Preparation and Characterization of Ni-Doped Calcium Silicate Hydrate Based on Steel Slag : Adsorption Capacity for Rhodamine B from Aqueous Solution

2 had great influence onmicrostructure of the composites, and SCSH possesses a mesoporous structure with slit-shaped pores, but NSCSH has narrow distributions of pore size. Furthermore, NSCSH has a higher adsorption capacity for Rhodamine B (RhB) than SCSHwith removal percentages of RhB of about 52.4% and 88.2%, respectively. In addition, the effects of NSCSHdosage and pH values on the adsorption of RhB were investigated. Adsorption isotherm parameters are obtained from both Langmuir and Freundlich analysis and showed a better fit to a Langmuir model. All results indicated that NSCSH has a great potential to be a safe, easily-made, and cost-effective material for the control of RhB contamination.


Introduction
The synthesis of calcium silicate hydrates (CSHs) in the hyperalkaline and hydrothermal environments has been extensively studied.Under different hydrothermal conditions, varieties of CSHs with different structures can be prepared, such as 11 Å tobermorite (Ca 5 Si 6 O 16 (OH) 2 ⋅4H 2 O) and xonotlite (Ca 6 Si 6 O 17 (OH) 2 ) which usually occur in nature as hydrothermal alteration products in contact zones of Ca bearing rocks, with the basic igneous ones [1][2][3][4][5].The structures of 11 Å tobermorite and xonotlite are built up of sheets of six and seven coordinated Ca polyhedra, which are ribbed on both sides by wollastonite-type silicate chains [6,7].Moreover, the structure of 11 Å tobermorite consists of a central layer of calcium octahedra which has silicate sheets on each side [8], and it can be described as "normal" or "anomalous" depending on its shrinkage behavior on heating [9].The structure of xonotlite is very similar to tobermorite, but it has double chains parallel to the b-axis which forms layers in the ab plane rather than the single chains in tobermorite [10].
Nowadays, calcium silicate hydrates are generally used as functional materials, especially as an adsorbent due to their high specific surface area and a large number of structural sites within the porous structure.Many studies have been devoted to the removal capacity of natural and synthetic crystalline CSHs for heavy metal ions.Thus, the uptake of by CSHs has been reported [11][12][13][14][15][16].Moreover, in the study of Chen et al. [5], xonotlite was synthesized and served as a seed crystal for the removal and recovery of phosphate from aqueous solution.In addition, the phosphate removal capacity of xonotlite was investigated depending on different dosages and pH values.Katsumata et al. [17] removed natural organic polyelectrolyte (humic and fulvic acids) and their metal complexes by adsorption onto xonotlite.Thus, xonotlite plays a role in fixing heavy metals by ion-exchange and adsorption due to its similarity with tobermorite [14,15].
Steel slag is a common by-product from iron and steel production and is little used, and its composites and mineral phases are similar to those of cement.As a result, it is available for preparing calcium silicate hydrate via hydrothermal reaction from steel slag [18].However, it has been reused as a coarse aggregate for concrete and raw material for cement, but the utilization is low [19].Hence, it is practical and meaningful to explore some new applications for steel slag with high utilization.Currently, steel slag has been studied as an adsorbing material, such as the removal of phosphate, ammonium ions, dissolved organic pollutant, and several hazardous ions from aqueous solution [20][21][22][23][24]. Asaoka et al. [25] revealed the removal mechanism of hydrogen sulfide in the pore water of eutrophic sediments by using carbonated steel slag which was produced through carbonation processes to alleviate alkaline impacts.Up till now, there have been few reports about the hydrothermal synthesis of calcium silicate hydrates using steel slag (SCSH) for the adsorption of contaminant.In this study, in order to accelerate the formation of more crystalline CSHs, small amounts of Ni(NO 3 ) 2 were introduced since it could affect the final product of calcium silicate hydrate (NSCSH).NSCSH has been used to bind hazardous dye from aqueous solution.The dye under consideration is Rhodamine B (RhB), which is widely used as a colorant in textiles [26]; it can cause irritation to the skin, eyes, and respiratory tract and has been found to be carcinogenic for human beings and animals [27].Nowadays, different kinds of adsorbents have been investigated for the removal of Rhodamin B as shown in Table 1 [28][29][30][31][32][33].The novelty of this research is that steel slag used to prepare CSHs can improve its utilization rate on one hand and the introduction of Ni 2+ on the other can enhance the adsorption capacity of CSHs, as compared with the common adsorbents presented in Table 1.Samples were analyzed by several methods, such as XRD, SEM, and N 2 adsorption/desorption.Moreover, the adsorption capacities of SCSH and NSCSH for the removal of RhB from aqueous solution were investigated and the adsorption mechanism of RhB on NSCSH was explained.mass of starting materials was 100 g, which were mixed in the mass ratio of steel slag/calcium oxide/silicon oxide = 1 : 0.05 : 0.38 to provide starting samples with Ca/Si = 1.The mixture was added to the water in the mass ratio of w/s = 10 : 1 and then put into a mixer and sufficiently mixed for 30 min.Subsequently, the mixtures were incubated at 185 ∘ C for 7 hours to manufacture SCSH.After being created, the sample was dried to constant mass in an oven (Chengdu Shengjie Technology Co., Ltd.) at 105 ∘ C and then milled to less than 45 m for the adsorption experiments.The same method was used to synthesize the NSCSH, and the mass ratio of steel slag/calcium oxide/silicon oxide and w/s remained unchanged; the only difference was the addition of another 5 percent of Ni(NO 3 ) 2 .The hydrothermal synthesis conditions and quantity of initial reactants of SCSH and NSCSH are shown in Table 3.

