaO, and traces of P and V. e red mud is treated with hydrazine sulphate and is doped in calcium alginate beads. us obtained beads (HRMCAB) are characterized and studied for its adsorption nature towards lead ions from water. e results are presented below.

HRMCAB Characterization
3.1.1. FESEM. FESEM images of the adsorbent (HRMCAB) before and after equilibration are noted and presented in Figure 2. It can be observed in SEM images that the adsorbent surface have many pores, edges, cavities, and corners before equilibration. Further, some crystalline aggregates are present, indicating di erent mineral phases such as gibbsite, goethite, and hematite.
After equilibration of the adsorbent with waters containing lead ions, there is an emphatic change on the SEM photographs.
e pores and cavities are clogged, edges disappeared, and phase boundaries are blurred. Further, some shiny patches appeared. All these changes in the surface features of the adsorbent indicate that the lead is "onto" the surface of the adsorbent. Moreover, the electronic images presented in Figure 6 also indicate that the lead is present on the adsorbent surface.  e FTIR spectrum of the adsorbent (before and after adsorption of the lead) is presented in Figure 3.
e band pertaining to the stretching -OH group of silanol groups and also adsorbed water appeared at 3445 cm −1 as broad band in the before adsorption spectrum. In the after adsorption spectrum, this band is shifted to 3429 cm −1 . e bending vibrations of -OH are assigned to the peak at 1624 cm −1 in both before and after adsorption spectra, and there is no change in their position. e peaks at 1020 cm −1 and 526 cm −1 in both spectra can be assigned to Al-O-Si (symmetric), Si-O-Si (asymmetric), and Fe-O stretching vibrations. e drastic di erence in the spectral peaks before and after adsorption of lead could be seen in the appearance of an intensive band at 2356 cm −1 and small peaks at 1321 cm −1 and 817 cm −1 in the after adsorption spectrum. ese features indicate that the lead is "onto" the surface of the adsorbent (HRMCAB).

EDX Spectrum.
e EDX spectra of adsorbent (HRMCAB) before and after adsorption of lead are presented in Figure 4. e presence of lead peak in the spectrum taken after adsorption equilibration and its absence before adsorption indicate that the lead ions are successfully adsorbed "onto" the surface of the adsorbent (HRMCAB).

In uence of Various Physicochemical Parameters.
e sorption ability of the adsorbent (HRMCAB) towards Pb 2+ is investigated using simulated waters containing Pb 2+ and varying di erent physicochemical conditions, namely, pH, equilibration time, adsorbent dosage, initial Pb 2+ concentration, temperature, and interfering co-ions. e results obtained are presented below.

E ect of pH.
Behaviour of the adsorbent (HRMCAB) towards lead ions mainly depends upon the solution pH. Hence, by varying the pH from 2.0 to 12.0, the optimum pH for the successful removal of lead is investigated while keeping the other conditions of extraction at other ideal levels, namely, sorbent dosage: 20 g/L, agitation time: 180 min, rpm: 300, initial concentration: 100 mg/L, and temperature: 303 K. e results are presented in Figure 7.
It can be seen from Figure 7 that the maximum removal of Pb 2+ to an extent of 91.5% can be removed at pH 6. But, as the pH is increased to 8, 10, and 12, the % removal decreased to 86.8%, 83.4%, and 78.2%, respectively. But, as the pH is decreased to 4 and 2, the % removal is decreased to 78.6% and 62.5%, respectively. It can be seen that on the acidic side of the pH, the fall of % removal is more pronounced than on the basic side. ese observations can be accounted from the view point of pH ZPC and the speciation of Pb 2+ ion at various pHs. From Figure 5, it may be inferred that pH ZPC is 5.8, and hence, at this pH, no ionic thrust prevails on HRMCAB surface. Below this pH, protonation to surface hydroxyl groups of HRMCAB occurs, thereby imparting +ve charge to the surface. e lead species in acidic conditions is Pb 2+ , and as it is a cation, it is to compete with H + ions for being adsorbed onto the surface of   . Hence, the negative surface charge will repel PbO 2 2− , and so, the % removal is decreased. e maximum % removal is found at pH 6 when neutral conditions prevail on the surface of the adsorbent and lead is in cationic speciation.

