3.1 The sorption performance of TMBC
The adsorption performance of TMBC composite (10g) on the removal of Cr (VI) (200mg/L) in potassium dichromate solution (pH = 4) was compared in Fig. 1(a), which showed the adsorption ability of TMBC-450, TMBC-550, TMBC-650 was 34.82, 37.69 and 41.10 mg/g, respectively. TMBC-650 exhibited the highest adsorption performance in that the thermally activated tourmaline has more specific surface area and negatively charged surface, which could enhance the adsorption of heavy metal ions (Li et al. 2015; Wang D et al. 2018). As a consequence, the adsorption of Cr (VI) would increase because of the enhancement of electrostatic repulsion and the strengthening of the polarity of tourmaline (Wang et al. 2019).
The sorption of Cr (VI) on TMBC-650 with different ratio of TM:BC (1:1, 1:2, 1:3, 2:1, 3:1) was compared in Fig. 1(b), showing different adsorption behaviors. When the proportion of TM was 50%, the adsorption efficiency was the lowest with a capacity of 33.43 mg/g. And when it decreased to 25%, the adsorption capacity increased to the highest level (42.86 mg/g). When the proportion of TM increased to 66.6% and 75% (w/w), the adsorption capacity decreased to 35.96 mg/g and 34.86 mg/g, respectively. The composite would produce agglomerate and disperse unevenly with increasing proportion of TM, so the adsorption efficiency of Cr (VI) on the composites decreased with the increasing of TM. However, TM in the composite would not get enough dispersed when TM increased too much. TM might be agglomerated and waken its ability of polarizes spontaneously. Besides, the composite might disperse easily and not bind tightly when BC in low proportion. Thus, the composite TMBC-650 with 25% of TM performed the optimal adsorption performance and it was employed for the following characterization and experiments.
3.2 Characterization of TMBC
Scanning electron microscopy (SEM) images of TM, BC and TMBC-650 were shown in Fig. 2. The surface of TM was smooth, while the BC showed loose and porous. TMBC-650 showed many clusters and the surface was rough and multilayer, which was effective for TMBC to absorb heavy metals.
The surface chemical analysis of TMBC was studied by energy dispersive spectrometer (EDS) as shown in Fig. 2. The main elements were C, O, Si, Fe, Al and Ca and the oxygen accounted for 26.88% of the total mass, which was about six times of the silicon content. It concluded that TMBC might contain a large amount of oxygen-containing functional groups and oxides, such as hydroxyl groups, carboxyl groups, aluminum oxide and ferrous oxide, which could combine with metal ions in wastewater (Jia et al. 2018).
The crystallographic structures of TM, BC and TMBC were identified by X-ray diffraction (XRD) in Fig. 3. TM has a complex structure, with different peaks at the diffraction angles of 14.35°,17.02°,18.75°, 20.933°, 22.018°, 26.62°, 28.768°, 34.501°, 44.569°, 46.876°, 55.763° and 64.785°, indexing to characteristic peaks of tourmaline (Hawthorne et al. 1999). BC has no obvious diffraction peaks due to amorphous material. TMBC possessed the same diffraction peaks of TM and BC, demonstrating that the sintering process did not damage the crystal structure of the raw material and tourmaline might be uniformly dispersed in biochar.
The FTIR spectra of the samples were recorded in Fig. 4, and TMBC have more absorptions bands than TM and BC between 400cm− 1-2000cm− 1. The main peak at 945 cm− 1 and 1047 cm− 1 were corresponded to Si-O-H stretching vibration and O-Si-O stretching, respectively (Wang F et al. 2018). The bands at 710 cm− 1 and 780 cm− 1 were ascribed to the bending stretching of M-O (M = Fe, Mg or Al), indicating that the surface groups were attached on the surface metallic ions, which were likely potential active sites during the adsorption (Yin et al. 2015). The absorption band at 1649 cm− 1 was ascribed to H-O-H (Barbier et al. 2000).
3.3 Kinetics and isotherms of Cr (VI) adsorption on TMBC
The adsorption kinetics of Cr (VI) g/L) on TMBC (g) in mL water (pH = 4) was carried out and the experimental data were fitted using pseudo-first-order and pseudo-second-order kinetic models in the following Eqs. (2)-(3).
Qt = Qe (1-e− k1t) (2)
Qt= Qe2k2t/(1 + Qek2t) (3)
Where Qt (mg/g) is the adsorption amount at time t (min), k1, k2 are the rate constants, corresponding to the first-order adsorption (1/min) and the pseudo-second-order adsorption (g/ (mg· min)), Qe (mg/g) is the maximum amount of adsorption per unit mass of adsorbent in equilibrium.
As shown in Fig. 5a, the adsorption capacity of TM, BC and TMBC increased rapidly within 200 minutes, and then slowed down to reach the equilibrium within 420 minutes. The maximum adsorption capacity of TMBC was 53.10 mg/g, which was much higher than that of TM and BC. The adsorption process was fast at early time due to many adsorption sites available at the initial stage of adsorption, low mass transfer resistance on the surface of adsorbent and fast adsorption reaction speed. As the surface adsorption sites were gradually filled up, the adsorption was more dependent on the transport of adsorbate from the external sites to the internal sites of the adsorbent, and the adsorption speed was reduced (Yu et al. 2000).