Characterization of Specimens. X-ray diffraction (XRD)
patterns of specimens were measured on a Rigaku D/max-1200 diffractometer equipped with a rotating anode and CuK irradiation, in the range of 5-70 ∘ 2 and at a step size of 0.02 ∘ .SEM micrographs were performed by using a TESCAN VEGA 3 scanning electron microscope, operating at the accelerating voltage of 20 kV and taking images at different magnifications.Brunauer-Emmett-Teller (BET) specific surface area, total pore volumes, and average pore diameter of the samples were measured from N 2 adsorption/desorption isotherms at 77 K, using automatic specific surface area measuring equipment (ASAP, 2020, Micromeritics, USA) after degassing under vacuum for 8 h at 383 K.

Adsorption Experiments.
Firstly, RhB stock solution (500 mg L −1 ) was prepared, and in order to prepare 10 mg L −1 solution, the stock solution was diluted to 100 mL with distilled water.The CSH specimen (100 mg) was added to 100 mL aqueous solution of RhB dye and placed in a 150 mL Erlenmeyer flask.The solution was magnetically stirred and was subsequently centrifuged at the time points of 2, 5, 10, 20, 30, and 60 min.The absorbance of the supernatant solution was measured on a UV-vis spectrophotometer (Beijing Puxi General Instrument Co., Ltd.) at the maximum absorption wavelength of 554 nm.The adsorption rate was calculated by (A.1), where  is the adsorption ratio of RhB and  0 and   , respectively, are the absorbency of RhB solution at initial time and at time  after UV irradiation at 554 nm [22].Furthermore, some experiments were performed at solution temperatures of 20 ∘ C to determine the adsorption capacity of SCSH and NSCSH with different time intervals and lasted 150 min.In addition, in order to determine the effect of adsorbent dosage, experiments were performed at adsorbent dosage of 40, 60, 80, 100, and 120 mg.Similarly, the effect of pH was studied by using different buffer solutions for maintaining the pH at 2-10.In order to determine adsorption isotherms parameters, experiments were carried out at solution temperatures of 20 ∘ C with different initial concentrations which varied from 5 to 25 mg L −1 .

