Facile Synthesis of Calcite-Impregnated Hydrochar with High Sorption Capacity for Cu(II) from Aqueous Solution.

Calcite-impregnated hydrochar (Ca-HC) was successfully synthesized by a one-step hydrothermal method and used as an adsorbent for Cu(II) remediation. Characterization techniques showed that Ca-HC contained calcite and oxygen-containing functional groups. A series of Cu(II) sorption experiments onto Ca-HC showed that the initial Cu(II) concentration, contact time, sorption temperature, and initial pH of the solution influenced the sorption of Cu(II). The actual achievable sorption capacity of Ca-HC for Cu(II) was 130.57 mg g-1 at 303 K, and the sorption process obeyed the Langmuir model and pseudo-second-order kinetic equation. The precipitation and surface complexation rather than ion exchange were mainly ascribed to the removal of Cu(II) onto Ca-HC. The calcite provided the active site to produce posnjakite precipitation during the sorption process and enhance the sorption capacity of the hydrochar. Therefore, these results demonstrated that Ca-HC is an effective sorbent that can remove Cu(II) from water.

Introduction

With the ongoing development of modern industry, water pollution caused by industrial sewage containing heavy metals has become a serious environmental problem, which is harmful to the health of human beings and ecological systems due to high toxicity, nondegradability, and bioenrichment of heavy metals in nature.[1,2] Therefore, it is of great significance to purify wastewater contaminated by heavy metals before it is discharged to the natural environment. Up to now, various technologies have been developed to remove the heavy metals including ionic exchange,[3] precipitation,[3] adsorption,[3] chemical reduction,[4] electrochemical methods,[3] and membrane separation.[3] Among these treatment methods, adsorption is a prevailing approach to remove heavy metals from wastewater due to its low cost and simple operation. However, the widespread application of adsorption has been largely restricted because the common adsorbents such as activated carbon,[5] zeolite,[6] and other carbon-based materials[7,8] are expensive and/or difficult to regenerate. Thus, it is very desirable to prepare an efficient, environmental-friendly, and economical adsorbent to isolate heavy metals from wasterwater.

Recently, hydrochar, a carbon-rich solid material derived from plant residues and agricultural wastes under hydrothermal technique, has been explored extensively as an environmental-friendly and cost-effective adsorbent for removing pollutants from aqueous environment because it is abundant in reactive oxygen-containing functional groups, such as hydroxyl and carboxylic acid functional groups.[9−12] Some studies suggested that hydrochar can remove heavy metals mainly through the surface complexation of heavy metals with the oxygen-containing functional groups on the surface of the hydrochar.[13−16] However, the sorption capacity of hydrochar for heavy metal is often lower than that of pyrolyzed biochar.[17,18] This is probably because hydrochar may contain fewer mineral components than pyrolyzed biochar. Many studies have indicated that both mineral fractions and carbon fractions with aromatic structures and oxygen-containing functional groups of biochar are involved in the sorption of heavy metals.[19−21] The different sorption mechanisms depend on the properties of both heavy metals and biochar produced by the types of feedstock and pyrolytic temperature. The sorption of heavy metals by biochar with low mineral contents is mostly attributed to surface complexations between heavy metals and the oxygen-containing functional groups on the biochar surface.[19,20] However, biochar with rich mineral components, such as manure biochar and sludge biochar, remove heavy metals mainly through four different possible interactions (precipitation, electrostatic interaction, cation exchange, and complexation) between heavy metals and mineral components.[19,22] Therefore, the mineral fraction of biochar plays an important role in the heavy metal sorption process. However, little attention is paid to the application of mineral-impregnated hydrochar to remove heavy metals from the aqueous solution.[23,24]

Carbonate minerals exhibit an excellent ability to remove heavy metals, and the adsorption mechanism is considered to be precipitation and ion exchange on the carbonate surface. It is well-known that calcium carbonate (CaCO3) is one of the most abundant and cheapest material found in nature and harmless to human being. Meanwhile, it is proved that calcium carbonate can remove heavy metals from water.[25−28] Therefore, it is highly desirable to develop a novel calcium carbonate impregnated hydrochar adsorbent material that can enhance the capability of hydrochar as an efficient and inexpensive adsorbent for the purification of heavy-metal-contaminated water. The objectives of the work were as follows: (1) synthesize calcite-impregnated hydrochar (Ca-HC) by a simple hydrothermal method as an adsorbent to enhance the sorption capacity of hydrochar for Cu(II); (2) compare the physicochemical properties of Ca-HC with and without Cu(II) sorption by energy-dispersive scanning electron microscopy with energy-dispersive X-ray (SEM–EDX), elemental analysis, Fourier transform infrared (FTIR), and X-ray powder diffraction (XRD); (3) evaluate the effect of solution pH, temperature, contact time, concentration, and salt concentration on the sorption capacity of Cu(II) by Ca-HC; and (4) analyze the sorption mechanisms of Cu(II) onto Ca-HC based on the characteristics of Ca-HC and sorption process.

