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Efficiency of Coconut Husk as Agricultural Adsorbent in Removal of Chromium and Nickel Ions from Aqueous Solution

Coconut husk is categorized as the agriculture waste product that can act as biosorbent material. In this study, coconut husk was used as an adsorbent to investigate the ability of this agricultural by-product to adsorb nickel and chromium. In this experiment, nickel and chromium ion were adsorbed in batch adsorption experiment using different parameters; effect of contact time (30, 60, 90, 120, 150) minutes, pH (3, 5, 7, 9,11), adsorbent dosage (1.0, 1.5, 2.0, 2.5, 3.0) g. An optimum level of adsorption for chromium by using raw coconut husk occurred at 90 minutes of contact time, pH 3, and dosage of adsorbent of 1.5g. Meanwhile, optimum adsorption of nickel occurred at contact time of 90 minutes, with pH 5, adsorbent dosage of 1.5g. Statistical analysis showed that there was a significant mean difference between percent removal of chromium (Cr) and nickel (Ni). There was a significant mean difference between dosage and percent removal of chromium ions. In conclusion, both raw and acid-treated coconut husk can adsorb nickel and chromium ions from aqueous solutions.


Introduction
Water pollution has affected the lives of humans and animals in all aspects. Most of the sources of water pollution are caused by human activity, in spite the fact that a few of them emanate from natural sources of pollution [1]. Example of sources of pollution from industry include energy and fuel production, mining and smelting of metalliferous, surface finishing industry, iron and steel, electroplating, electrolysis, metallurgy, leather working, fertilizer and pesticide industry, photography, and aerospace [2,3].
Chromium (Cr) and Nickel (Ni) are used primarily in metallurgic industries for producing alloys and in electroplating industry for making batteries and paints [4]. Heavy metals are highly toxic and non-biodegradable. It withal considered carcinogenic to human and cause many health effects on human. The excessive exposure toward Ni may lead to a few types of cancer including lungs, nose, and bone [5]. Hence, Ni and Cr should be treated at their permissible level before it is ready to be discharged to the environment.

Characterization of adsorbents
Flame Atomic Absorption Spectrophotometer (FAAS) by Perkin Elmer was used to determine the concentration of Cr and Ni in the adsorption process ( Figure. 1(c)). Fourier-Transform Infrared Spectroscopy (FT-IR) was used to detect the presence of functional groups in a coconut husk beside to analyze the content of carbon, hydrogen, oxygen, and sulphur. The FT-IR absorbance data were collected in the range of wave number 400-4000cm -1 .

Batch adsorption study
Stock solution of 1000 mg/L was prepared by dissolving nickel sulphate, NiSO4 and chromium oxide, CrO2 in deionized water that contains Cr(VI) metal ions. The aqueous solution of CrO2 solution were prepared from in five concentrations: 10, 20, 30, 40 and 50 mg/L using serial dilution of the stock solution. Flame Atomic Absorption Spectroscopy (FAAS) was utilised to determine the concentration of the aqueous solution.
During the experiment, four parameters were measured; effect of contact time, pH, adsorbent dosage and the initial concentration of Cr (VI) towards adsorption process. Firstly, 1.0g of adsorbent was added in 250 mL beaker containing 100 mL of 10 mg/L Cr(VI) solution. Then, the solution was stirred at 400 rpm using magnetic stirrer until it reached equilibrium. During the process, 10 mL was withdrawn from the solution at pre-determined time interval and was centrifuged before the concentration of Cr(VI) being analysed by FAAS. The different contact time was carried out at 30, 60, 90, 120, and 150 minutes. To study the effect of pH, the sample pH (3,5,7,9,11) was adjusted with 0.1 M NaOH and 1. 0 M HCl. Various amounts of adsorbents (1.0, 1.5, 2.0, 2.5, 3.0) g were used to study the effect of adsorbent dosage. The experiment was performed in triplicate.

Percentage of removal of Cr (VI)and Ni
The removal percentage was calculated by using the following equation. Removal (%) = 0 − 0 100 , where 0 and are the concentration of initial and after adsorption respectively.

Data analysis
Statistical Package of Social Science (SPSS) software was used to analyze the data. The frequency and percentage were obtained through descriptive statistics. Non-parametric test (Mann Whitney) was performed to determine if there is significant difference between the percent removal of heavy metal by raw and acid-treated coconut husk. Figure. 2 displayed on how the adsorption occurred with different time for Cr and Ni solution using 1 g dosage of adsorbent and 10 mg/L initial concentration. For raw coconut husk, it was observed that the removal percentage of both ions in aqueous solution were increasing for the first 30 minutes, are increasing. Then, the trend becomes constant afterward. Between contact times of 90 to 150 minutes, the percent removal of Cr ions ranged 60% to 65%, meanwhile for Ni ions range 80% to 100%. The explanation behind this is due to the vacant adsorbent sites of the coconut husk. At minute 90, the absolute concentration peaked and the removal of Cr and Ni become constant. Meanwhile, acid-treated coconut husk has lower percent removal of Cr and Ni as compared to raw coconut husk. The acidic pH of the coconut husk was contributed by the acid. This was due to the electrostatic interaction between Cr and Ni ions and the raw coconut husk structure and composition [21]. This equilibrium state of time at 30 minutes was due to all of the active sites of acid treated coconut husk had been occupied led to less efficiency in adsorbing Cr and Ni ions after 30 minutes [18]. Thus, for the next procedure of determining optimum level, 90 minutes of contact time was chosen for removing Cr and Ni ions using raw coconut husk.  Figure 2. Raw coconut husk performance towards removal of both ions with different time

