J. Cent. South Univ. (2015) 22: 4184−4192 DOI: 10.1007/s11771-015-2966-z
Characteristics and mechanisms of Ni(II) removal from aqueous solution by Chinese loess WANG Yan(王艳)1, TANG Xiao-wu(唐晓武)2, WANG Heng-yu(王恒宇)2 1. Faculty of Architectural, Civil Engineering and Environment, Ningbo University, Ningbo 315211, China; 2. Research Center of Costal and Urban Geotechnical Engineering, Zhejiang University, Hangzhou 310058, China © Central South University Press and Springer-Verlag Berlin Heidelberg 2015 Abstract: Nickel is a toxic heavy metal among trace elements which has a detrimental impact on living organisms. There is growing need of finding an economic and effective solution for Ni(II) immobilization in environments. Chinese loess was selected as adsorbent to remove Ni(II) from aqueous solution. Adsorbent dosage, reaction time, solute concentration, temperature, and solution pH also have influences on efficiency of Ni(II) removal. The monolayer adsorption capacity of loess towards Ni(II) is determined to be about 15.61 mg/g. High temperature and pH favor the removal of Ni(II) using Chinese loess soil and the optimal dosage of loess is determined to be 10 g/L. The kinetics and adsorption isotherms of the adsorption process can be best-fitted with the pseudo second order kinetics and Langmuir isothermal model, respectively. The thermodynamic analysis reveals that the adsorption process is spontaneous, endothermic and the system disorder increases with duration. Nickel ions can be removed with the removal efficiency of 98.5% at pH greater than or equal to 9.7. Further studies on loess and Ni(II) laden loess (using X-Ray diffraction, Fourier transform infrared spectroscopy) and Ni(II) species distribution at various pH have been conducted to discuss the adsorption mechanism. Loess soils in China have proven to be a potential adsorbent for Ni(II) removal from aqueous solutions. Key words: adsorption mechanism; kinetics; isotherm; loess; nickel removal
1 Introduction Heavy metal contamination in surface and subsurface environments has driven a lot of scientific researches all over the world. Accumulation of excessive amounts of trace elements (such as nickel) in soils and waters renders severe toxic effects on living organisms. Nickel can be released into environments via nickel mine smelting, industrial waste disposal, electroplating and battery manufacturing processes [1−4]. Once entering soil or water, nickel ions can easily be bioaccumulated and spread along the food chain. Excessive exposure to nickel-containing materials can result in detrimental chronic disorders to human lungs, nose and bone [2, 5]. Therefore, limiting the levels of nickel in environments seems vitally important. Over the past few decades, many techniques have been developed to remove heavy metals, including oxidation, reduction, precipitation, ion exchange, membrane separation, solvent extraction, ultrafiltration, reverse osmosis, adsorption, etc [5−9]. Precipitation and ion exchange are the most commonly adopted among these methods. The major problem with precipitation is
the disposal of the precipitated waste. In addition, the precipitation itself cannot reduce the contaminant far enough to meet current environmental quality standards. Ion exchange can reduce the metal ion to a very low level, but the high cost limits its application. In recent years, adsorption has been found to be both efficient and cost effective since the process does not involve the production of sludge which will add to the cost of recovery of metal ions [10−11]. Activated carbon is an adsorbent with high efficiency due to its high specific surface area and convenient regeneration from spent carbon [12−13]. However, its high cost inhibits largescale application in developing countries. As a result, the need for effective and economical new sorbents aroused interest of many researchers. The removal of heavy metals using natural adsorbents has been studied intensively. Clay minerals, as important constituents in soil, play the role of scavenger for metals during the process of surface runoff and underground flow. Many kinds of minerals such as zeolite [14−15], goethite [16−19], bentonite [20−21], kaolinite [22−25], montmorillonite [26−28] and calcite [29−30] have been reported to remove heavy metals. Factors that influence the adsorption of Ni(II) have also
Foundation item: Projects(51179168, 51308310) supported by the National Natural Science Foundation of China; Project(LQ13E080007) supported by Zhejiang Provincial Natural Science Foundation of China Received date: 2014−10−20; Accepted date: 2015−01−14 Corresponding author: WANG Yan, PhD; Tel: +86−574−87600337; E-mail:
[email protected]
