E � cient Removal of Cd ( II ) From Aqueous Solution By Chitosan Modi � ed Kiwi Branch Biochar

Yuehui Tan Northwest Agriculture and Forestry University College of Resources and Environment Xirui Wan Northwest Agriculture and Forestry University College of Resources and Environment Xue Ni Northwest Agriculture and Forestry University College of Resources and Environment Le Wang Northwest Agriculture and Forestry University College of Resources and Environment Ting Zhou Northwest Agriculture and Forestry University College of Resources and Environment Huimin Sun Northwest Agriculture and Forestry University College of Resources and Environment Nong Wang Agro-Environmental Protection Institute Xianqiang Yin (  xqyin@nwsuaf.edu.cn ) Northwest Agriculture and Forestry University https://orcid.org/0000-0002-0831-4181


Introduction
With the development of factories, environmental problems are getting serious, including potentially toxic metal pollution, which has been a threat to the ecology, environment and human health due to the characterizations of high toxicity and bioaccumulation. Among them, Cd (II) is a typical pollutant, and it sources from electroplating, paints, electronics and so on (Mohan et al., 2014). It is well known that Cd (II) Tomczyk et al., 2020). Because these contradictions limit the use of biochar as an effective adsorbent, biochar needs to be modi ed to improve its performances (Hu et al., 2015;Jin et al., 2018;Lu et al., 2015a).
Novel biochar modi cation methods are used to boost biochar adsorption capacity to pollutants from wastewater. The methodologies of surface modi cation are concerned widely due to its simple modi cation procedure and sorption effectiveness. Biochar-based materials can be obtained by Chitosan is a low-cost material, which is prepared by hydrolysis of chitin with sodium hydroxide, and the amide functional group was converted into amino functional groups. Because amino polysaccharide is renewable and biodegradable worldwide, chitosan is relatively cheap. What's more, chitosan have been used to remove heavy metal ions and have fantastic adsorption ability to heavy metals from soil and water as an adsorbent (Gerente et al., 2007;Pontoni & Fabbricino, 2012). In addition, Chitosan contains a large number of amino functional groups, which have strong binding ability to different heavy metals.
Therefore, chitosan can be introduced into the surface of adsorption materials to improve adsorbents' adsorption ability to heavy metals (Bhatnagar & Sillanpaa, 2009;Gerente et al., 2007;Pontoni & Fabbricino, 2012). There is a hypothesis that chitosan accumulates easily, which can become more disperse by combinating chitosan with biochar. The combination is very advantageous, both materials are cheap and environmentally friendly. What's more, chitosan modi ed biochar will combine the larger surface area and porous network structure of biochar as well as the higher adsorption capacity of chitosan, so the chitosan modi ed biochar obtained will have a stronger adsorption capacity for heavy metals. However, no previous studies explored in detail the adsorption characteristics, in uencing factors and potential mechanisms of Cd (II) on kiwi biochar modi ed with chitosan.
In this research, a novel chitosan-modi ed kiwi branches biochar (CHKB) was synthesized and used as an adsorbent for the removal of Cd (II). Batch sorption experiments were used to examine the sorption behaviors of Cd (II) on the CHKB under various conditions. The objectives of this study were to: (a) synthesize and characterize an effective CHKB adsorbent that can be used for Cd (II) removal; (b) determine the adsorption mechanisms of the CHKB to Cd (II); (c) investigate the kinetics and equilibrium isotherms of Cd (II) adsorption on CHKB; and (d) identify the effect of solution pH, dosage and regeneration cycles of the Cd (II) adsorption on the CHKB.

