Abstract

Phosphate is an important macronutrient for the growth of aquatic plants and algae but its excessive supply in water bodies causes eutrophication. The wide range of phosphate utilisation also affects the substantial measure of phosphate-bearing waste. This study aims to synthesise and characterise the bio-adsorbent form rice husk for phosphate removal. Rice husk ash was prepared by heating the rice husk at 105 °C for 24 h while the activated carbon from rice husk was further carbonised in a muffle furnace at three different temperatures from 450 to 750 °C. Efficiency of adsorbent was analysed using synthetic phosphate solution. Overall findings show that the thermally treated rice husk at 750 °C promoted the best phosphate removal of around 97% with 2 mg/L initial phosphate concentration and pH value of 9.0. The adsorption behaviour was fixed with Langmuir adsorption isotherm when R2 for the fitting of the experimental data was 0.991 with low chi-square value and the results indicated a monolayer adsorption mechanism. The findings show that the rice husk can be an efficient and eco-friendly adsorbent for phosphate removal and promotes a good alternative use of abundant agriculture waste.

INTRODUCTION

The discharge of effluents from industries, domestic wastewaters, and agriculture runoff have reduced the portability of natural water sources which further intensifies the issue of water pollution. The mentioned sources produce excess biogenic substances which deteriorate the quality of natural waters and cause changes in the structure of water body ecosystems (Ruzhitskaya & Gogina 2017). High concentrations of biogenic substances such as phosphorus can invigorate the algal bloom and anoxic conditions, trigger the growth of cyanobacteria, reduce dissolved oxygen content, suffocate fish populations, create murky water, and complicate water purification. The deteriorating quality of water in the water bodies is directly related to the evolution of eutrophication-like conditions (Hussain et al. 2011; Kilpimaa et al. 2014). Eutrophication is a natural process whereby lakes, estuaries, and slow-moving streams receive excessive amounts of nutrients as a consequence of rocks and soil weathering from the surrounding watershed. To resolve this complex issue, the discharge of biogenic substances into the water bodies should be minimised and one needs to identify the sources of phosphorus compounds that reach the water bodies.

Currently, industrial and residential drains are the main source of phosphorus contaminating water reservoirs. Studies conducted by Kozaki et al. (2017) documented a strong phosphate contamination (11.1 mg/L) detected from industrial waste in Pahang compared to a sample collected from the housing area (1.64 mg/L). Water sampling was also carried out at Kuala Gula, Perak, where the highest concentration of soluble reactive phosphorus in the surface intertidal waters was recorded in June, July, and August (0.055 mg/L) coinciding with the paddy planting season (Lomoljo et al. 2009). River water samples from nine states in Malaysia were collected and the concentration of both phosphate and nitrate were measured (Khan et al. 2007) and the highest phosphate concentration was detected at Penang, Malaysia (0.66 mg/L). Industrial and residential wastewaters contain phosphorus as a result of human excretion (30–50% of phosphorus comes with residential waters) and after wide use of synthetic detergents (50–70%) containing polyphosphate components (Ruzhitskaya & Gogina 2017). Other than that, the utilisation of phosphorus as fertiliser in agriculture leads to the release of high amounts of phosphate into the water stream (Rout et al. 2014). Leaching of phosphate into the groundwater through the sub-soil also reduces drinking water quality which leads to a high risk to human health, including kidney damage and osteoporosis (Oliveira et al. 2012). Therefore, it is necessary to determine a reasonable, economic, and highly effective method to remove the phosphate until the standard prior to discharge is achieved.

Advanced technology such as flocculation, electrodialysis, nano-filtration, and reverse osmosis are considered successful but require high operational and maintenance costs. A few other techniques for phosphate removal are chemical precipitation (Oguz et al. 2003), advanced oxidation process (Crittenden et al. 2005), and ion exchange (Kuzawa et al. 2006). The conventional technology for phosphate removal in industrial wastewater treatment consists of treating the wastewater with iron oxide tailing (Zeng et al. 2004). In this process, the phosphorus reacts with the iron to form iron phosphates. A significant drawback of this process is the large amount of sludge formation which is contaminated with metal salts. This sludge can only be disposed of by landfill, incineration or dumping at sea. To overcome secondary pollution problems, various phosphate removal technologies were developed including adsorption (Yadav et al. 2015).

