Study on Chromium Ion Adsorption from Wastewater by Sugarcane Residues

Ruigang WANG Hangfeng CHEN

Abstract With the application of chromium increasing， wastewater shows an increase of chromium pollution. In particular， Cr （VI） has become a major concern because of its high toxicity. Cr （VI） is recognized to be much more toxic than Cr （III）. Sugarcane residue is a byproduct of the sugar industry， and it is an important renewable biomass resource. In this paper， sugarcane residues were used to remove chromium ions from wastewater in order to develop cheap and efficient heavy metal adsorption materials. The effects of pH， sugarcane residue dosage， sugarcane residue particle size and chromium ion initial concentration on chromium ion removal and the kinetics of chromium ion removal at normal temperature were investigated. The results showed that the removal rate of Cr （VI） and Cr （III） increased with an increase of sugarcane residue concentration， and decreased with an increase of particle size and the initial concentration of chromium. The removal effect of Cr （III） increased with increasing pH， and the removal effect of Cr （VI） decreased with increasing pH. The removal kinetics of chromium fitted well with a pseudosecondordermodel. Sugarcane residues had a higher adsorption capacity for Cr （III） than for Cr （VI）. This paper provides a basis for the treatment of chromium containing wastewater or other heavy metal wastewater in the future.

Key words Sugarcane residue; Adsorption; Cr （III）; Cr （VI）

With increasing industrialization， chromiumcontaining substances are getting widely used， resulting in the pollution of chromiumcontaining wastewaters. In particular， Cr （VI） has become a major concern because of its high toxicity[1]. Therefore， the removal of chromium ions from wastewater has attracted great attention. At present， there are several methods to remove chromium， but most of them have some shortcomings， such as high cost， large dosage of reagents， incomplete removal， secondary pollution and other issues[2]. Adsorption treatment of heavy metal wastewater is an ideal and practical technology with good removal effect and simple operation[3-4]. Several reports are provided about substances that are directly used with modification as adsorbent materials[5-13]. However， due to the high price of adsorbents used in industry， regeneration of adsorbents and secondary pollution， it is difficult to popularize the application of adsorption methods. Therefore， it is of great significance to develop cheap and efficient adsorbents for heavy metal wastewater.

China is a major sugarcanegrowing country. The annual output of sugarcane exceeds 7≠107 t， and the output of sugarcane residues exceeds 7≠106 t， which are discharged from sugar mills， occupying a large amount of land and causing environmental pollution[14]. There are also reports on the comprehensive utilization of sugarcane residues， but only limited studies deal with the adsorption mechanism of heavy metals[15]. Therefore， if sugarcane residues are used to treat heavy metal wastewater， waste water pollution can be eliminated， but also to solve the problem of sugarcane residues dumping in certain areas. Sugarcane residues are a kind of renewable biomass with high cellulose content and large surface area， which can be used to remove heavy metals from wastewater. However， there is only limited research on the removal of chromium by sugarcane residues. Under certain specific environmental conditions， Cr （III） and Cr （VI） can coexist in wastewater， and Cr （III） can also be converted into more toxic Cr （VI）.

The effects of pH， dosage of sugarcane residues， particle size of sugarcane residues， and initial concentration of chromium on the removal of chromium from simulated wastewater were investigated at room temperature.

Experimental Materials and Methods

Sugarcane residues

Sugarcane residues were harvested from sugarcane by pressing the stems. Sugarcane residues were dried for 2 h in a constant temperature drying box （model： 10103BS） at 100. Then， they were divided into 60 meshes by mortar and sieving.

Adsorption of chromium ions

The solution of Cr （III） and Cr （VI） with mass concentration of 1 g/L was prepared by dissolving 5.125 g of CrCl·6H2O and 2.833 g of K2Cr2O7 into 1 L of deionized water using a balance. 1.0 g of sugarcane residue and 30 ml of chromium ion solution with known initial concentration were put into 200 ml of polyethylene plastic bottles. The pH of the sample was adjusted to the desired size by hydrochloric acid or sodium hydroxide solution. The oscillator was oscillated for 24 h at room temperature. After the adsorption reached equilibrium， the adsorbent was removed by filtration and the supernatant was left. Then the concentration of total chromium in the supernatant was determined by atomic absorption spectrometry and the concentration of chromium （VI） was determined by spectrophotometry. The concentration of Cr （III） was calculated by the difference between the concentrations of total chromium and Cr （VI） in the filtrate. pH was determined by pH （model： PHS3s）. In this study， the effects of pH， sugarcane residue dosage， sugarcane residue particle size and initial chromium concentration on the removal efficiency of Cr （III） and Cr （VI） were investigated.