Materials
Kiwi branches were obtained from a kiwi orchard in Shaanxi Province to make biochar. And the kiwi branch from kiwi fruit was separated, chopped, washed and dried. Then it was pyrolyzed at the temperature of 500 ℃ in a Vacuum tube furnace (SG-GL1200K, Shanghai, China) under N 2 ow conditions. The biochars prepared were broken and sifted to keep a uniform size of 0.053-1 mm and collected. The CHKB composites could be obtained according to Zhou et al (Zhou et al., 2013). Brie y, putting 5.0 g of chitosan into 250 mL acetic acid (2 %), then 5.0 g of kiwi biochar was added, and the admixture was stirred about 1 h. The above suspension was added dropwise into a 1500 mL NaOH (1 %) solution, then keeping about 24 h. Finally, the CHKB was obtained by washing, drying, milling and sieving (0.053-1 mm) above compound. The concentration of Cd (II) adsorbed was known according to the aqueous concentrations of incipient and eventual.

Sorption isotherm experiments
Cd (II) sorption on KB and CHKB were measured through putting 0.01 g CHKB into 20 mL diverse concentration Cd (II) solutions, which were from 0.5 to 100 mg L − 1 and 0.5 to 500 mg L − 1 in 50 mL centrifuge tubes respectively. The IS and pH of experiments were set to 0.01M and 6.0 ± 0.2. The centrifuge tubes were vibrated in an oscillator at 190 rpm for 24 h at the temperature of 25 ℃. Then the mixtures were quickly ltered to determine Cd (II) concentration according to the same method and procedure of sorption kinetic. The Cd (II) concentrations adsorbed were known through the aqueous concentrations of incipient and eventual. The post-adsorption KB and CHKB materials were gathered, cleaned and dried at 80 ℃ for characterizations after the experiments.

Effects of pH and dosage
The in uence of incipient solution pH which was from 3 to 7 (3, 4, 5, 6 and 7) on the CHKB adsorption to Cd (II) was tested. In addition, the in uence of the dosage was tested by putting a range of 0.01-0.05 g

Sorption-desorption cycles experiments
Sorption-desorption cycle studies were conducted by using Cd (II) saturated CHKB through sorption isotherm experiment. Then putting Cd (II) saturated CHKB (0.01 g) were added in 20 ml of desorption medium (0.2 mol L − 1 EDTA-2Na) in 50 mL centrifuge tubes. EDTA-2Na was used to elute sorbents which were shaken at 190 rpm for 24 h in a shaker, and then desorbed adsorbents were washed. All of the above tests were repeated.

Materials characterization
The characterization experiments were tested about KB and pre-and post-sorption CHKB. V-Sorb 2800P surface area and pore size analyzer (GAPP-spectrum, Beijing, China) determined the KB and CHKBs' Brunauer-Emmett-Teller (BET) speci c surface area. X-ray diffraction (XRD) analysis was used to study structure using the X-ray diffractometer (D/RAPID II, Rigaku, Japan). Fourier transform infrared spectroscopy (FTIR) (Tensor27, Bruker, Germany) was used to study the functional groups impact in the period of sorption. Scanning electron microscopy (SEM) coupled with Energy dispersive spectrometer (EDS) (S-4800, Hitachi Limited, Japan) con rmed the component elements' morphology and content of biochars. Composition and speciation of surface elements of biochar samples were analyzed by using Xray photoelectron spectroscopy (XPS) (Thermo Scienti c K-Alpha+, America). The characterizations of KB, pre-and post-sorption CHKB were compared to clarify the mechanisms of adsorption.

