Heavy metals ’ removal from aqueous environments using silica sulfuric acid

There is a growing environmental and health concern associated with contamination by heavy metals. It has also been intensi ﬁ ed due to an increase of the exposure to such pollutants as a result of industrial and technological growth. Therefore, it is necessary to remove heavy metals in contaminated water to eliminate the associated risks. This study focused on the removal of heavy metal ions using silica sulfuric acid (SSA). A comprehensive study was conducted to assess the effect of different factors on the adsorption by SSA as well as selectivity properties of the adsorbent, kinetic and thermodynamic studies of the adsorption process. A batch test was used to remove heavy metals from a multi-element solution containing Ni(II), Pb(II), Mn(II), Cu(II), and Cd(II). The results showed that removal rate reached its peak at pH, string time, and adsorbent amount equal to 8, 60 min, and 0.04 g/mL of solution, respectively. The removal ef ﬁ ciency of Ni 2 þ , Cd 2 þ , Mn 2 þ dropped by increasing the volume of solution and smoothed at 150 mL while the removal of Pb 2 þ and Cu 2 þ did not vary with the volume. The removal ef ﬁ ciency by SSA was decreased as Pb > > Mn > > Ni (cid:2) Cu > Cd. In general, SSA successfully removed heavy metals from contaminated water.

Membrane filtration successfully removes heavy metal ions, but high maintenance cost is needed in operation (Foong et al. ). Chemical precipitation would result in massive sludge and, consequently, secondary pollution (Wu et al. ). Electrodeposition requires energy with insufficient performance on low concentration wastewater (Allioux et al. ). In comparison with metal removal methods, adsorption has a high removal efficiency, quick adsorption kinetics, and possible metal ion selectivity depending on the synthesis methods; therefore, it is an ideal candidate for the treatment of complex metalcontaining wastewaters or industrial effluents (Gao et al. ). Adsorption is the result of the chemical bond between the adsorbent and adsorbed molecules. Therefore, this type of specific adsorption is a chemical type. At irreversible adsorption atomic properties, the sorbent change consequently (Ding et al. ). So far, numerous studies have been done to remove heavy metals using a variety of adsorbents. Pitokezka et al. (1998) used ferric chloride to remove trace amounts of Pb, Cd, Ni, Cr, and Cu of three different waste streams and proved ferric salts were one of the selective sorbents which were able to decrease heavy metals' concentration levels below 1 mg/L in complex industrial wastewater (Patoczka et al. ). The results of this study showed that different elimination processes could lead to the removal of heavy metals up to ppb levels in the case of soluble, complex, colloidal, emulsion compounds. On the other hand, conventional methods failed to control pH and, consequently, were unsuccessful in the removal of heavy metals below 1 mg/L. However, in all cases, adsorption/co-precipitation with ferric chloride was effective to remove heavy metals. The range of concentrations for Ni and Cr was obtained as 0.01 mg/L and it was 0.04, 0.1, and 0.75 mg/L for Cd, Pb, and Cu, respectively. Adsorption properties of natural zeolite for metal finishing removal from wastewater were studied by Erdem et al. (). A batch method was used in the range of metal concentration of 100-400 mg/L. The ratio of metal cations extracted to clinoptilolite was fitted by Langmuir adsorption isotherm, Freundlich, and Dubinin-Kaganer-Radushkevich models.
Sequence selectivity of metal cation extraction was obtained as Co 2þ > Cu 2þ >Zn 2þ > Mn 2þ . Mohammadi et al. () estimated and compared the efficiency of sand, soil (lime and limestone), and organic matter (rice husk and leaf compost) for the removal of heavy metals Cu(II), Zn(II), Ni(II), and chromium from industrial wastewater. A pilot study was carried out with four treatments and four repetitions. This study indicated that low soil cation exchange capacity of the organic matter did not necessarily reduce its adsorption power.
Rice husk was a suitable adsorbent of Cr(II), and leaf compost was appropriate for Zn(II) and Ni(II) removal. Moreover, sand was effectively able to adsorb heavy metals in the wastewater of an electroplating factory. One of the adsorbents used for heavy metal removal is silica, which is a solid mineral. This adsorbent is cheap, non-toxic, and recyclable. The adsorbent is reusable up to five times and has high thermal stability (up to 200 C).
Using silica sulfuric acid (SSA) benefits from specific reactor volume compared to other adsorbents, dry synthesis, absence of solvents and byproducts, and ease of handling  water. Their study showed that Cu 2þ and Pb 2þ ions were adsorbed on silica gel for less than 2 h to reach the equilibrium. Pb 2þ -containing samples were adsorbed in the presence of Cu 2þ . The use of silica gels with a large specific surface area and smaller pore leads to a slight increase in Cu 2þ and Pb 2þ adsorption. Also, the dissolution of the silica from the adsorbent surface was kinetically controlled, unlike Cu and Pb adsorption. Therefore, they argued that the measured silica dissolution changed with time, temperature, and specific surface area of silica. Green chemistry focuses on using a process with minimum hazardous substances generation.
Moreover, there is a growing interest in heterogeneous systems due to the critical role in industrial activities, especially in developing technologies (Shaterian et al. ). Adsorption is a green and environmentally friendly method to remove heavy metals from water and wastewater due to its simplicity and low cost (Gao et al. ). The adsorbents can be easily prepared, functionalized, separated, and regenerated; however, low adsorption capacities and adsorption rates are their disadvantages. Therefore, it is essential to conduct studies on the adsorbents and find the sorbents with high adsorption capacity. SSA is safe, easy to handle, environmentally benign with fewer disposals problems and has been used as an efficient adsorbent and catalyst in chemical processes. To the best of the authors' knowledge, there is no study on heavy metal removal by SSA.
The main objective of this study is to assess the removal efficiency of heavy metals using SSA. We examined the effect of initial pH, stirring time, solution volume, initial concentration of metal ions, simultaneous effect of initial pH and their concentration as well as study of selectivity properties of the adsorbent, kinetic and thermodynamic studies of the adsorption process. Moreover, SSA was used for heavy metal removals from samples in two case studies.