Adsorption kinetics of chromium ions

Samples of 30 ml volume were taken respectively， mass concentration of Cr （III） was 200 mg/L and mass concentration of Cr （VI） was 50 mg/L. 1.0 g of sugarcane residues were added to the polyethylene bottle of 200 ml volume. The polyethylene plastic bottles were placed on an oscillator to take the reading at room temperature. After a certain time of oscillation， sugarcane residues were separated from the filtered solution. The supernatant was selected to determine the concentration of total chromium and Cr （VI）.

Characterization of sugarcane residues

The surface structure of sugarcane residues was analyzed by scanning electron microscopy （model： JSM6360LV）， and the functional groups of sugarcane residues were analyzed by infrared spectroscopy （model： AVATA360）.

Results and Discussion

Characteristics of sugarcane residues

Scanning electron microscopy （SEM） was used to observe the surface morphology of sugarcane residues. The results showed that sugarcane residues had many pores， larger surface area and a rough structure， which was beneficial to adsorption of ionic chromium. A large number of new particles appeared on the surface of sugarcane residues after adsorption of Cr （III） and Cr （VI）， which could be inferred to be caused by ion adsorption.

The changes of functional groups in sugarcane residues were determined by infrared spectroscopy， and the spectrum wavelength range was determined. The results showed that the peak values of 3 383.2， 2 918.1， 1 730.5 and 1 054.2 cm-1 were hydroxyl， alkyl， carboxyl and ether groups， respectively. The peak bands of carboxyl， ether and hydroxyl groups were shifted to 1 731.5， 1 055.4 and 3 407.2 cm-1 respectively when Cr （VI） was absorbed by sugarcane residues. When Cr （III） was adsorbed by sugarcane residues， only the peak displacement of hydroxyl group was observed. The results showed that different functional groups of sugarcane residues participated in the adsorption of Cr （III） and Cr （VI）， resulting in different removal behaviors of Cr （III） and Cr （VI）.

Effect of pH on chromium removal

The initial concentrations of Cr （III） and Cr （VI） were 100 and 50 mg/L， respectively. The amount of sugarcane residue was 1.0 g， and the oscillating time was 24 h. The effect of pH on the removal behavior of chromium was investigated. The results are shown in Fig. 1. The removal of Cr （III） and Cr （VI） was greatly influenced by pH. When the pH changed from 2 to 5， the removal efficiency of Cr （VI） decreased rapidly and then increased slightly. At pH 2， more than 80% of Cr （VI） in the solution was removed by sugarcane residues. At pH 5.5， the removal rate was 29.1%. The adsorption effect of pH on Cr （VI） may be attributed to the characteristics of sugarcane residues and the types of Cr （VI）. There was competition between Cr （VI） and protons on the surface binding sites of adsorption materials. The presence of Cr （VI） in water may be due to dichromate ions or chromate ions， or both， which was related to the acidity and alkalinity of water. Acidic chromate ions were the main components at low pH. When pH increased， the ratio of acid chromate ions were decreasing.

Unlike Cr （VI）， the removal efficiency of Cr （III） will increase with the increase of pH. When pH was 5， more than 90% of Cr （III） in the solution was removed. When the pH was 2， the removal rate of Cr （III） was only 42.5%. An adsorption trend is attributed to the competition of Cr （III） and proton on the surface of sugarcane residues. At low pH， the excess protons can compete with Cr （III）， resulting in a small amount of Cr （III） being adsorbed. Increasing pH can reduce the competition of protons in the system. Precipitation may occur at high pH. In order to avoid the precipitation of Cr （III）， the maximum pH of all Cr （III） adsorption experiments was controlled below 6.0.

Effect of the amount of sugarcane residues

The initial mass concentrations of Cr （III） and Cr （VI） were 100 and 50 mg/L， respectively. The oscillation time was 24 h. The amount of sugarcane residues was changed from 0.4 to 2.4 g. The effect of the amount of adsorbent material on the removal behavior of chromium was investigated. The results are shown in Fig. 2.

The removal rate of chromium increased with an increase of the amount of sugarcane residues. The maximum removal rate of chromium （III） exceeded 90%， the maximum removal rate of chromium （VI） exceeded 60%， and the amount of sugarcane residues was 1.6 g. When the amount of sugarcane residues was not large enough， the possible adsorption sites were not enough compared with a large number of chromium ions， resulting in poor removal. When the amount of sugarcane residue was large， there were many adsorption sites. Further increase of the amount of sugarcane residues was not increasing the removal efficiency. The effective contact surface area of the adsorbent material will also decrease with a higher amount of sugarcane residues， resulting in a decrease of the unit adsorption capacity.