Main adsorption mechanism
The KB's BET surface is 1.5 m 2 g − 1 , lower than the CHKB's BET surface of 3.3 m 2 g − 1 . In a way, the speci c surface area was a key parameter which is generally desirable for Cd (II) adsorption. The SEM-EDS analyses of the obtained sorbents composites are presented in Fig. 1a-c. The morphology described no regular or amorphous characteristics using SEM (Fig. 1a-c). Compared with the KB's glazed surfaces (Fig. 1a), CHKB composites showed the surface's pimpling (Fig. 1b) through SEM images, which indicated the chitosan was modi ed on biochar matrix. The KB used as an excellent matrix to disperse chitosan and CHKB sorption sites for Cd (II) increased, so the chitosan modi cation is a good in uence on improving Cd (II) adsorption (Fig. 1b). EDS demonstrated abundant O and Na elements in CHKB rather than KB's, which may be the reason for existing abundant oxygenated functional groups on the CHKB composites' surface and it was further supported that the CHKB was modi ed with chitosan successfully ( Fig. 1a-b). And CHKB can improve the adsorption ability to Cd (II), one of the mechanisms might be the ions substitution of cations (Na + , K + , Mg 2+ and so on) with Cd (ΙΙ) (Wang et al., 2018a).
The CHKB's XRD pattern showed its amorphous properties, and the chitosan is represented by dominant peaks (Fig. 1d). The XRD pattern of CHKB in the range of 10-80° is shown in Fig. 1d. Chitosan was found in CHKB pre-and post-Cd (II) sorption, the broad diffraction peaks of CHKB at 2θ = 19.8º matched to chitosan (Gartner et al., 2011). XRD analysis of the pre-and post-CHKB con rmed the hypothesis that the chitosan particles were introduced on the CHKB surface through XRD. In addition, CHKB contains calcium carbonate, which forms cadmium carbonate precipitate after adsorbing Cd (II) (Fig. 1d). Compared with other sorption mechanisms, the precipitation also plays a key role in Cd (II) removal.  (Fig. 1e).
This result further suggests that the surface of CHKB has been covered by chitosan. These peaks altered after the chitosan particles introduction and the Cd (ΙΙ) sorption (Fig. 1e)

Adsorption kinetics
The Cd (II) adsorption kinetics of KB and CHKB were depicted in Fig. 2a and b. The Cd (II) sorption on KB and CHKB composites augmented with the increase in time employing the incipient Cd (II) concentration (50 mg L − 1 of KB and 200 mg L − 1 of CHKB), and sorption of Cd (II) by KB and CHKB reached equilibrium within 120 min. The data of adsorption kinetics experiments were simulated by some models. Moreover, the pseudo-rst-order (Lagergren, 1898) and pseudo-second-order model (Blanchard et al., 1984), Elovich model (Low & M., 1960) and Constant double model (Peng & Robinson, 1976) were applied to describe the Cd (II) sorption kinetics on KB and CHKB were summarized ( Table 1) et al., 2002). The slow process might be related with biochar's pore network and surface, which promoted the dispersion of CHKB to effectively augment its reaction with Cd (II). The high linear correlation (R 2 > 0.72) was known by sorption of pre-equilibrium Cd (II) on the square root of time ( Fig. 2c  and d). The consequence showed intraparticle surface diffusion was signi cant in deciding Cd (II) sorption on the CHKB (Moral-Rodríguez et al., 2016).

Adsorption isotherms
The equilibrium adsorption isotherms of Cd (II) by KB and CHKB are presented in Fig. 3a and b. The Cd (II) sorption capacity of KB and CHKB are 4.24 and 118.43 mg g − 1 respectively. Langmuir (Schmid et al., 1999), Freundlich (Yang, 1998) and Temkin (Johnson & Arnold, 1995) models were studied to explain the Cd (II) sorption on biochars. The corresponding parameters were enumerated in Table 2, these data are modeled and these results were shown in Fig. 3. For the CHKB, the Langmuir maximum capacity of CHKB was 126.58 mg g − 1 larger than the KB's of 4.25 mg g − 1 , and CHKB determination coe cients (R 2 ) around 0.9497. As shown, the Cd (II) sorption on KB is best tted with the Temkin model, but on CHKB suits the Langmuir model. This result indicates the adsorption mechanism of Cd (II) removal with CHKB by singlelayer adsorption onto the chitosan particles on CHKB composites surface. However, determination coe cients (R 2 ) of Freundlich model about CHKB is also high around 0.9299, and it indicates the single and double-layer adsorption play the key roles on Cd (II) adsorption on CHKB. The Table 3 shows a summary of Cd (II) adsorption capacity on modi ed biochars in other studies and the CHKB in this study. Through comparing different modi ed biochars reported previously, it could be known that the maximum adsorption capacities (Q m ) of CHKB in this study (126.58 mg g − 1 ) is notably higher than the other modi ed biochars in literature (0.98-81.1 mg g − 1 ). What's more, it indicates that the kiwi branch is a valuable precursor for the production of engineered biochars and kiwi biochar-based chitosan composites have obvious advantages to Cd (II) removal.