Contact Time.
e % removal of lead at di erent time intervals is shown in Figure 8. As seen from the gure, the extraction of lead increases from 58.4% to 91.5% until it reaches the steady state value 180 min with increase in time. After reaching the equilibrium state, the % removal remains almost constant even when the agitation time is increased to 240 min. Initially, rapid adsorption is occurred due to availability of more vacant sites. But with increase in time, the vacant sites are used up, and hence, the removal is slowed down.

Adsorbent Dosage.
e variation of % removal with increase in the adsorbent dosage is investigated when all other extraction conditions are maintained at optimum levels. e results are presented in Figure 9. It can be noted that as the dosage increased from 0.5 g to 2.0 g of beads/100 mL, the % removal is increased from 61.5% to 91.5%, but after that, the % removal remains constant. e optimum dosage is 2.0 g/100 mL (in terms of beads).

Journal of Analytical Methods in Chemistry
Pb 2+ is investigated. e results are presented in Figure 10. It can be inferred from the gure that as the Pb 2+ concentration increases from 25 mg/L to 200 mg/L, the % removal decreases from 100.0% to 57.0%. At lower concentrations of lead ions, the availability of binding sites of the adsorbent (HRMCAB) is more, and hence, extraction of lead ions is more. With the xed amount of adsorbent, only a de nite amount of adsorption sites is available, and hence, as the initial concentration of lead ion increases, there is a multiple competition to secure the sorption sites, and this results in the decrease in the % removal.

Co-Ions.
e interference of commonly found co-ions ( vefold excess) in water on adsorption of the Pb 2+ is studied, and the results are depicted in Figures 11(a) and 11(b).
It can be seen in the case of co-cations that the interference is of the order Cu 2+ > Fe 3+ > Zn 2+ > Ca 2+ > Mg 2+ . e interference in the case of anions is in the order

ermodynamic Parameters.
e e ect of temperature on the % of extraction of Pb 2+ is studied by increasing the temperature in the intervals of 10 K in the range 303 to 333 K, while keeping all other conditions of extraction at optimum levels, namely, pH: 6, contact time: 180 min, initial concentration of lead: 100 mg/L, rpm: 300, and adsorbent dosage: 20 g/L. e obtained results are presented in Figures  12(a) and 12(b) and Table 2. e % of extraction is found to be increased from 91.5% to 99.5% as the temperature increases from 303 to 333 K. ermodynamic parameters, namely, enthalpy change (∆H) (kJ/mole), entropy change (∆S), and free energy change (∆G) (kJ/mole), are evaluated at di erent temperatures using the equations ΔG 0 ΔH 0 − TΔS 0 ; K d q e /C e , ΔG 0 −RT ln K d ; ln K d ΔS 0 /R − ΔH 0 /RT, where C e is the equilibrium concentration of lead ion solution, q e is the amount of adsorbed adsorbate Pb 2+ , K d is the distribution coe cient of the adsorption, R is the gas constant, and T is the absolute temperature in Kelvin as described in the literature [30,31]. e values obtained are presented in Table 2.
e positive ΔH value (79.656) indicates the endothermic and physisorption nature of the adsorption process. e negative ΔG values re ect that the nature of the adsorption is spontaneous. Moreover, the "+" value of ΔS reects the increase in randomness at the boundary of solid and liquid during the equilibration leading to more adsorption of lead ions.