Effect of sugarcane residue size

The initial concentrations of Cr （III） and Cr （VI） were 100 and 50 mg/L， respectively. The amount of sugarcane residues was 1.0 g and the oscillation time was 24 h. The effect of the size of sugarcane residue particles on the removal of chromium was studied. The results are shown in Fig. 3. With a decrease of sugarcane residue particle size from 100 mesh to 200 mesh， the removal rate of Cr （VI） increased from 25% to 46%， and the removal efficiency of Cr （III） increased from 68% to 83%. The removal rate of chromium ion can increase by reducing the particle size of sugarcane residues， and the surface area of the adsorption material can increase by small particles. In general， under the same dosage of sugarcane residues， small particles can provide more sites to adsorb ions， resulting in higher removal rate.

Effect of initial concentration of chromium ion on chromium removal

The amount of sugarcane residue was 1.0 g， and the oscillation time was 24 h. We studied the removal of Cr （III） and Cr （VI） with initial concentration. The results are shown in Fig. 4 and Fig. 5. When the initial concentration of Cr （III） was changed from 40 to 200 mg/L， the adsorption capacity increased from 1.12 to 2.85 mg/g of the initial concentration of Cr （III）. Thereafter， the adsorption capacity of Cr （III） increased little. On the contrary， the removal efficiency of Cr （III） decreased with an increase of the initial concentration， due to the limited adsorption sites of sugarcane residues. The adsorption of Cr （VI） also had such a similar pattern. However， the adsorption capacity and removal rate of Cr （VI） were significantly lower than those of Cr （III）.

Adsorption kinetics of chromium ions

The initial mass concentrations of Cr （III） and Cr（VI） were 200 and 50 mg/L， respectively， and the amount of sugarcane residue was 1.0 g， oscillating for 24 h. The relationship between the removal behavior of Cr （III） and Cr （VI） and the removal time was investigated. The results are shown in Fig. 6. Within 24 h， the removal rates of Cr （III） and Cr （VI） increased rapidly， more than 36% Cr （VI） and more than 57% Cr （III） were removed. After that， the removal rate of Cr （III） and Cr （VI） increased slowly. It can be concluded that the adsorption of Cr （III） and Cr （VI） on sugarcane residues can reach adsorption equilibria after 24 h.

Different kinetic models were used to fit the experimental data， and the rate control steps of sugarcane residue removal process were discussed. The mechanism of chromium adsorption by sugarcane residue was analyzed. The specific parameters of the kinetic model are shown in Table 1. From table 1， the correlation coefficient of the pseudofirstordermodel of Cr （III） was 0.936 and the adsorption capacity was 1.085 mg/g. The predicted adsorption capacity of Cr （III） was much lower than that of experimental data 3.528 mg/g. Therefore， it can be considered that the pseudofirstordermodel was not suitable for the adsorption of Cr （III） from sugarcane residues.

The results showed that the pseudosecondordermodel was fit well with the Cr （III） adsorption well， and the correlation coefficient was larger， with the value of 0.998， which was better than the pseudofirstordermodel. Moreover， the calculated adsorption capacity of Cr （III） was 3.546 mg/g， which was close to the experimental data of 3.528 mg/g. It can be considered that it was appropriate to describe the adsorption behavior of Cr （III） by the pseudo second order model. The model assumed that the rate control step was a chemical adsorption through the sharing or exchange of electrochemical forces between adsorbed materials and heavy metal ions[16-17]. Through the fitting of the model， we can think that the rate control of Cr （III） was mainly chemical adsorption. Through data fitting analysis， the kinetic data of Cr （VI） were similar to those of Cr （III）.

Conclusions

（1） Different functional groups on the surface of sugarcane residues participated in the adsorption of Cr （III） and Cr （VI）， resulting in different removal behaviors of Cr （III） and Cr （VI）.

（2） Because of the strong adsorption of active sites of sugarcane residues， chromium ions in wastewater can be removed. Sugarcane residues had a stronger adsorption capacity for Cr （III） than Cr （VI）. The adsorption of Cr （III） wastewater was better than that of Cr （VI） wastewater.

（3） The removal of chromium ion by sugarcane residue followed the pseudosecondorderkinetic model， and the adsorption rate was mainly controlled by chemical adsorption. The calculated equilibrium adsorption capacity of Cr （III） was 3.546 mg/g， and the calculated equilibrium adsorption capacity of Cr （VI） was 0.619 mg/g， which was close to the experimental data.

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