XRD Characterization.
The XRD pattern of the SCSH and NSCSH are shown in Figure 1; peaks for CaCO 3 decrease to the fact that during the hydrothermal reaction and desiccation process, SCSH consists of disordered calcium silicate hydrate gels with X-ray peaks at 0.303, 0.292, 0.3346, 0.290, and 0.1913 nm.The peak of C-S-H gel at 0.303 nm is strong and sharp, but the peaks at 0.292, 0.3346, 0.290, and 0.1913 nm are weak, indicating a poorly crystalline structure.Peaks with low intensity at 0.308 and 0.298 nm would indicate a tobermorite component but are weak, which is consistent with the main phase of SCSH being amorphous calcium silicate hydrate.However, the NSCSH sample is obviously different from SCSH.The intensities of amorphous C-S-H peaks are smaller and some new peaks appear, as is shown at 1.13, 0.308, 0.298, 0.280, 0.184, and 0.167 nm, especially at 1.13 nm (corresponding to 7.8 ∘ , 2), which is the most characteristic peak of tobermorite as 002 orientation [34].Moreover, peaks at 0.308, 0.183, 0.424, and 0.282 nm (resp., corresponding to 28.9, 49.5, 20.9, and 31.6 ∘ ) represent the development of the 320, 040, 400, and −321 reflections, indicating the formation of xonotlite.In particular, the sequence in which the Bragg peaks form during the crystallization reactions shows a clear pattern that the first peaks to form are at ≈3.0 Å, which is in the position as the xonotlite 320 peak.At the same time, a peak at ≈1.8 Å forms, which is in the position as the xonotlite 040 peak.Hence, the xonotlite 320 and 040 peaks are more strong and sharp [9].The results suggest that the main components of NSCSH are well-crystallized tobermorite and xonotlite, which can be attributed to the addition of Ni 2+ .3.1.2.SEM Characterization.SEM micrographs of SCSH and NSCSH are shown in Figure 2, which illustrate that the addition of Ni 2+ significantly impacts the crystallinity and morphology of samples.The major product of SCSH is agglomerated with some flakey grains and fibrils accompanied by flocculation [35], which indicates C-S-H gel in Figure 2(a).It can be seen from Figure 2(b) that the microappearance of SCSH is structured by the flake-like hydrates (tobermorite) interlocked together with C-S-H [36].However, the flakelike structure of tobermorite in Figure 2(b) was low and disordered compared with Figure 2(c) which presented a better crystalline tobermorite.In addition, a small amount of floccules are observed in the micrograph of NSCSH due to the residual C-S-H gels, which did not turn into tobermorite [37].
At the same time, the xonotlite crystals synthesized consist of tiny needle-like microfibers together with orbicular particles which was observed in Figure 2(d) [38].Therefore, it can be inferred that part of tobermorite converted to xonotlite depending on the addition of Ni(NO 3 ) 2 , and the fibrous xonotlites are closely associated with tobermorite plates.It is clear that the better crystalline tobermorite and xonotlite are the main components of NSCSH.As it is known, nickel existing in CSH gel is mainly in the form of nickel hydroxide [39].With the addition of Ni 2+ , some of the Ca 2+ ions presenting in the crystal lattice of NSCSH release to solution and exchange with Ni 2+ ions [40].Moreover, Komarneni et al. [41] have shown by TEM and EDS that Ni 2+ could be detected at the edge but not at the core upon reaction of CSHs with Ni 2+ , and Ni 2+ in the solution of hydrating system is capable of binding some hydroxides.Therefore, some excess of calcium ions would be free in solution [35], and it indicates that the mechanism of reaction takes place from edge to core and the partial replacement of Ca 2+ to Ni 2+ will slow down the dissolution and subsequent further incorporation of calcium ions into the crystal structure of NSCSH, but calcium ions would be almost totally incorporated into the structure so that the reaction would be more complete and full.As a result, the dissolution of silicon component is promoted due to the adequate hydrothermal.Hence, the better crystalline tobermorite and xonotlite can be obtained with better pore structure and adsorption capacities.