Results and Discussion

Characterizations of HC, Ca-HC, and Cu-HC

To characterize their surface morphology and surface elemental composition, Ca-HC and HC before and after adsorption were observed by SEM–EDX. The SEM images showed that the surface of Ca-HC was rougher and covered with more small particles than that of HC (Figure S1a,b). The SEM–EDX analysis showed that the content of the Ca element increased on the surfaces of Ca-HC compared with that of the Ca element in HC (Figure S1e,f), which indicated the formation of calcite particles on the surface of Ca-HC. After sorption of Cu(II), the atomic percentage of Ca on the surface of Ca-HC obviously decreased while the elemental Cu showed a marked increase through the EDX spectrum (Figure S1f,h). Based on the findings, Cu(II) was adsorbed on the Ca-HC surface and calcite was involved in the sorption process.

The XRD profiles of HC, Ca-HC, and Cu-HC are shown in Figure a. The peak at 2θ = 26.6° of the hydrochars was assigned to graphite (JCPDS 26-1080).[29,30] The weak diffraction peaks of calcite (JCPDS 47-1743) was observed in HC. The increase in the diffraction peaks of calcite (JCPDS 47-1743) in Ca-HC indicated that the calcite was successfully incorporated into the hydrochar.[31] After the sorption of Cu(II), the diffraction peaks of calcite clearly decreased, while the strong peaks of posnjakite (JCPDS 43-0670) and Cu4(SO4)(OH)6·2H2O, appeared.[32] The possible precipitation reaction (eq ) is shown belowThe C, H, S, and N contents of HC were 47.24, 5.48, 0.63, and 2.85%, while those of Ca-HC were 44.18, 5.00, 0.58, and 2.47%, respectively. After the sorption of Cu(II), the C, H, S, and N contents of Ca-HC were 40.02, 4.79, 1.41, and 1.72%, respectively. In comparison to Ca-HC, the surface C content of Cu-HC decreased, which might cause the dissolution of some CO32– from Ca-HC. However, the S content of Cu-HC increased greatly after Cu(II) sorption. It could be explained that SO42– was introduced to the Cu-HC surface as a component of a newly formed precipitate, which was consistent with the XRD data.

Figure 1

(a) XRD patterns of HC, Ca-HC, and Cu-HC. C, carbon; Q, calcite; and P, posnjakite. (b) FTIR spectra of HC, Ca-HC, and Cu-HC.

Figure b illustrates the FTIR spectra of HC, Ca-HC, and Cu-HC. For Ca-HC, the broad and strong peak around 3342 cm–1 denote the −OH stretching vibration.[33] The band at 2917 cm–1 was typical of the stretching of aliphatic C–H.[9] The peak at 1607 cm–1 could be specified as the asymmetrically stretching vibration of carboxylate (−COO–).[30,33] The bands of 1160–1000 cm–1 were related to the C–O stretching vibrations.[34] Besides, the bands at 1430 and 875 cm–1 could be attributed to the presence of carbonate, which increased significantly compared with those in HC.[33,35] After sorption of Cu(II), the function groups undergo some significant changes. The asymmetric stretching vibration of the −COO– group at 1607 cm–1 was changed to 1625 cm–1 and the symmetric stretching of the −COO– group at 1379 cm–1 became obvious, which showed that chemical interactions occurred between the carboxyl group and Cu(II) on the Ca-HC surface.[30] The intensities of bands at 1430 and 875 cm–1 became weak after sorption, indicating that carbonates played a key role in Cu(II) sorption.[36] The increase in the intensity of the band at 1060 cm–1 could be attributed to the introduction of sulfate after sorption.[37] The bands at 1112, 793, 603, 515, and 434 cm–1 were attributed to the presence of posnjakite.[37] The results of the FITR spectra, together with the XRD and elemental analysis, demonstrated that sulfate as the main constituent of the precipitation existed on the Cu-HC surface.