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been investigated by researchers [30−33]. As a typical soil containing abundant clay minerals in China, the loess soil has abundant porous structure and clay minerals. Correspondingly, a thought has occurred to us that loess soil may be a potential appealing material for retention of heavy metals. Removals of Cu(II), Zn(II), Pb(II) and Cd(II) from aqueous solutions using Chinese loess have recently been reported and the loess shows a high affinity for heavy metals [34−37]. In China, loess soil is easily available and cost effective. As an adsorbent removing heavy metals from wastewater, loess soil will have a promising future, and our studies can also provide some reference meaning for loess soil applied as a pollutant containment material in geoenvironmental engineering. Taking into account of the detrimental effect of Ni(II) and the excellent character of Chinese loess, the present work is aimed at investigating the adsorption behavior of Ni(II) on loess and revealing the adsorption mechanism. In practice, the constituents existing in heavy metals contamination usually are very complicated having not just only one heavy metal but in combination with other inorganic and organic pollutants. In this work, we have prepared an aqueous solution containing Ni(II) only. Adsorption behavior of single heavy metal Ni(II) on loess soil should be investigated first before we know how other heavy metals or organic matters influence Ni(II) removal. Based on the adsorption behavior of single metal on loess soil, adsorption behavior of multiple heavy metals coexisting in one pollution source will be further studied in the near future. Considering the factors that affect efficient uptake of nickel ions such as reaction time, temperature, solute concentration, loess dosage and pH value, batch experiments under different conditions were performed. The interaction mechanisms between Ni(II) and loess were analyzed with X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) spectra of both fresh and Ni(II)-loaded loess.
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ignition at 650 oC for 8 h in an oven in the presence of O2 gas. The mass loss was assigned to the organic content. The BET specific surface area was measured to be 24.1 m2/g from N2 adsorption isotherm with Autosorb 1-MP apparatus (Quantachrome Corporation, USA). Its cation exchange capacity (CEC) was determined to be 11.2 cmol(+)/kg. The pHpzc of loess was determined by acid-alkaline titration of the sorbent. The solution pH was measured with a glass electrode potentiometer (Shanghai Precision & Scientific Instrument Co., Shanghai, China). The basic parameters of loess are summarized in Table 1. Table 1 Basic parameters of loess CEC/ Organic Specific surface area/ pHna pHpzc content/% density (m2·g−1) (cmol(+)·kg−1) 55
2.75
24.1
11.2
8.16 2.82
The chemical constituents, mineral components and surface functional groups of loess were tested by ICP-MS (ICP-MS PQ3, Thermo Electron Corp. USA), XRD (D/MAX-RA, Rigaku Corp., Japan, equipped with a Cu K tube and Ni filter) and FT-IR (Nexus-670, Nicolet, USA), respectively. The Ni(II) loaded loess soil sample was prepared following the procedure: A group of batch test was carried out by equilibrating solutions (40 mL) including loess soil (0.4 g) and nickel ions (100 mg/L) in a thermostat at 25 °C for 24 h and the supernatant was poured out when the equilibrium solution was centrifuged. The bottom sludge was oven dried at 100 oC and then was sent to the Analytical and Testing Center of Zhejiang University for XRD and FT-IR test. The chemical compositions of loess are presented in Table 2. Table 2 Chemical compositions of loess (mass fraction, %) SiO2
Al2O3
CaO MgO
K2O
Fe2O3
Na2O
FeO
63.68
12.77
9.56
3.01
2.74
2.35
0.89
3.14
2 Materials and methods 2.1 Preparation of adsorbent and adsorbate The adsorbent, Chinese loess, was sampled from the suburban area of Xi’an, China, which is typical Quaternary loess located on the Chinese Loess Plateau. The soil was oven-dried at 105 oC for 24 h to remove bulk water, cooled to room temperature, and then sealed in plastic bags for storage. The reagent nickel chloride hexahydrate (NiCl2·6H2O) used in this work was of analytical grade. Ni(II) stock solution (1 g/L) was prepared in deionized water (DW). 2.2 Characterization of loess The organic content in loess was determined by