Effect of pH
The adsorption behaviour of both ions upon the change of pH is portrayed in Figure 3. As the pH affects the degree of ionization of the adsorbate during reaction, concentration of the counter ions on the functional groups of the adsorbent, and the solubility of the metal ions, is it an important parameter to be considered in the adsorption process [22]. From Figure.2, the highest percent removal of the Cr by the coconut husk is at pH 3 (86%). The Cr showed low affinity for electrostatic interaction with the positively charged composite, hence the lowest removal of Cr was observed at pH 7 [23]. This is almost similar to the previous report, which stated that pH 4 is the optimum pH for the maximum removal of both ions but significantly decreased at pH 5-6. However, their pattern slightly increased at higher pH values [24]. In this study, the removal was decreased after pH changed to 10 and onwards.

Figure 3. Effect of pH for adsorption of Cr and Ni using raw coconut husk
In addition, based on Figure. 3 it was found that the highest removal of Ni ions was at pH 5 which had 99% percentage of removal. This indicated that pH 5 was the most suitable pH for the adsorption of Ni on the raw coconut husk. At pH more than 5, the Ni precipitated due to hydroxide anions forming a nickel hydroxide precipitate. As reported by the previous study, H+ ions competed with Ni on the surface of the adsorbent in pH less than 3. The repulsive forces exist caused the Ni to be rendered from reaching the binding sites of the sorbent [25]. Thus, the optimal pH value was selected to be 5 [26].   Figure. 4 illustrates the percentage removal of Cr and Ni ions increased with increase of dosage of adsorbent until reaching 1.5 g of adsorbent. According to the graph, 1.5 g of adsorbent had the highest percent removal of Cr and Ni ions. After 1.5 g, the percent removal of Cr and Ni became constant and decreased because the adsorbent was saturated and it decreased the surface area [27]. In agreement with previous study, more availability of adsorption sites resulted to the increase of adsorption [20,25]. In this study, the removal of Cr and Ni was achieved at 1.5g of raw coconut husk.  Figure. 5A shows that raw coconut husk had a higher peak at 3363.71 cm -1 indicated the presence of bonded hydroxyl group in stretching mode (O-H group). Acid-treated coconut husk had a lower peak at 3336.21 cm -1 because during acid hydrolysis, major components of the functional group were removed because hydrochloric acid changes the structural and composition of the adsorbent. From the Figureure, peaks of (C-H), (C-O), and (C-O-C) for raw coconut husk were higher than acid-treated coconut husk which at 2912.46 cm -1 , 1606.64 cm -1 , and 1033.80 cm -1 respectively.  between the functional group of coconut husk and Cr/Ni [19]. After adsorption, the peaks of the functional group shifted to a lower wavelength. This also indicated that the raw coconut husk adsorbs the metal ions efficiently and has a high adsorption capacity. From this study, the percentage of adsorption varies non-linearly with pH, on the contrary, the adsorption varied linearly with the amount of adsorbent and concentration with time but [20].

Comparison of mean difference between percent removal of heavy metal adsorbed by raw and acid treated coconut husk
Mann-Whitney test was used to compare the mean difference of percent removal of Cr and Ni between raw and acid-treated coconut husk, since the data was not normally distributed. Based on Table 1, the p-value was 0.016 indicating that there was a significant difference in the amount of metals reduced by raw and acid-treated coconut husk. Raw coconut husk had higher percent removal as compared to acidtreated coconut husk. For Ni, the p-value was 0.009. The result again shown significant difference in the amount of the metal reduced using raw and acid-treated coconut husk. Raw coconut husk recorded a higher percent removal as compared to acid treated coconut husk.

Conclusion
In summary, coconut husk was the adsorbent that has a high ability to adsorb metal ions. This was due to the surface of coconut husk that contain functional groups which can bind to the metal ion. Coconut husk has a good adsorption as all the parameters achieve their optimum state. In this study, the optimum conditions for adsorption of Cr using raw coconut husk were 90 minutes contact time, pH 3, 1.5g adsorbent dosage at 20 mg/L initial concentration of Cr aqueous solution. Meanwhile, the optimum conditions for adsorption of Ni were 90 minutes of contact time, pH 5, and at 1.5g adsorbent dosage. Statistical analysis shown that there was a significant mean difference of percent removal of Cr and Ni between raw and acid-treated coconut husk, whereby raw coconut husk has a higher ability to adsorb Ni and Cr. In conclusion, raw and acid-treated coconut husk had a good ability to adsorb heavy metals especially Cr and Ni using method of adsorption.