Efficiency of activated carbon from palm kernel shell for treatment of greywater

Abstract The potential of activated carbon from palm kernel shell (PKS-AC) to improve the quality of surface water and greywater based on the measurements of the parameters pH, turbidity, chemical oxygen demand (COD), total dissolved oxygen (TDS) and total suspended solids (TSS) was investigated in the present study. The PKS was acid treated with aqueous H3PO4 (1 N) for overnight at room temperature and then subjected to heat treatment at 550 °C for 2 h. The efficiency of the PSK-AC samples of 30 and 40 mm thicknesses was examined for reduction of the above parameters in surface water and greywater samples for 5, 15, 30 and 60 min of the filtration process. The efficiency of COD reduction by 50 and 56.44%, that of TDS by 57.81% and 22%, and that of TSS by 83.11 and 42.11% were detected using the PSK-AC samples above, respectively. The OH group contributed most to the removal of pollutants among the OH, N–H, C=O, C=C, C–O–C, C–O–H main functional groups pointed out via Fourier Transform Infra-red (FTIR) analysis on the surface. The scanning electron microscopy (SEM) images revealed that the surface of the raw PSK-AC appeared smooth with holes on the external surface, while the grains were filled in the PSK-AC after the adsorption process.


Introduction
Activated carbon (AC) is the term used to represent a group of absorbing substances of crystalline form with large surface area (1 g of AC has a surface area in excess of 500 m 2 ) and internal pore structures which make the AC more efficient; it is a commonly used absorbent for removal of a wide range of pollutants from wastewater (Askalany et al., 2012). Many materials such as silica gel, zeolites, molecular sieves, activated alumina and synthetic resins are used for the preparation of AC (Zhou et al., 2010). Recently, researchers have moved to using low-cost materials such as cotton silk and coconut shell, saw dust, palm shell, olive stone, walnut shell, grape stalk, bamboo, olive mill, pistachio shell, tropical wood and almond shell (Jankowska et al., 2010). Palm kernel shell (PKS), a by-product of palm oil processing, has a high carbon content, high density and low ash content, and it is produced in large quantities (2 million tonnes annually) in Malaysia. The preparation of AC from natural materials has been reported in the literature (Regti et al., 2017;Selim et al., 2017;Noreen et al., 2017;Bhatti et al., 2017;Shoukat et al., 2017;Tahir et al., 2017). A summary for preparation of AC from coconut and palm shell and its application in the removal of pollutants from wastewater are presented in Table 1. However, the removal efficiency of PKS-AC with regard to pollutants from greywater has not been reported in detail before. Greywater has a different chemical composition than other wastewaters because of the presence of detergents and nondegradable compounds from soap, shampoo and personal care products. This composition might negatively or positively affect the main reduction parameters of greywater, such as chemical oxygen demand (COD), total dissolved solids (TDS), total suspended solids (TSS) and turbidity. Morever, the direct discharge of greywater into the environmnet is asscoiated with adverse effects on ecosystems. Some negative effects occur for a short time while others have long-term effects. Nonetheless, the hazard risk for greywater discharge is more associated with the chemical contamination of water bodies which receive released greywater from various sources due to the potential to persist for a long period in the environment (Noman et al., 2019).
In the present study, PKS was used for preparing AC and then the effectiveness of PSK-AC for the removal of pollutants from greywater was investigated. The PKS-AC samples were characterized by scanning electron microscopy (SEM) and Fourier Transform Infra-Red (FTIR) analysis techniques.

Preparation of PKS-AC
The raw oil palm shells were obtained from a manufacturing company located at Batu Pahat, Malaysia, and dried under sunlight for 3 days. The dried PKS sample was ground first and then sieved to obtain 1-2 mm-sized fractions. Then 30 g of PKS particles were soaked in 100 mL of H 2 SO 4 , (1 N) solution for 24 h in order to reduce the amounts of fibre and traces as well as ash content. The PKS particles were washed in distilled water to remove the excess of H 2 SO 4 and dried at 130 C for 4 h, and then kept at room temperature overnight. The activation process of PKS was carried out by using H 3 PO 4 (1 N) overnight at room temperature. Afterwards, the sample was subjected to heat treatment in a furnace (550 C) for 2 h, followd by cooling down to room temperature and washing with distilled water three times. Finally, the PKS sample was air dried for 2 h and heated at 130 C for 4 h.

Water and wastewater sampling
The water and greywater samples used in this study were obtained at a village area in Tamanu, Batu Pahat, Malaysia. The study area was chosen because the discharge of wastewater into the drainage system is a common practice there. Greywater is discharged directly into the drains as a result of the lack of an effective drainage system. The greywater samples used for the investigation were obtained from the discharge point pipes before the final discharge into main drainage. The quantity of bathroom greywater for each house was estimated by the bucket method (100 L of capacity). The raw surface water samples were taken from drainage and a lake situated at Universiti Tun Hussein Onn Malaysia (UTHM). The pH, turbidity, COD, TDS and TSS of the water and greywater samples were determined according to APHA (2005).