2.3 Adsorption experiments using batch method To ensure the accuracy and reliability of data collection, all batch experiments were repeated once and blank experiments were also conducted in the same concentration range as used in adsorption experiments. 2.3.1 Effect of adsorbent dosage Five different dosages of loess (i.e., 2, 5, 10, 20 and 40 g/L) were investigated in order to determine appropriate adsorbent dosage. The initial Ni(II) concentration was separately set at 25, 50, 100, 200 and 300 mg/L and so 5 groups of experiments were performed simultaneously. pH values of solutions were not adjusted in the duration. The sample flasks were put into a thermostatic box agitated at 180 r/min at 25 °C for
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24 h. Afterwards, the slurry was centrifuged at a speed of 3000 r/min for 5 min to obtain the supernatant. The atomic absorption spectroscopy (AAS) was then used to determine the equilibrium Ni(II) concentration. The Ni(II) removal ratio, R (%) was calculated by R Ci Ce Ci 100%
(1)
where Ce (mg/L) and Ci (mg/L) are the equilibrium and initial concentrations of nickel ions, respectively. 2.3.2 Kinetics experiments The adsorbent dosage was fixed at 10 g/L, and the initial solute concentration was 50 mg/L. The solution pH was not adjusted and the reaction temperature was maintained at 25 °C. The reaction time was set in the range of 15−1440 min. Specific samples were taken out at the end of predetermined time intervals. The samples were then centrifuged at 3000 r/min for 5 min. The obtained supernatant liquids were detected using AAS to determine the concentration of Ni(II) in solution. 2.3.3 Isotherms experiments Five sets of isothermal adsorption tests were conducted separately at 5, 15, 25, 35 and 45 °C. The loess dosage was 10 g/L and the initial solute concentration ranged from 25 to 200 mg/L. The solution pH was not adjusted. All samples were equilibrated for 24 h in the controlled temperature shaking box and the equilibrium Ni(II) concentrations were measured by AAS. 2.3.4 Effect of pH The same amounts of sorbent (i.e., 0.4 g) and Ni(II) solution (100 mg/L, 40 mL) were put into several pretreated PVC (Polyvinylchlorid) tubes. The initial pH values, pH0, of the solutions were adjusted from 5.4 to 11.5 by adding 0.1 mol/L HCl or NaOH solution. The sample tubes were then placed into a thermostatic box agitated at 180 r/min at 25 °C for 24 h. The pH values of solutions were measured at the end of the test using a glass electrode potentiometer (pH 213, China). The solutions were then centrifuged at 3000 r/min for 5 min. The supernatants were sampled to determine the Ni(II) concentration by AAS.
concentration of 25 mg/L. The effect of loess dosage on the amount of Ni(II) adsorbed per unit mass (qe) of loess is shown in Fig. 2. Contrary to the removal ratio, qe decreases with increasing the dosage of loess at each specific initial solute concentration and the largest qe occurs at the largest solute concentration. The number of available adsorption sites increases by increasing the adsorption dosage and it, therefore, results in the increase of removal ratio. The decrease in qe with increase in the loess dosage is mainly due to unsaturation of adsorption sites through the adsorption reaction. In addition, high adsorbent concentration could create particle aggregation, which would lead to decrease in total surface area of the sorbent and an increase in diffusional path length [9, 24].
Fig. 1 Removal ratio of Ni(II) at different loess dosages
Fig. 2 Amount of Ni(II) adsorbed per unit mass of loess
3 Results and discussion 3.1 Effect of adsorbent dosage Figure 1 shows the effect of loess dosage on the removal ratio of Ni(II) from aqueous solutions. The removal ratio of Ni(II) increases rapidly with the amount of loess dosage and reaches maximum at dosage of 10 g/L. Thereafter, the removal ratio keeps still with no obvious change against continuous increase of loess dosage. It is also found that the removal ratio is larger at a smaller initial solute concentration and the removal efficiency can reach 86.8% at an initial solute
3.2 Adsorption kinetics Figure 3 shows the adsorption of Ni(II) by the loess as a function of contact time. The plot indicates that the adsorption progresses in two steps, consisting of a rapid adsorption process within the initial 15 min and a slow adsorption step during which the equilibrium is obtained. In order to investigate the rate law of Ni(II) adsorption by loess, the kinetic data obtained from the batch experiments were analyzed using three kinetic equations, i.e., the pseudo first order kinetics, the pseudo second order kinetics and the intraparticle diffusion
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time is similar with that by WANG et al [3]. The adsorption amounts per unit mass fitted with the pseudo first order and the pseudo second order are 3.16 and 3.46 mg/g, respectively, very close to each other.