The BET surface area and pore volume of HC were 11.54 m2 g–1 and 0.025247 cm3 g–1, while those of Ca-HC were 21.62 m2 g–1 and 0.064501 cm3 g–1, respectively, which indicated that Ca-HC has a larger surface area and a higher pore volume than HC. The porosities of HC and Ca-HC measured by mercury porosimetry were 80.56% and 83.48%, respectively. Bulk densities of HC and Ca-HC at 0.51 psia were 0.3096 and 0.2815 g mL–1, respectively. These results showed Ca-HC has a looser structure than HC.

Sorption Performance of HC and Ca-HC

We compared the adsorption capacities of HC and Ca-HC to three different initial concentrations of Cu(II) (30, 60, and 90 mg L–1), and the results are shown in Figure . It can be seen from Figure that the adsorption capacities of HC for the three different concentrations of Cu(II) were about 5 mg g–1, while those of Ca-HC were 33.67, 62.72, and 86.79 mg g–1, respectively. These results showed that the adsorption capacities for Cu(II) on the hydrochar loaded with calcite were obviously improved.

Figure 2

Uptakes of HC and Ca-HC (Co = 30, 60, 90 mg L–1, time = 12 h, T = 303 K, dose = 40 mg/50 mL).

Sorption Isotherms for Cu(II) on Ca-HC

The Cu(II) sorption isotherms onto Ca-HC are depicted in Figure a. The equilibrium sorption capacities increased rapidly with increasing equilibrium Cu(II) concentrations in the range from 0 to 60 mg L–1 and increased slowly above this range. It was obvious that increase in temperature was conducive to increase in sorption capacities, which showed that the sorption was an endothermic process.

Figure 3

(a) Sorption isotherms of Cu(II) onto Ca-HC. (b) Linear fitting of the sorption using the Langmuir isotherm model.

In general, sorption isotherm models have been widely applied to describe the phenomenon of controlling the release or movement of adsorbate onto the solid from the solute and show the essence of interactions between adsorbents and adsorbates. The Freundlich model (eq )[38] and Langmuir model (eq )[39] are the two isotherm models applied to describe the sorption characteristic of Cu(II) onto Ca-HC. The linear forms for the two models are expressed in the following equationswhere qe (mg g–1) and Ce (mg L–1) are the sorption capacity of Cu(II) at an equilibrium and the Cu(II) equilibrium concentration, respectively. n and KF (L mg–1) are the exponential parameter of the Freundlich model and the Freundlich constant, respectively. Qo (mg g–1) and KL (mg1–1/ L1/ g–1) are the maximum sorption capacity of Ca-HC and the Langmuir constant, respectively. The linear fitting sorption of the Freundlich and Langmuir models are displayed in Figures S2 and 3b, respectively. The corresponding parameters and the correlation coefficients (R2) of the applied sorption isotherm models are generalized in Table . It was apparent that the Langmuir sorption model showed higher R2 values than the Freundlich model at different temperatures. This result suggested that the sorption isotherms were fitted well by the Langmuir equation in which the monolayer coverage of Cu(II) was predominantly on the sorbent surface.[38] The actual achievable sorption capacity of Ca-HC for Cu(II) was found to be 130.57 mg g–1 at 303 K, which was much higher than that of most biochars (Table ).

Table 1

Isotherm Parameters of Cu(II) Sorption by Ca-HC

		Langmuir equation			Freundlich equation
T (K)	qe,exp	Qo (mg g–1)	KL	R2	KF	1/n	R2
303	130.57	138.50	0.09116	0.99959	28.71	0.3198	0.90198
313	136.69	143.27	0.12304	0.99973	32.82	0.3089	0.83559
323	144.65	149.92	0.17068	0.99962	39.44	0.2878	0.76312

Table 2

Maximum Cu(II) Sorption Capacities of Biochars

adsorbent	qe (mg g–1)	references
earthworm manure-derived carbon material	24.27	(40)
activated magnetic biochar	85.93	(41)
biochar derived from green macroalgae	137	(42)
banan peels biochar	75.99	(43)
cauliflower leaves biochar	53.96	(42)
MnO2/biochar composites	124	(44)
amino-modified biochar	17.01	(45)
Ca-HC	138.50	this study
pistachio green hull biochar	19.84	(46)
biochar derived from red macroalga Porphyra tenera	75.1	(47)
biochar derived from KMnO4-treated hickory wood	34.2	(48)

Sorption Kinetics of Cu(II) onto Ca-HC

Figure a shows the Cu(II) sorption kinetics onto Ca-HC at different temperatures (303–323 K). The Cu(II) sorption increased sharply during the first 120 min, followed by a slower increase till reaching an sorption equilibria at about 480 min. Thus, the sorption equilibrium was achieved approximately within 8 h.