Water and wastewater treatment process
The treatment unit was designed with (1 L of capacity) three layers consisting of PKS-AC (diameter 5 mm and depth 80 mm located at the lower portion of the filter), sand (diameter 3 mm; depth 600 mm) and gravel (diameter 5 mm; depth 50 mm). The water and wastewater samples (1 L) were passed through the designed system at a hydraulic loading rate of 400 mL/min. The loading was continued for 2 h, and the samples were collected after 5, 15, 30 and 60 min intervals. They were subjected to the analyses of turbidity, COD, TSS and TDS. The removal percentage for each parameter was calculated using the equation given by Adeleke et al. (2017) and Al-Gheethi et al. (2017), where C o is the initial concentration of the parameter, C e is the final concentration of the parameter in filtrated water and greywater.

Characterization of PKS-AC
The chemical and physical surface characteristics of PKS-AC before and after the treatment process were determined by FTIR and SEM techniques. FTIR analysis was used to determine the surface chemical functional groups on PKS-AC, referring to the work of Rugayah and Nuraini (2014). The physical morphology of PKS-AC was obtained by SEM to obtain the configuration figure of raw sample and activated sample structure, based on the description by Donnet et al. (2010).

Characteristics of raw water and greywater samples
The characteristics of surface water and greywater samples including the pH, turbidity, COD, TDS and TSS data are presented in Table 2. As can be seen  Pogonoski et al. (2016) and Keshavarzifard et al. (2014). In contrast, the concentration of TSS in the present study was much lower than the value (305.0 mg/L) reported by Eze et al. (2015). These differences might be related to the source of greywater; the greywater in the current study was collected from bathroom which has inherent properties such lower amount of TSS in comparison to that generated in kitchen or laundry greywater. COD in the greywater samples tested in the present work was found to be 506 mg/L, which is within the range detected by Mohamed et al. (2013) in Malaysia (445-621 mg/L) but higher than that reported by and Eze et al. (2015). The turbidity value found in the present study was 258 NTU. Therefore the greywater samples should be subjected to further treatment process before final disposal into the surface water systems.

Efficiency of PKS-AC in reducing parameters of water and greywater
The pH of water and greywater samples during the treatment process using PKS-AC (3 mm and 40 mm of thicknesses) is presented in Figure 1. It can be noted that the pH of the water samples remained constant between 6.2 and 6.8 during the treatment process. In contrast, the pH of the greywater samples decreased from 8.46 to below 6.8 after 30 s of treatment with PSK-AC of 40 mm thickness. The decrease in the pH value of the greywater might be due to the removal of detergent compounds from the waste via adsorption by PSK-AC. Abugu et al. (2015) indicated that AC has high potential to remove organo-chemical compounds containing detergents and colorants from wastewater. The reduction in the turbidity of the surface water and greywater is shown in Figure 2. The maximum reduction of the turbidity in the surface water (70%) was recorded with PSK-AC of 40 mm thickness. There was no significant difference in the reduction percentage when the period of the treatment process was increased from 5 to 60 min. The maximum lowering of the turbidity from the greywater was noted after 60 min with the PSK-AC samples of 3 mm and 40 mm thicknesses; however, the removal efficiency with the PSK-AC sample of 40 mm thickness was better than that with the PSK-AC sample of 30 mm thickness (55.3 vs. 44.55%). The turbidity reduction ability of AC from wastewater has been reported previously by Ademiluyi et al. (2009), who claimed that AC from Nigerian-based bamboo removed the turbidity from the refinery effluents completely after 1 h of treatment. In the present study, the maximum removal percentage reached was 55%, which is less than that reported by Ademiluyi et al. (2009). These differences might be ascribed to the different compositions of greywater and refinery effluents.
The maximum removal of COD from surface water and greywater was achieved using the PSK-AC sample of 40 mm thickness (Figure 3). These findings may be related to the thickness of the PSK-AC having more functional groups which contribute effectively in the filtration process, with more efficient adsorption of pollutants than with the 30 mm thickness sample. The COD reduction in surface water reached the highest ratios (50%) after the treatment for 5 and 15 min. Increasing the treatment period to 30 and 60 min reduced the removal of COD, probably due to the interactions occurring between the chemisorbed species on the surface of PSK-AC as well as degradation. The maximum reduction of COD in the greywater was achieved by PSK-AC (thickness: 40 mm) after 30 and 60 min (55.84 vs. 56.44%, respectively). These results revealed the role of PSK-AC in the removal of chemical substances from greywater. It has been demonstrated that the acid-activated coconut shell carbon had higher adsorption for organic matter (Ademiluyi et al., 2009). The higher performance of the AC prepared from bamboo waste (Ahmad and Hameed, 2009) on the reduction of COD from textile mill effluent (75.21%) might be related to the composition of greywater and textile mill effluent. It seems that greywater has less complicated composition than textile mill effluent. However, the chemical composition of the substances in greywater which contribute to the amount of COD is different from that in textile mill effluent. In greywater, most chemical substances are soluble in water, while they remain as suspended solids in textile mill effluent (Jamrah et al., 2006). The TDS removal efficiency of PSK-AC from surface water and greywater is depicted in Figure 4. This figure shows that the reduction of TDS from surface water was more efficient than that from the greywater sample. The highest reduction was achieved after 5 and 15 min (56.25 vs. 57.81%, respectively) with the PSK-AC sample of 4 mm. In the greywater sample, the maximum TDS reduction values recorded after 5 and 15 min with the PSK-AC sample of 4 mm were 22.14 and 21.97%, respectively. The highest reduction of TSS was achieved after 15 and 60 min from surface water (83.11%) and greywater (42.11%), respectively ( Figure 5), revealing that PSK-AC acted more efficiently in the the reduction of TSS than that of TDS. This behaviour may be explained by the higher affinity shown by TSS than TDS towards the surface of PSK-AC, which facilitates the adsorbate-adsorbent interactions.
The high efficiency of PSK-AC for reduction of COD and TDS may be related to the presence of the function groups acting as adsorbent for the ion cations of the metals as well as the organic compounds, both of which are the main reasons for high COD and TDS in the water. These findings are explained further based on the analysis of functional groups on PSK-AC using FTIR as discussed in section 3.3.