Fig. 3 Effect of contact time on uptake of Ni(II) by loess
model. The pseudo first order equation can be represented by [38] ln(qe qt ) ln qe k1t
(2)
where qe (mg/g) and qt (mg/g) are the amounts adsorbed per unit mass at equilibrium and at any time t, respectively, and k1 (min−1) is the pseudo first order adsorption rate constant. The pseudo second order equation can be expressed as [39]
3.3 Adsorption isotherms Ni(II) adsorption is greatly influenced by the initial solute concentration and reaction temperature, as observed in the adsorption isotherms presented in Fig. 4. The isotherms seem to be of the “S” type at 278 and 288 K, and be of the “L” type at 298−318 K according to the classification of the solute adsorption isotherm by GILES and SMITH [41]. The nickel ions amount adsorbed per unit mass loess increases as the equilibrium solute concentration increases. The adsorption isotherms of Ni(II) were simulated by the mathematical equations of Langmuir, Freundlich and Dubinin-Radushkevich (D-R). The Langmuir model assumes that the removal of metal ions occurs on an energetically homogenous surface by monolayer adsorption, and the Langmuir adsorption isotherm equation is given as follows [42]: qe QbCe 1 bCe
(5)
where k2 (g·mg−1·min−1) is the adsorption rate constant of pseudo second order. The intraparticle diffusion equation is given by [39]
where qe (mg/g) and Q (mg/g) denote the amount adsorbed at equilibrium and the monolayer adsorption capacity, respectively, Ce (mg/L) is the equilibrium concentration in solution, and b (L/mg) is the Langmuir constant which is related to the energy of adsorption. The linear form of the Langmuir isotherm is given by
qt kint t1/ 2 C
Ce qe Ce 1 b Q
t qt 1 (k2 qe2 ) 1 qe t
(3)
(4)
where kint (mg·g−1·min−1/2) is the intraparticle diffusion rate constant and C is the intercept. The kinetic parameters for adsorption of Ni(II) by loess are given in Table 3. The correlation coefficients obtained by the pseudo first order kinetics, the pseudo second order kinetics and the intraparticle model are 0.969, 0.999 and 0.963, respectively. The test data are well fitted with the three models above, and the pseudo second order kinetics is the most appropriate model for Ni(II) adsorption by loess with the largest correlation coefficient of 0.999. Several previous studies on the adsorption of Ni(II) from aqueous solutions show that the kinetics generally also follows the pseudo second order rate law [3, 24, 31, 40], and the process of two steps development of Ni(II) adsorption with reaction
(6)
where Q and b can be calculated from the intercept and slope of the linear plot, with Ce/qe versus Ce. The Freundlich model, which assumes the surface heterogeneity and exponential distribution of active sites, provides an empirical relationship between the adsorption capacity and equilibrium constant of the adsorbent. The mathematical representation of this model is [42] qe K FCe1/ n
(7)
where KF (mg/g) and n are the Freundlich constants that are related to the adsorption capacity and intensity, respectively. The Freundlich equation can be written in the linear form as
Table 3 Kinetic parameters for Ni(II) adsorption by loess Pseudo-first order kinetics −1
qe/(mg·g ) 3.16
−1
k1/min
0.101
Pseudo-second order kinetics −1
−1
−1
Intraparticle diffusion model
R
qe/(mg·g )
k2/(g·mg ·min )
R
k/(mg·g−1·min−1/2)
C
R
0.969
3.46
0.009
0.999
0.026
2.499
0.963
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their high correlation coefficients. The Langmuir model has a better fit with the test results than the Freundlich and the D-R models since all the Langmuir correlation coefficients are over 0.940, and accordingly, the latter shows very good fit with the test results at temperatures higher than 288 K. The monolayer adsorption capacities estimated with the Langmuir model are 10.39, 12.73, 13.84, 13.94 and 15.61 mg/g at 278, 288, 298, 308 and 318 K, respectively, showing an increasing trend with increasing the reaction temperature. The natural loess is quite a good adsorbent for removal of Ni(II) in contrast to the adsorption capacity of 4.51, 1.26 and 4.73 mg/g reported by WANG et al [3], BOUJELBEN et al [5] and CARVALHO et al [31] using nanoscale magnetite, natural iron oxide-coated sand and silylated clay, respectively. The Langmuir constant b is found to have the same variation law increasing from 0.021 to 0.149 L/mg when the temperature varies from 278 to 318 K. The larger b indicates that the loess surface is covered with more nickel ions as a result of the stronger affinity of Ni(II) towards the surface at a higher temperature, which is consistent with the adsorption capacity at different temperatures. The parameter KF related to adsorption capacity obtained using the Freundlich equation is found to have the similar variation trend with the adsorption capacity Q obtained with the Langmuir model. The Freundlich constant n ranges from 1.752 to 2.746 at 278−318 K, all greater than unity, suggesting some degree of heterogeneity of this Ni(II)/loess system. The qm predicted with the D-R model is 29.88, 38.10, 39.07, 29.28 and 29.96 mg/g at temperatures ranging from 278 to 318 K, much larger than the adsorption capacity Q obtained from the Langmuir model. This is due to the assumption of the volume filling of micropores in the D-R model which describes