N 2
Adsorption/Desorption.BET surface area, total pore volume, and pore diameter of the samples were measured.The data are summarized in Table 4 and nitrogen adsorptiondesorption isotherms and pore size distribution curves of SCSH and NSCSH are shown in Figure 3. Based on the pore diameter, samples are characterized as mesoporous material (2 nm < pore diameter < 50 nm).As is seen from Figure 3, the obtained N 2 adsorption isotherms of samples exhibit Type II behaviors (according to IUPAC classification), characteristics of major mesoporous, and a few microporous adsorbents.Type II isotherm represents unrestricted monolayermultilayer adsorption.Meanwhile, Point B of curves always exist in the area of low relative pressure, indicating the stage at which monolayer coverage is complete and multilayer adsorption is about to begin [42].Besides, N 2 absorbance of mesoporous SCSH does not have any limits in the area of high relative pressure, which fits to the H3 hysteresis loops, and it implies that the plate-like particles give rise to slitshaped pores.However, the hysteresis type of NSCSH is H1, which is often associated with porous materials that consist of   agglomerates or compacts of approximately uniform spheres in fairly regular array, hence having narrow distributions of pore size [42].From Table 4, the BET surface area of NSCSH increases compared with SCSH, but the average pore volume and average pore diameters both decrease, indicating that the addition of Ni 2+ is beneficial to prepare the sample, having more narrow pores instead of slit-shaped pores, indicating a better adsorption capacity of NSCSH.It can be seen that the adsorption of RhB for both SCSH and NSCSH occurs from the beginning of the experiment, and, as time progresses, the adsorption does not drastically increase.Furthermore, the adsorbent concentrations used in this study are 100 mg L −1 for both samples.The maximum adsorption takes place at around 30-50 min for both samples.After this time, the adsorption rate is slow, leading to a single, smooth, and continuous saturation curve [43].With the addition of Ni 2+ , the maximum removal rate of RhB correspondingly increases from 52.4% to 88.2%.The major product of NSCSH is flake-like structure of tobermorite and tiny needle-like microfibers together with orbicular particles structure of xonotlite which is attributed to a larger specific surface area of NSCSH; hence, the adsorption performance is more stable and efficient.As the adsorption efficiency of NSCSH is much better than that of SCSH, the following discussion will be focused on the adsorption capacity of NSCSH.

Effects of Adsorbent Dosage.
Adsorbent dosage is an important factor in the adsorption process, which significantly influences the removal effect of adsorbate.The effect of different adsorbent dosages on the uptake of RhB by NSCSH is shown in Figure 5 for dosages of 40-120 mg in a volume of 100 mL at the initial dye concentration, 100 mg L −1 , and all experiments are at the same temperature and at optimum pH constant for 30 min time intervals.As the dosage of adsorbent increases, the removal rate of RhB correspondingly from 34.8% to 90.6%.It is attributed to the greater surface area and availability of more active sites for adsorption while the increasing of adsorbent dosage, which lead to the increase of the removal rate of dye from the solution finally [44].However, when the adsorbent dosage is 100 mg L −1 , more than 80% RhB has been removed from the solution.Thus, considering the efficiency of experiments, the optimum dosage of adsorbent is selected at 100 mg/L for the following experiments.

Effects of pH.
The initial pH of dye solution is an important parameter that affects the adsorption property of an adsorbent.In particular, the pH of the solution changes due to (1) surface charges of the adsorbent; (2) the degree of ionization of the adsorptive molecule; (3) the dissociation extent of functional groups on the active sites of the adsorbent [45].To study the effects of the initial solution pH on RhB adsorption of the synthesized NSCSH, different initial pH between 2.0 and 10.0 was used.Figure 6 shows that the adsorption capacity of RhB decreases with the increase of the initial pH, which reaches a plateau value with pH from 3.0 to 8.0, and then decreases with pH above 8.0.However, the removal process of RhB for NSCSH is very complex and is related to the dissociation properties of the solution and the nature of the substrate surface.Thus, for a value of 3.0 for the RhB pKa, the surface charge value is positive at pH < 3.0 while the carboxyl group is unionized [46].The electrostatic attraction between cationic RhB and negatively charged NSCSH leads to the increase of adsorption.When pH value is above 3.0, the carboxyl group starts to become ionized, which conduces to the competition between COO − and hydroxyl group of CSHs, leading to the decrease of RhB adsorption onto NSCSH.