Figure 4

(a) Influence of time on Cu(II) sorption onto Ca-HC; (b) and (c) kinetic analysis of Cu(II) sorption based on the linear plot of pseudo-second-order rate equation and pseudo-first-order rate equation; and (d) two-step intraparticle diffusion model simulation for the sorption of Cu(II) onto Ca-HC.

To confirm the process of sorption kinetics and the rate-controlling steps of Cu(II) onto Ca-HC, the experimental data were simulated by the pseudo-first-order (eq ),[49] pseudo-second-order (eq ),[50] and intraparticle diffusion models (eq ).[51] The equations involved in the calculation are as followswhere qe and q are the quantity of Cu(II) absorbed by Ca-HC at equilibrium and time t, respectively. k1 (min–1), k2 (g min–1 mg–1), and kid (mg g–1 min1/2) are the pseudo-first-order and the pseudo-second-order and intraparticle diffusion rate constants, respectively. Besides, C (mg g–1) is proportional to the thickness of the boundary layer.

Figure b,c displays the linear kinetic sorption of Cu(II) on Ca-HC by the pseudo-first-order and pseudo-second-order models, respectively. The calculated and R2 values from the kinetic models are shown in Table . Table shows that the pseudo-second-order model might have higher R2 values compared with those of the pseudo-first-order model. Besides, the experimental qe values (qe,exp) were close to those (qe,cal) calculated in the pseudo-second-order model. Based on this data analysis, it was likely that the pseudo-second-order model was in line with the kinetic process, and this justified the fact that the interaction with the sorbent and sorbate was chemisorption.[50]

Table 3

Sorption Kinetic Parameters of the Pseudo-First-Order and the Pseudo-Second-Order Models Derived from Linear Regression for Cu(II) Sorption on Ca-HC

		pseudo-first-order model			pseudo-second-order model
T (K)	qe,exp	qe,cal (mg g–1)	k1 (min–1)	R2	qe,cal (mg g–1)	k2 (mg g–1 min–1)	R2
303	104.75	73.22	0.00893	0.98388	112.49	2.119 × 10–4	0.99976
313	113.39	50.33	0.00854	0.93120	119.76	3.031 × 10–4	0.99910
323	121.91	43.21	0.01155	0.93542	126.26	4.718 × 10–4	0.99926

Interestingly, the intraparticle diffusion equation could describe the sorption kinetics of Cu(II) onto Ca-HC. The plots of q versus t1/2 are shown in Figure d and the process of sorption was fitted by two straight lines with high R2.[51] The calculated values are shown in Table S1. The higher slope of the first region originated from the fast sorption of Cu(II) on the available external surface of Ca-HC. The second region with a lower slope was controlled by intraparticle diffusion. The regression linear curves during two stages failed to cross the origin, which meant that the intraparticle diffusion could not be considered as the only step to control the rate during the sorption process.[52]

Initial pH and Cation Concentrations on the Sorption of Cu(II)

The influence of the initial pH value of Cu(II) solution is presented in Figure a. The sorption of Cu(II) increased from 84.45 to 149.97 mg g–1 with the increase of initial pH value of Cu(II) solution from 3.0 to 6.5. Moreover, the equilibrium pH values were higher than the initial pH values. As the initial pH values were low, the low level of Cu(II) uptake by Ca-HC could be ascribed to the increase of protonation of these organic functional groups,[12] and thus the ability to form surface complexation between organic functional groups on the surface of the Ca-HC with Cu(II) decreased. Moreover, as the initial solution pH decreased, calcite on the surface of Ca-HC was gradually converted to soluble calcium bicarbone, which remained as solid in the solution.[37] Thus, the contribution of carbonate of Ca-HC to remove Cu(II) gradually disappeared. It provided another explanation for the decreased uptake of Cu(II) by Ca-HC at a lower initial pH.

Figure 5

(a) Effects of pH on the sorption of Cu(II) by Ca-HC. (b) Effect of cations on Cu(II) sorption by Ca-HC.