Fig. 4 Isotherms of Ni(II) adsorption on loess
ln qe ln K F 1 n ln Ce
(8)
where Freundlich constants KF and n can be calculated from the slope and intercept of the linear plot, with ln qe versus ln Ce. The D-R isotherm model assumes a uniform porefilling adsorption and can predict the free adsorption energy change. The D-R model can be written as [42] ln qe ln qm k 2
(9)
where qm (mol/g) is the maximum adsorption capacity, k is the model constant which is related to the free adsorption energy and ε is the Polanyi potential which is related to the equilibrium concentration as follows:
RT ln 1 1 Ce
(10)
The mean free energy of adsorption E is obtained by E 1
(11)
2k
The isothermal parameters obtained using the three isotherm models are presented in Table 4. All these three isotherm models well fit the experimental data due to
Table 4 Predicted isothermal parameters for Ni(II) adsorption on loess Model
Parameter
278
288
298
308
318
10.39
12.73
13.84
13.94
15.61
b/(L·mg )
0.021
0.022
0.037
0.057
0.149
R
0.947
0.940
0.981
0.977
0.994
KF/(mg·g )
0.57
0.62
1.00
1.64
3.00
n
1.847
1.752
1.909
2.332
2.746
0.893
0.947
0.991
0.999
0.999
Q/(mg·g−1) Langmuir
−1
−1
Freundlich
R −1
qm/(mg·g )
29.88
38.10
39.07
29.28
29.96
−2
0.0067
0.0064
0.0052
0.0037
0.0026
−1
E/(kJ·mol )
−8.63
−8.83
−9.84
−11.67
−13.79
R
0.903
0.955
0.996
0.996
0.996
2
D-R
Temperature/K
k/(mol ·kJ )
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an ideal state almost impossible to realize. The adsorption energy is estimated to be −8.63, −8.83, −9.84, −11.67 and −13.79 kJ/mol at 278−318 K. The absolute value of the adsorption energy is found to increase along with increasing the temperature, and all lie within 8− 16 kJ/mol, suggesting an ion exchange mechanism [43−44]. 3.4 Adsorption thermodynamics In order to explain the effect of temperature on the adsorption process, thermodynamic parameters, i.e., Gibbs free energy change (ΔGΘ), enthalpy change (ΔHΘ) and entropy change (ΔSΘ), were determined. The thermodynamic parameters are summarized in Table 5. The Gibbs free energy change is negative and decreases with increase in temperature at each specific solute concentration, indicating that adsorption of Ni(II) by loess is spontaneous and spontaneity increases with increasing the temperature. The value of the enthalpy change is positive and decreases from 74.41 to 19.60 kJ/mol with increasing the solute concentration from 25 to 200 mg/L, implying that the adsorption process is endothermic. The entropy change is positive and decreases from 283.23 to 80.26 J·mol−1·K−1 as the solute concentration ranges from 25 to 200 mg/L, indicating an increase in the degree of freedom of the adsorbent-adsorbate system and less disorder at higher initial solute concentration. Table 5 Thermodynamics parameters for Ni(II) adsorption on loess ΔHΘ/ ΔSΘ/ C0/ ΔGΘ/ T/K R −1 −1 −1 (mg·L ) (kJ·mol ) (kJ·mol ) (J·mol−1·K−1)
25
100
200
278
−5.34
288
−6.50
298
−9.05
308
−12.15
318
−16.94
278
−3.62
288
−4.29
298
−5.83
308
−6.59
318
−9.44
278
−2.71
288
−3.55
298
−4.37
308
−4.88
318
−6.07
74.41
283.23
0.967
34.85
136.91
0.956
19.60
80.26
0.991
3.5 Effect of pH The effect of pH on Ni(II) removal from aqueous solutions is shown in Fig. 5. It is seen that the pH values of solutions greatly influence the adsorption amount of Ni(II) on loess. The adsorption behavior of Ni(II) on loess is very similar with that on Na-rectorite in aqueous solution studied by CHANG et al [32] within the same pH range. Only a small amount of Ni(II), 8.3%, is removed from the solution in acidic condition when pH= 5.4. As the initial pH of the solution increases from 5.4 to 8.8, the removal rate of Ni(II) increases to 92.1% rapidly. Then, as can be seen from the slope in Fig. 5, the rate of Ni(II) removal becomes a bit slower when continuously increasing the pH values of the solution. The removal efficiency of Ni(II) reaches 98.5% at pH=9.7 and basically remains constant at higher pH values. Therefore, nickel ions in aqueous solutions can be nearly completely removed under a strong alkaline condition.