Synthesis of silica sulfuric acid
A 500 mL round-bottom flask containing 60 g silica gel was used to produce SSA. The flask was equipped with a constant pressure dropper containing 13.3 mL chlorosulfonic acid. This flask was connected to a gas outlet tube to allow transfer of produced HCL gas into water. Chlorosulfonic acid was dropped on silica gel for 30 min at room temperature. HCL gas was released rapidly from the reaction vessel. During and after completion of the reaction, the reaction mixture was stirred for 30 min. Finally, a 76 g white solid, called silica sulfuric acid, was obtained. It should be noted that the preparation of SSA could generate acidic wastewater and the hazards associated with waste generated from SSA production need to be considered in practical purposes.

Batch tests
Batch tests were conducted for removing heavy metals by mixing 1 g adsorbent in 25 mL of a multi-element solution containing 5 mg/L of Ni(II), Pb(II), Mn(II), Cu(II), and Cd(II). A series of batch tests was carried out to find the effect of pH, equilibrium time, kinetics of adsorption, the effect of temperature, thermodynamic characteristics, and effect of interfering ions. All tests were repeated three times to avoid data inconsistencies, and to obtain reasonable data deviation, heavy metals were not buffered to simulate industrial operation (Afkhami et al. ). The percentage of metal ions' removal was calculated by Equation (1): where R is the removal efficiency of metal ions, C 0 is initial concentration of metal ions, and C t is the concentration at time t.

Effect of initial pH of solution, stirring time, and solution volume
To evaluate the effect of pH on cation adsorption, the experiment was done at pH 1.7, 3, 5, 7, 8, 8.5, and 9. At each pH, the experiment was repeated with different stirring times ranging from 10 to 75 min, and the volume of metal-containing solution changed from 25 to 1,000 mL. Therefore, the optimum condition of adsorption was obtained at this stage.

Effect of the initial concentration of metal ions
Under optimum condition, the batch test was done by adding 5-25 mL of the multi-metal solution. Regarding mass balance law, the amount of adsorbed metal on adsorbent was calculated using Equation (2): where C 0 is initial concentration of metal ions, q t is mass of metal adsorbed per unit mass of adsorbent at time t, C t is concentration of metal ions at time t, W is adsorbent weight, and V is volume of the multi-metal solution.
Simultaneous effect of initial pH and concentration and study of selectivity properties of the adsorbent To investigate the simultaneous effect of initial pH and concentration, solutions with 50 mg/L, 10 mg/L, and 30 mg/L of Ca, Ni, Pb, Mn, and Cu at pH of 3, 5, and 7 were tested by the batch method.
In this study, Kurbatov curve was used to assess the selectivity properties of the adsorbent.

Kinetic studies of the adsorption process
The batch tests were implemented at 20, 30, 40, and 50 C and thermodynamic parameters such as changes in Gibbs free energy, enthalpy, and entropy were estimated as a function of temperature.
Since industrial wastewater contains a variety of ions which makes adsorption difficult, samples with different cations and anions including sulfate, nitrate, and chloride cations and sodium anions were prepared to study.