In uence of pH and dosage
The CHKB's sorption ability to Cd (II) from pH 3 to 7 was shown in Fig. 4a. The Cd (II) sorption ability increased with the increase of pH, and the results showed the high pH is bene cial for Cd (II) removal. It can be seen from the Fig. 4a that pH has a great in uence on adsorption. With the increase of pH from 3 to 7, the removal e ciency increased. When pH increased from 3 to 5, the removal e ciency of Cd (II) didn't change obviously. This is because of the competition between Cd (II) and H 3 O + at pH = 3 . With the deprotonation of hydroxyl group, the adsorption of Cd (II) increased. When pH increased from 5 to 6, the removal e ciency of Cd (II) on CHKB also didn't change signi cantly. This might be because the deprotonation of hydroxyl group had reached the maximum. When pH increased to more than 6, the removal rate of Cd (II) increased rapidly, which is the reason for the precipitation formation of Cd (OH) 2 (Liang et al., 2017). There are some different charges hydrolysates of Cd(OH) 2, Cd(OH) + and Cd 2 (OH) 3+ . Thus, Cd (II) may be removed through precipitation and electrostatic interaction (Su et al., 2014).
The optimum dosage of CHKB for Cd (II) removal in adsorption process is vital for its e cient use. In order to maximize the interaction between active sites of CHKB and Cd (II), the experiments about the dosage effect of CHKB to the Cd (II) removal rate and sorption ability have been conducted, and the data were shown in Fig. 4b

Regeneration cycles
The regeneration cycles tests were studied with 0.2 mol L − 1 EDTA-2Na solution as eluent. Figure 5 showed that the adsorptivity of the CHKB and desorption ability of EDTA-2Na to Cd (II) saturated CHKB had no appreciable decrease after ve cycles, and the CHKB could be regenerated and reused for Cd (II) sorption. The consequence described the CHKB's excellent stability, indicating CHKB is a low cost and effective material in the treatment of Cd (II) polluted wastewater.

Conclusions
In this work, a novel chitosan-modi ed kiwi branches biochar (CHKB) was successfully fabricated and was developed for the removal of Cd (II) from aqueous solutions. The adsorption experiment results of Cd (II) on CHKB showed that the adsorption isotherms can be described best by the Langmuir model and that the pseudo-second-order model ts the Cd (II) adsorption kinetics well, indicating that the process was monolayer and controlled by chemisorption. Compared to KB, Cd sorption ability of CHKB (118.43mg g − 1 ) to Cd (II) was about 28 times higher than that of KB (4.24 mg g − 1 ). And CHKB showed the maximum sorption ability of Langmuir for Cd (II) (126.58 mg g − 1 ), owing to larger micropore sizes, bigger speci c surface areas, more adsorption sites and the great deal of oxygen-containing functional (-OH, -NH, C = O and so on) groups compared with KB. The main mechanisms of CHKB's higher adsorption capacities may be cation exchange, electrostatic interaction, surface complexation and precipitation. The adsorption ability can be changed in various conditions. When CHKB dose increased, the adsorption ability dwindled and the removal e ciency increased. What's more, the high pH is bene cial for Cd (II) sorption on CHKB. By comparing various biochar adsorbents, CHKB has excellent advantages of a simpler preparation method and higher adsorption ability. And CHKB can be regenerated and reused for Cd (II) sorption by the eluent of EDTA-2Na. Therefore, CHKB would be a promising, low-cost, and effective adsorbent for Cd (II) removal from wastewater in practical applications. Availability of data and materials The data and materials will become available upon individual requests.

Declarations
Ethical Approval Not applicable.