Adsorption Isotherms.
The equilibrium adsorption isotherm is important in reflecting the interactive behavior between adsorbate and adsorbent.Therefore, two main equations of adsorption isotherm are analyzed, namely, the Langmuir [47] and Freundlich isotherm [48].The Langmuir isotherm theory assumes a monolayer coverage of adsorbate over a homogenous adsorbent surface; it is represented by (A.2), where   is the concentration of dye solution at equilibrium (mg L −1 ),   is amount of dye absorbed at equilibrium (mg g −1 ),  0 is the maximum absorbance (mg g −1 ), and  is the Langmuir constant related to energy of adsorption (L mg −1 ).
The plots of   /  versus   for RhB with concentrations of 5, 10, 15, 20, and 25 mg L −1 are shown in Figure 7. Values of  0 and  are obtained from the slope and intercept of plot of   /  versus   and are given in Table 5.It can be seen that the correlation coefficients ( 2 ) are close to unity, indicating the monolayer adsorption of RhB on the outer surface of adsorbent.
The essential characteristics of the Langmuir isotherm can be expressed in terms of a dimensionless constant separation factor   called equilibrium parameter which is given by (A.3), where values of  and  0 are obtained from the Langmuir isotherm.
The value of   indicates that the type of the isotherm is either to unfavourable (  > 1), linear (  = 1), favorable (0 <   < 1), or irreversible (  = 0) [43].It is clear that the values of   are 0.148 (Table 5), indicating a favorable adsorption in this case.The adsorption isotherm is also applied to the Freundlich model.This model is based on an adsorbent surface which has a heterogeneous energy distribution of active sites.Theoretically, the Freundlich model is suitable for use with heterogeneous surfaces and, with this expression, an infinite amount of adsorption can occur [49].The Freundlich isotherm is described by (A.4), where   is the dye concentration for a solid at equilibrium (mg g −1 ),   is the dye concentration in solution at equilibrium (mg L −1 ),   is the measure of the adsorption capacity, and  is a constant related to adsorption intensity [50].
The plot of log  versus log  for RhB is shown in Figure 8.The values of  and   are calculated from slope and intercept of this plot and  values are given in Table 5.It shows that the values of n fall between 1 and 10, indicating

Conclusion
This paper describes the characteristics of Ni-doped calcium silicate hydrate based on steel slag and investigated the performance of NSCSH for the adsorption of RhB from aqueous solution.The SEM and XRD results indicate that the addition of Ni(NO 3 ) 2 promoted the formation of better crystalline tobermorite and xonotlite, resulting in a higher adsorption of RhB.The following conclusions can be drawn.
(1) Hydrothermal synthesized SCSH and NSCSH products both absorb RhB from solution, but NSCSH was much more effective.This is due to better pore structure and lager surface area of flake-like structure of tobermorite and tiny needle-like microfibers together with orbicular particles structure of xonotlite onto NSCSH.
(2) The adsorption capacity of RhB onto NSCSH was almost constant when pH value was in the range of 3.0 to 8.0, but it decreased when pH value is above 8.0.
(3) The experimental data of NSCSH was treated with the Langmuir and Freundlich isotherm equations.The analysis of the obtained parameters and correlation coefficients for both models showed that the adsorption behavior of RhB onto NSCSH is slightly better explained using a Langmuir model than a Freundlich model, which showed that in general the type of the isotherm was favorable (0 <   < 1).
(4) Steel slag, a common industrial waste material, could be a useful, simple, and inexpensive source of material for producing CSH samples which could have great potential for the removal of dye from aqueous solutions.

3. 2 . 1 .
Effects of Reaction Time.The effects of reaction time on the removal of RhB for SCSH and NSCSH are shown in Figure 4.

Figure 4 : 4 Figure 5 :Figure 6 :
Figure 4: Effect of time on the adsorption of RhB onto SCSH and NSCSH.

Table 1 :
Adsorption capacity of RhB onto natural materials.

Table 2 :
Chemical components of samples.

Table 3 :
Parameters for hydrothermal synthesis of calcium silicate hydrate from steel slag.

Table 4 :
The structure characteristics of samples.

Table 5 :
Isotherm parameters for RhB adsorption onto NSCSH.Figure Freundlich adsorption isotherm for RhB onto NSCSH.abeneficial adsorption.However, the dimensionless constant (  = 0.148) falls within the favorable limit in adsorbent.Meanwhile, compared with the values of  2 , it is noted that the Freundlich model is slightly smaller than the Langmuir model indicating that the Langmuir model is the better fit model for the adsorption of RhB onto NSCSH.