Removal of heavy metals from case study samples
The samples used in this study were collected from

RESULTS AND DISCUSSION
Effect of initial pH of the solution Figure 1 shows the effect of pH on heavy metal removal by SSA. As seen in the figure, adsorption of nickel, cadmium, manganese, and copper was low in an acidic environment.
The minimum adsorption was observed for Cu ions.  However, the removal of lead ions was high even in the acidic condition and reached its peak at pH equal to 7.
The low Cu removal rate may be related to stable copper complexes with water molecules based on Irving-Williams series. Hydroxide ions in an alkaline environment can form a complex with the majority of heavy metals and precipitate. Results also verified this as removal of copper suddenly rose at pH above 5.5 (Figure 1). The sudden increase could be due to the formation of hydroxyl complexes and precipitation of copper hydroxide as a solid.
Metal hydroxide formation depends on pH and concentration so that an increase in pH and concentration intensifies this process. In the studied concentration range, 5-100 mg/L, at 5.5-6 pH, solid copper hydroxide began to form and for heavy metals ions, this occurred at pH greater than 8. Despite the fact that hydroxide compounds can be in aquatic form, deposition of metallic hydroxides in pores and on the surface of the adsorbent is hardly possible because adsorption is quicker from precipitation kinetically. Therefore, it can be concluded that even though there was a possibility of precipitation, the dominant process for the removal of heavy metals was adsorption.
Even though the optimum pH was obtained at 8.5-9, pH equal to 8 was selected as the optimum pH to avoid precipitation of metal hydroxide except for copper and possible silica destruction at pH higher than 8. At optimum pH,

Effect of adsorbent amount
The adsorbent amount is a crucial parameter which deter-

Effect of stirring time
Contact between the adsorbent and adsorbate increases the probability of the adsorption and it increases with longer contact time. In this study, stirring time varied from 10 to 75 min. The results are illustrated in Figure 3. As shown in the graph, Cu 2þ , Pb 2þ , and Mn 2þ were removed at 10 min.
Nickel and cadmium determined the reaction and equilibrium time. Ni 2þ and Cd 2þ removal rates peaked after The standard deviation was less than 0.005.

Effect of the initial concentration of metal ions
Solutions with different concentration were tested by the batch method, as illustrated in Figure 5. Consequently, none of the Langmuir and Freundlich models could explain the adsorption process. Therefore, it can be concluded that adsorption of multiple metal solutions changed the adsorption process compared to that of single metal solutions, and the maximum adsorption capacity cannot be estimated. In these experiments, the standard deviation of the results was less than 0.004.

Effect of both pH and concentration
During this stage of the study, the selectivity of adsorbent was determined, and it was adjusted such that adsorbent tended to adsorb lead. Lead removal was greater than other metals, even at high concentrations and acidic condition. At the concentration of 50 mg/L and pH 3, lead removal adsorption efficiency was 85.4% ( Figure 6). The sharp increase of Cu 2þ reduction at an elevation of pH from 5 to 7 testified to the participation of copper in the

Kinetics studies of adsorption process
Adsorption of cadmium and nickel ions on the surface of SSA from the solution was as a function of mixing time.
As shown in Figure 8(a) and 8(b), adsorption of these ions occurred quickly so that more than 95% of nickel and 54% of cadmium were removed after 60 min. However, there was no significant change in the adsorption at times longer than 60 min. Morris kinetics equation was used to investigate the adsorption. K i was determined as 0.055 and 0.109 μmol·g À1 ·min À½ for cadmium and nickel, respectively.
Kinetics of nickel and cadmium adsorption were analyzed by first order Lagrangian equations (Aguado et al.

;
Ahmed ). The correlation coefficient was low, and this model could not explain the adsorption of these ions.
Morris-Weber transition diagrams of the particle, shown in Figure 8, could prove the multistage adsorption of nickel and cadmium on SSA. As stated in the following, ΔH values obtained for nickel, cadmium, and manganese cations showed both physical and chemical adsorption.

Effect of temperature and thermodynamic parameters
The experiment was implemented at 20, 30, 40, and 50 C temperatures to study the effect of temperature on the adsorption of cadmium, nickel, manganese, lead, and copper cations by SSA. As seen in Table 1, removal of Cd, Ni, and Mn increased as temperature rose. The increase in the adsorption of cadmium, nickel, and manganese cations

CONCLUSION
In this study, silica sulfuric acid was used as an adsorbent to remove heavy metals including nickel, cadmium, magnesium, lead, and copper ions. The experiment was done on laboratory samples and case study samples by the batch method. SSA peaked in its efficiency for heavy metal at weak alkaline conditions and 0.04 g of adsorbent/g of solution. The highest tendency for adsorption was for lead, and the least was for chromium. SSA adsorption is an inexpensive method for purifying wastewater contaminated with low to moderate amounts of heavy metals.
The reaction kinetics of adsorption followed the firstorder kinetic model. Thermodynamic studies showed that the removal of cations enhanced with increase in temperature. According to negative values, ΔG obtained for cadmium, nickel, and manganese, surface adsorption process on SSA was a spontaneous reaction. Positive values of ΔS represent an increase of irregularities during the process, and positive ΔH means that the reaction on the surface of the adsorbent nature was endothermic. Also, the amount of change in obtained enthalpy (ΔH ) showed that Cd(II) ions on SSA were physical adsorption, but adsorption of Ni(II) and Mn(II) ions were chemical ones.

DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.