Adsorption Isotherms.
Freundlich [32], Langmuir [33], Temkin [34], and Dubinin-Radushkevich [35] models related to adsorption equilibrium are investigated as described in the literature to understand the nature and mode of adsorption. e linear equations used for Freundlich model is log(q e ) log k f + (1/n)log C e and for Langmuir model is (C e /q e ) (a L /k L )C e + 1/kL, where C e , n, and q e are adsorption capacity (mg/g), empirical parameter, and amount of Pb 2+ adsorbed, respectively, and k L and a L are the Langmuir constants. e obtained results are presented in Figures 13(a) and 13(b) and Table 3. e dimensionless separation factor (R L ) is calculated using the equation R L 1/(1 + a L C i ) [36]. R L values indicate the nature of adsorption: R L 1, linear; R L > 1, unfavourable; R L 0, irreversible; and 0 < R L < 1, favourable.
e R L values of the present equilibration system are found to be between 0 and 1 (Table 3) that Langmuir adsorption isotherm model is more acceptable, indicating the homogeneous nature of the adsorbent and the monolayer formation of lead onto the HRMCAB surface. Further, Temkin and Dubinin-Radushkevich isotherms are also used in analysing the equilibrium process. Temkin equations used are q e B ln C e + B ln A and B RT/b, where A is the Temkin isotherm constant (L/g), B is the heat of sorption (J/mol), b is the Temkin isotherm constant, T is the temperature (k), and R is the gas constant.
Linear form of Dubinin-Radushkevich equation is ln q e −βε 2 + ln q m , where ε RT ln (1 + 1/C e ), β is the energy constant, and q m is the Dubinin-Radushkevich monolayer adsorption capacity (mol/g). e linear plots of these two isotherms are presented in Figures 13(c) and 13(d), and the correlation coe cient and isothermal constants are presented in Table 3. By using the formula E 1/ 2 √ β and by the slope of the Temkin isotherm constants, the heat of sorption (B) and mean free energy (E) are calculated. As E is <8 kJ/mol (i.e., 0.408) and B is <20 kJ/mol (i.e., 12.701), the mechanism of adsorption is "physisorption" in nature, that is, nonspeci c adsorption attributed to the weak Van der Waals forces between HRMCAB and lead ions.
e results are presented in Figures 14(a)-14(d) and Table 4. e correlation coe cient value (R 2 ) is found to be in the following order: pseudo-second order (0.9934) > Bangham's pore di usion model (0.9768) > Elovich model (0.9654) > pseudo-rst order (0.8373). Hence, the pseudo-second-order model is better t to explain the adsorption process.

Applications.
e methodology developed in the present work is applied to the e uent samples collected at lead-based industries.
e results are presented in Table 5. It can be inferred from the table that substantial amounts of lead can be removed from the industrial samples.

Comparison.
e adsorption ability of the adsorbent developed in this work is compared against various hitherto reported adsorbents in the literature as presented in Table 6. It may be inferred that the present developed adsorbent has good adsorption ability than many adsorbents developed so far, and further, the immobilization of the hydrazine sulphate-treated red mud in calcium alginate beads renders the ltration easy.

Regeneration and Reuse.
Of the various leaching agents tried for the extraction of lead from the adsorbent, 0.1 M HCl is found to be good and hence adopted in this work.
A number of regenerations of the adsorbent are made, and thus regenerated adsorbents are used for the removal of lead. e results obtained are presented in Figure 15. It is seen that, as the number of regenerations increases, the adsorption nature decreases. But even with 10th regenerated adsorbent, Pb 2+ is successfully removed to an extent of 84.3%. Hence, by repetitive use of the regenerated adsorbent, the lead can be completely removed from water samples at the comfortable natural water pH 6. is indicates that the present developed methodology is successful.

Conclusions
Hydrazine sulphate-activated red mud doped in calcium alginate beads (HRMCAB) are prepared and are used as sorbent to extract lead ions from water by optimising various physicochemical parameters. Batch methods of extractions are adopted. 91.5% of lead is found to be removed from simulated water at pH 6, sorbent dosage of 0.066 g of red mud (doped in 2.0 g of HRMCAB beads)/100 mL, agitation time of 180 min, initial concentration of 100 mg/L, 300 rpm, and 303 K temperature. e adsorption capacity of HRMCAB is 138.63 mg/g. e interference of the co-cations is of the order Cu 2+ > Fe 3+ > Zn 2+ > Ca 2+ > Mg 2+ while the anions interfered in the order Cl − > SO 4 2− > NO 3 − > PO 4 3− > HCO 3 − . Surface morphological studies adopting FTIR, FESEM, and EDX con rm that lead is onto the surface of the adsorbent. By evaluating thermodynamic parameters, it is inferred that the sorption process is spontaneous and "physisorption" in nature.
On analysis of the various adsorption isotherms, the adsorption follows the Langamuir isotherm model, thereby con rming the homogeneous surface of the adsorbent and monolayer formation. Further, adsorption kinetics is analysed adopting various models, and it is observed that it follows pseudo-second order with R 2 0.9934.
By immobilizing the activated red mud in the beads, the ltration process is made easy. Successive regeneration of the adsorbent with 0.1 M HCl has not markedly a ected the adsorption nature of the adsorbent. Even after 10 cycles of regeneration of the adsorbent, substantial amount of lead is removed to an extent of 83.4%. Hence, by repetitive use of the same adsorbent, it is possible to remove lead completely from the water. Further, the immobilization of activated red mud in the beads facilitated the easy ltration. e methodology developed is successfully applied for removing lead from industrial waste waters.

Conflicts of Interest
e authors declare that there are no con icts of interest.