The influence of cations on the relative Cu(II) sorption gave further insight into the mechanisms of Cu(II) binding by Ca-HC. The values of qe were little affected by coexisting cations (Na+, K+, Mg2+, and Ca2+) (Figure b). The results revealed that the cations had negligible influence on the sorption of Cu(II) onto Ca-HC, and the Cu(II) sorption mechanism was not governed by any cation-exchange mechanism.

Study of Sorption Thermodynamics

The enthalpy (ΔH°), entropy (ΔS°), and Gibbs free energy (ΔG°) values of the sorption of Cu(II) onto Ca-HC could be derived by the following equationswhere T is the absolute temperature, R (8.314 J mol–1 K–1) is the universal gas constant, and KL is the constant of the Langmuir equilibrium. The values of ΔH° and ΔS° could be obtained through the van’t Hoff plots (Figure S3). The calculated parameters are shown in Table . The negative values of ΔG° showed that Cu(II) sorption onto Ca-HC was spontaneously and thermodynamically favorable. The negative values of ΔG° increased as the temperatures increased, which meant that the sorption was favored at higher temperatures.

Table 4

Thermodynamic Analysis Data of Cu(II) Sorption by Ca-HC.

temp. (K)	ΔG (kJ mol–1)	ΔH (kJ mol–1)	ΔS (J mol–1 K–1)	R2
303	–21.84	25.49	155.85	0.99619
313	–23.35
323	–24.97

The positive values of ΔH° showed the endothermic nature of sorption, that is, as the temperature increased, the capacities of the Cu(II) sorption also increased.[53] The positive value of ΔS° indicated a rise in the randomness at the interface of solid solution in the Cu(II) sorption process. As a result, the driving force for Cu(II) onto Ca-HC was due to the enthalpy and entropy effects.

Sorption Mechanisms of Cu(II) on Ca-HC

To delve on the removal mechanism of Cu(II), the FTIR spectra of Ca-HC before and after Cu(II) sorption are shown in Figure b. After the Cu(II) sorption, the asymmetric stretching vibration of carboxylate at 1607 cm–1 was blue-shifted and the symmetric stretching of the carboxylate group at 1379 cm–1 increased, indicating that surface complexes possibly occurred between Cu(II) and the carboxylate group on the Ca-HC surface.[30] The intensities of the carbonate at 1427 and 879 cm–1 clearly decreased after Cu(II) sorption, indicating that carbonate was involved in the Cu(II) sorption reaction.[36] The band intensity at 1060 cm–1 increased and new bands appeared at 1112, 793, 603, 515, and 434 cm–1, which could be attributed to the introduction of new components on the surface of Ca-HC.[37] To further confirm the contribution of the chemical precipitation in Cu(II) sorption. The Ca-HC before and after sorption of Cu(II) was analyzed by XRD (Figure a). The characterization diffraction peaks of calcite in the XRD pattern decreased, while a new diffraction peak of posnjakite appeared, suggesting that calcite played an important role in Cu(II) sorption. However, the effect of cations on Cu(II) sorption was negligible (Figure b), which confirmed the no cation exchange and electrostatic interaction occurred between Cu(II) and calcite on the Ca-HC surface. Based on the XRD and FTIR analysis, it was concluded that the possible precipitation reaction (eq ) was proposed for Cu(II) sorption on calcite.

The elemental analysis showed that the S content increased from 0.58 to 1.41% after the sorption of Cu(II). This assumption of the increase of S content came from the precipitate of Cu4(SO4)(OH)6·2H2O. One gram of Cu-HC contained 66.4 mg of Cu(II) according to elemental analysis, which suggested that precipitation was an important mechanism driving Cu(II) sorption.

Conclusions

Calcite-impregnated hydrochar sorbent was synthesized by a facile one-step hydrothermal technique for the removal of Cu(II). The as-prepared Ca-HC as a dual sorbent has high efficiency of removing Cu(II) from aqueous solution. The sorption mechanisms were involved in the surface complexation of Cu(II) by the oxygen-containing functional groups and precipitation of Cu4(SO4)(OH)6·2H2O from copper sulfate and hydroxide from the hydrolysis of carbonate. The calcite can increase the sorption capacity of the hydrochar for Cu(II) and the maximum sorption capacity of Ca-HC for Cu(II) calculated obtained from the Langmuir model was 138.50 mg g–1 at 303 K. Therefore, this study suggested a novel strategy to enhance effectiveness of hydrochar as a sorbent for the removal of Cu(II) from the aqueous solution.