FTIR analysis of PSK-AC
FTIR analysis was performed to identify the main functional groups in the composite adsorbent before and  after the treatment with surface water and greywater, as well as to follow the intensities of the absorption peaks. The spectra were recorded in the wavelength region between 400 and 4000 cm À1 . The peaks were obtained through the plot of transmittance against the wave length. There are a number of peaks belonging to different functional groups at characteristic positions with differing intensities (Figure 6). The changes took place in the vibrational stretches of OH, N-H, C¼O, C¼C, C-O-C, C-O-H out of plane bending in the substituted alcohol and CH¼CH. The envelope at 3339 cm À1 revealed the presence of host-guest interactions of a hydrogen-bonded nature (Garg et al., 2008;Koksal et al., 2011). The peak at 1646 cm À1 can be attributed to the C¼C and C¼C aromatic bonds.
The peak located at 1647 cm À1 is assigned to the asymmetric carboxylate stretch. The peak seen in the range 1644-1622 cm À1 can be attributed to C¼O stretching of carboxylic groups, which may significantly influence adsorption process. Montes-Moran et al. (2004) attributed the spectral features between 1700 and 1500 cm À1 to the C¼C symmetrical stretching group and C¼O carboxylic groups. The C¼O stretching peak at 1647 cm À1 is an indication of stable binding as a result of the chemisorption process. The O-H bonding peak detected in the region between 3000 and 3500 cm À1 for the raw material disappeared in the PSK-AC after the adsorption process, which highlights the role of the OH group in the reduction of pollutants from the wastewater.

Textural Characterization of PSK-AC
SEM images were obtained to elucidate the surface morphology of the PSK-AC before and after treatment (Figure 7). The samples were coated with platinum    target before scanning of the images at 1000Â magnification. The results showed that the types of processes applied for the preparation of PSK-AC affected the surface morphology. It can be observed that Figure 7a and b show different textural features; Figure 7a has a smooth surface, whereas the surface appears rough in Figure 7b with the observation of dense external surface and closer pores. Some grains were filled, and also cases of evolution of volatiles may be responsible. The PSK-AC was seen to have an uneven, dense texture before and after treatment, which may result from the release of volatile matter during oven drying (Pezoti et al., 2016). The surface of the raw PSK-AC appeared smooth with holes on the external surface, while the grains in PSK-AC were filled during adsorption. The decrease of the pore size was attributed to the occupation of the pore spaces by the solute species in the POME (Peter et al., 2010). Therefore, altering the chemistry, and in particular the pore structure, could lead to significant changes in the surface area, porosity and reactivity of the end product and directly influence the adsorptive properties of AC (Mangun, 2001;Caglar et al., 2013).

Conclusion
This paper presents an experimental study on the applicability of AC prepared from PKS as a low-cost adsorbent for treatment of wastewater. PKS-AC exhibited an efficiency of >50% in removing COD from surface and greywater samples. The findings indicated that the removal mechanism of the pollutants by PKS-AC is dependent on the adsorption process proceeding via the surface functional groups available on PSK-AC, as well as the the efficacy of sand and gravel which might contribute to the treatment of water. Hence, future work is suggested with further experiments without sand and gravel to establish the efficiency of the PKS-AC. Morever, the kinetics models for COD, TDS and turbidity removal  could be performed in a future work. The FTIR analysis of PSK-AC confirmed the OH group as the main functional centre contributing to the reduction of the pollutants from water and greywater. However, in order to make the removal process more efficient by the use of PKS-AC, the optimum treatment parameters, such as the reduction period and costs of removing the pollution from the contaminated water and wastewater, should be tackled in detail.