The adsorption kinetics models fitted the experimental data and the parameters were listed in Table 1. It showed that the pseudo-second order is better than the pseudo-first order kinetic model for Cr (VI) adsorption by TM, BC and TMBC. This indicated that the adsorption behavior of Cr (VI) on TMBC was more related to chemical absorption (Cheng et al. 2008). During heat treatment at higher temperature dehydrated hydroxyl groups was incorporated in the crystal lattice, and tourmaline may undergo chemical reaction. Different from Cr (VI)) adsorption on TM and BC, the composite of TMBC mainly depends on the ion exchange sites and specific adsorption sites (Barbier et al. 2000). The adsorption of Cr (VI) on TMBC is mainly due to its special adsorption mechanism and structural characteristics of TM, such as spontaneous polarization, release of negative ions and resolution of metal bonds (Zhang et al. 2011). The results showed that the adsorption capacity of TMBC was greatly improved, and the combined process had a positive effect on the adsorption. SEM results showed that there are many pores in the biochar structure, which is helpful for tourmaline dispersion. Thus, the addition of biochar can reduce the aggregation of tourmaline, which improve the spontaneous polarization of tourmaline. In addition, the increase of specific surface area of micro-size tourmaline can provide many sites for Cr (VI) adsorption. Moreover, high temperature treatment can activate the negative surface charge and specific surface area of tourmaline (Chen et al. 2019), improve the thermal stability of tourmaline, and enhance the adsorption properties of TMBC composites.
Table 1 Model fitting parameters for adsorption kinetics of Cr (VI) by TM, BC and TMBC.
Langmuir model Eqs. (4) and Freundlich model Eqs. (5) were used to describe the isothermal adsorption of Cr (VI) on TMBC.
Langmuir isotherm: Qe = (4)
Freundlich isotherm: Qe = KFC1/ne (5)
Where Qe (mg/g) is the equilibrium adsorption capacity, Ce (mg/L) is the concentration of adsorption equilibrium, Qmax (mg/g) is the maximum sorption capacity, KL, KF are the Langmuir constant (L/mg) and Freundlich constant ((mg/g) (mg/L)−n), respectively, and 1/n is the adsorption intensity.
The adsorption isotherms of Cr (VI) on TMBC were shown in Fig. 5(b) and the model fitting parameters were listed in Table 2. The Langmuir model was more consistent with the experimental data, and the R2 value of the former is closer to 1 than that of the Freundlich model. In addition, the adsorption of Cr (VI) on TMBC surface is monolayer adsorption. The molecular weight distribution was even, and the adsorption energy on the surface was uniform (Awual et al. 2014). Due to the great contribution of ions, this led to the inhomogeneity of surface exchange sites (Zhou et al. 2015). At the same time, the 1/n value in Langmuir isotherm was less than 1, indicating that the adsorption of Cr (VI) by TMBC was a chemical adsorption (Duan et al. 2018). The results showed the maximum adsorption capacity of TMBC for Cr (VI) was 55.10 mg/g.
Table 2 Model fitting parameters for isothermal adsorption of Cr (VI) by TMBC.
3.4 Effects of solution pH on Cr (VI) adsorption by TMBC
The adsorption of Cr (VI) (200mg/L) on TMBC (24mg) in 50mL water was compared under the effect of different initial pH values from 4.0 to 10.0 as shown in Fig. 6. When the pH increased from 3.0 to 7.0, the removal efficiency of Cr (VI) by TMBC decreased, while increased from pH 7.0 to 10.0. The maximum adsorption was only 19.89 mg/g at pH 7, While the maximum of adsorption was 43.01mg/g at pH 4. In addition, the adsorption efficiency of TMBC for Cr (VI) in acidic condition is better than that in alkaline or neutral condition. Such behavior is attributed to the neutral pH of the solution associated with the polar mechanism of tourmaline (Xia et al. 2006). The results suggested that TMBC is a nice absorption for the removal of Cr (VI) in acidic wastewater.
3.5 Adsorption mechanism
In this study, TMBC composite with strong adsorption capacity for the removal of Cr (VI) was obtained by sintering tourmaline and biochar. The adsorption mechanism of TMBC can be explained by the following factors. Firstly, tourmaline can be polarized automatically in TMBC and produce negative ions as Eqs. (6)-(9) (Li et al. 2016). Meanwhile, the M-O bond (m = Na, Mg or Fe) of metal ions on tourmaline surface is easy to decompose, and then exposed to a large range of aqueous solutions. These ions are easily attracted by polar water molecules, leaving the crystal surface and entering into water phase, which results in many negative potentials on the mineral surface.
H2O → H++OH− (6)
OH− +H2O → OH− (H2O)n (7)
Cr6++ OH− → Cr (OH−)5+ (8)
Cr6++ OH− (H2O)n → Cr (OH−)5+ (H2O)n (9)
Cr6++ 6Si-O-H− → 6Si-O-Cr + 6H+ (10)
Secondly, there are a lot of siloxanes and silanol groups on the surface of TMBC as shown in Fig. 4, which can significantly improve the adsorption performance of Cr (VI). Some Si-O bonds can be directly broken and interact with water molecules. So the surface of TMBC can be hydroxylated and Cr (VI) can directly replace the protons as Eqs. (10). On the other hand, the oxygen atoms in the solution can directly complex with Cr (VI) or Cr 6+ (H2O) n to reduce the concentration of Cr (VI) (Wang et al. 2012). As FTIR spectrum of TMBC-Cr in Fig. 7, the absorption peaks of 719cm− 1 and 783 cm− 1 represent the spectral vibration of Cr-O, and the absorption peak of 1045 cm− 1 represents the spectral vibration of Si-O-Cr.
Thirdly, the cation exchange mechanism plays an important role in the removal of heavy metals. The cations such as Fe 3+and Cu2+ on the surface of TMBC can be replaced by Cr (VI), then the concentration of TMBC can be reduced by complex adsorption. Therefore, TMBC can adsorb Cr (VI) by electrostatics and complexation, such as metal bonds on composite surface, hydroxylation and self-polarization.