Fig. 5 Effect of pH on Ni(II) removal
No buffer solution was used to maintain a constant pH of the solutions. It is noted that the pH of all the solutions changes at the end of 24 h exposures. The unadjusted pH of all the solutions is 7.2. The equilibrium pHe increases to 7.2 for the solutions whose initial pH0≤ 6.6, but decreases for those pH0≥7.6, indicating that loess has a strong buffering effect on acid solution rather than on alkaline solution. 3.6 Discussion of mechanisms involved Figure 6 shows the XRD patterns of loess and Ni(II) loaded loess. The main mineral constituents of loess are quartz, calcite, albite, goethite, and kaolinite as determined from the characteristic bands on the XRD spectra [36]. The patterns have some changes when being loaded with Ni(II). New peaks are observed at 2=24.28, 30.47 and 36.66 which are related to nullaginite (Ni2(CO3)(OH)2). A new peak related to Ni2SiO4 is found at 2=57.54 as well according to the
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Fig. 6 XRD spectra of loess (a) and Ni(II)-loaded loess (b)
Fig. 7 FT-IR spectra of loess (a) and Ni(II)-loaded loess (b)
MDI Jade software. By contrast, the intensities of some peaks, i.e., those of kaolinite at 2=19.80, albite at 2=35.00, calcite at 2=48.57, goethite at 2=61.55 and quartz at 2=45.80, 64.06, 73.49 and 79.91 in Ni(II)-loaded loess are found to become stronger than those in loess, indicating that minerals in loess play an important role in adsorption of Ni(II). The formation of Ni2(CO3)(OH)2 can be ascribed to the calcite in loess, and the reaction between Ni(II) and calcite can be written as
—COOH+Ni2+→—COONi++H+
2Ni 2 CaCO3 2H 2 O Ni 2 (CO3 )(OH) 2 Ca 2 2H
(12)
Accordingly, the formation of Ni2SiO4 can be ascribed to the hydrolysis of quartz in loess, and the process can be written as SiO 2 2H 2 O SiO 44 4H
(13)
2Ni 2 SiO 44 Ni 2SiO 4
(14)
No peaks regarding to Ni(OH)2 are found in XRD spectra. The solution pH of the Ni-loess system is around 7.2, and the solution is undersaturated with respect to Ni(OH)2 under this reaction condition according to MATTIGOD et al [45], which is consistent with our study. ROBERTS et al [46] and ELZINGA and SPARKS [47] studied Ni adsorption under similar condition with our experiments, and found that mixed Ni-Al layered double hydroxide precipitate was formed on Al-containing clay minerals. This could be another adsorption mechanism in our work in terms of the abundance of Al element in loess. Figure 7 shows the FT-IR spectra of loess and Ni(II) loaded loess. The absorption bands at 1596 and 1352 cm−1 in loess disappear in Ni(II)-loaded one. Both bands can be assigned to the bending vibration of the carboxyl group [48] which is originally contained in loess in the form of organic matter. The carboxyl groups in the surface of loess can complex with Ni(II) in the following form:
(15)
The surface functional groups related to quartz (i.e., absorption bands at 796, 777, 694, 524 and 472 cm−1) and calcite (i.e., absorption bands at 1797, 1436, 873 and 713 cm−1) have no changes in the FT-IR spectra. They are not detected probably due to the limited adsorption amount. In addition, as can be seen from Fig. 5, the adsorption of nickel ions is greatly influenced by pH values of solutions. As known, the precipitation of heavy metals is greatly dependent upon the pH values of solutions. Therefore, species distribution of Ni(II) at various pH values calculated by Visual MINTEQ shown in Fig. 8 is also used to investigate the adsorption mechanism.
Fig. 8 Species distribution of Ni(II) at various pH values
The results in Fig. 8 are obtained considering 100 mg/L Ni(II) at 25 °C, the same condition with experiments in Fig. 5. The precipitation of Ni(OH)2 can be neglected at pH