Materials and Methods

Materials and Chemicals

All chemicals were of analytical purity and applied as received. Hydrochloric acid, sodium carbonate, copper(II) sulfate pentahydrate, sodium hydroxide, calcium chloride, sodium chloride, magnesium chloride hexahydrate, and potassium chloride were used as received from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Wood powder (particle size less than 250 μm) provided by Linan Mingzhu Bamboo and Wood Powder Co., Ltd as feedstock was used to produce hydrochar.

The stock solution of copper (1000 mg L–1) was prepared through the dissolution of 3.9293 g of CuSO4·5H2O in 1000 mL of deionized water and then diluted to appropriate concentrations.

Preparation of Ca-HC and HC

Forty grams of dried wood powder (less than 60 mesh), 5.0 g of CaCl2, and 5.0 g of Na2CO3 were mixed with 200 mL of deionized water and stirred for 15 min at room temperature. Then, the mixture was placed in a 500 mL Teflon autoclave, heated for 12 h in an electric oven at 200 °C, and cooled down to room temperature. Afterward, the product was filtered, washed with 1000 mL of deionized water, and dried at 80 °C. The products obtained before and after the sorption of Cu(II) were denoted as Ca-HC and Cu-HC, respectively.

The preparation process of wood hydrochar (HC) was the same as that of Ca-HC except that CaCl2 and Na2CO3 were not added as reactants.

Characterizations of Ca-HC and Cu-HC

The concentrations of Cu(II) after sorption were obtained by a Z-2000 Frame Atomic Absorption Spectrophotometer (HITACHI, Japan). The carbon, hydrogen, sulfur, and nitrogen contents of hydrochars were determined by an Elementar Vario ELIII elemental analyzer. Fourier transform infrared (FTIR) spectra of hydrochars were carried out using a Spectrum 65 FTIR instrument (PE, America) in the range of 400–4000 cm–1 for 32 scans with the resolution of 4 cm–1 using KBr pellets. The morphological structures of hydrochars were examined by a field emission scanning electron microscope (SEM) (FEI Quanta F250). The composition of the elements on the surface of hydrochars were determined by an energy-dispersive X-ray analyzer (EDX, Oxford X-max 80). The phase structures were identified by a Bruker D8 advance X-ray diffractometer with a Cu Kα radiation (Germany) at a resolution of 0.02°. The solid phases were identified using Jade 6.5 software. A AutoPore IV 9500 Hg porosimeter (Micromeritics) was applied to determine the structural properties. The BET surface area and pore volume were determined using Micromeritics ASAP 2020.

Sorption Experiments

The batch experiments in this work were executed by adding 40 mg of Ca-HC to 50 mL of Cu(II) solution in a 150 mL conical flask, and the mixtures were horizontally shaken on a mechanical shaker (SHY-2A, China) at a speed of 110 rpm for the predetermined time at the desired temperature. After sorption, the mixtures were filtered and the residual concentrations of Cu(II) were analyzed by atomic absorption spectrophotometer with the wavelength of 324.8 nm. The quantity of Cu(II) that was adsorbed onto Ca-HC was worked out according to eq shown below[54,55]where Co and Ce (mg L–1) are the initial and equilibrium concentrations of Cu(II) in solution, respectively. V (mL) is the solution volume. W (mg) is the weight of Ca-HC. qe (mg g–1) is the amount of Cu(II) adsorbed at equilibrium.

The sorption isotherms were obtained at different concentrations of Cu(II) solutions (30, 60, 90, 120, 150, 200, 250, and 300 mg L–1) at the natural pH of solution and the temperatures of 303–323 K for 24 h.

The sorption kinetics of Cu(II) onto Ca-HC were studied at different time intervals (30, 60, 90, 120, 180, 240, 360, 480, and 600 min), and the concentration of Cu(II) was 120 mg L–1 at the initial pH and temperatures of 303–323 K.

To study the influence of pH on the Cu(II) sorption, Cu(II) solution of 120 mg L–1 was adjusted to varied values (3.0–6.5) by 0.1 mol L–1 of HCl and NaOH and shaken at 303 K for 24 h.

The effect of cations was studied by adding 40 mg of Ca-HC to 120 mg L–1 Cu(II) solution containing different concentrations of NaCl, KCl, MgCl2, and CaCl2 (0.0–0.2 mol L–1) at natural pH and 303 K for 24 h.