The use of waste materials in developing various economic adsorbents has gained much attention because of the increase in sustainable awareness. Adsorption is one of the most effective, easily handled, and economic methods for water and wastewater treatments. Various types of adsorbent such as activated carbon, aluminium hydroxide (Tanada et al. 2003), nano-alumina and charcoal (Mor et al. 2016), and nano-scale zero-valent iron (Khalil et al. 2017) have been studied for phosphate removal. However, the direct addition of adsorbent in water treatment systems may result in the rapid loss of adsorbent due to the very small particle size of the powder (Xiong et al. 2017). To all intents and purposes, these alternatives have some limitations such as high cost, labour-intensive operation, and low efficiency (Tarley & Arruda 2004). Therefore, more approaches have been investigated for the development of low-cost adsorbents with a good sorption capacity to remove phosphate from water bodies.

In recent years, considerable attention has been devoted to the study of natural materials application as adsorbents. Natural materials have the advantages of large quantities, low cost, and good sorption capacity. They have a high potential to be utilised as adsorbents for pollutant removal rather than wasted. Various studies have shown the application of natural and different waste materials such as carbon fibre (Zhang et al. 2014), calcine egg (Köose & Kıvanç 2011), fruit juice residue (Yadav et al. 2015), cow-dung ash (Kaur et al. 2016), and activated charcoal (Mor et al. 2016) for phosphate removal from aqueous solutions and wastewater. Adsorption by activated carbon is the most preferred method due to its highly developed internal surface area, porosity, and high adsorption capacity as well as its simplicity of design and ease of operation (Bhatnagar & Sillanpää 2017). Nevertheless, the commercial activated carbons are expensive (Dias et al. 2007) and commonly derived from non-renewable materials such as bituminous coal, anthracite, lignite, and peat (Ali et al. 2012). These limitations have prompted numerous research efforts with the aim of finding low-cost as well as environmentally friendly alternative materials that can be transformed into low-cost adsorbents for pollutant removal in water and wastewater.

The Food Agriculture Organisation reported that 582 million tonnes of paddy is produced (Foo & Hameed 2009) and ∼120 million tonnes of rice husk biomass waste is generated annually (Foo & Hameed 2011). Therefore, it is crucial to fully utilise the rice husk to become a value-added product to reduce environmental pollution and waste treatment costs. The open burning of rice husk can become a serious issue when rice husk ash releases carbon dioxide (CO2) and produces several toxic gasses including nitrogen oxides, volatile organic compounds, carbon monoxide, and particulate pollution into the atmosphere. The CO2 concentration in the atmosphere contributes significantly to global warming (Huang & Tan 2014).

Rice husk is a fibrous material containing about 50% cellulose, 30% lignin, and 20% silica (Siddique 2008). Besides, rice husk contains plentiful floristic fibre, protein, and some functional groups such as carboxyl and amidogen. Thus, rice husk has the potential to be excellent for removing ionic dyes from aqueous solutions (Chuah et al. 2005) and pre-treated rice husk has been used for the sorption of cadmium from effluents (Kumar & Bandyopadhyay 2006). Moreover, rice husk is widely used in many industrial applications due to its chemical stability, high mechanical strength, insolubility in water, and its local availability at almost no cost (Hegazi 2013).

The present study deals with the utilisation of rice husk as an adsorbent for phosphate removal from aqueous solutions. The adsorbent was prepared via chemical activation using sodium hydroxide (NaOH) followed by heat treatment at 450–750 °C. The prepared adsorbent was characterised using scanning electron microscopy (SEM) and Fourier transformed infrared spectroscopy (FTIR). Batch adsorption was performed to study the effects of different experimental parameters such as pH value, contact time, adsorbent dosage, and initial phosphate concentration on the adsorption performance. This study also reveals the mechanism of phosphate adsorption onto the prepared adsorbent by fitting the experimental data to the isotherm model. The novelty of the present work is that the bio-adsorbent produced can be easily scaled up since it used a non-complicated pre-treatment and has a lower heating temperature (less than 1,000 °C) which consequently reduces the cost of production. Other than that, the phosphate loaded adsorbents after adsorption can be used as the feedstock for biogas production using anaerobic digestion as phosphate which is used as an essential inorganic nutrient for microbes during the biomethanation process, while the rice husk can be a good source of lignocellulosic material. Therefore, the disposal problem of phosphate loaded adsorbents can be reduced and will create a good industrial ecology for sustainable development.

METHODOLOGY

Materials

Fresh rice husk (raw material) was obtained from a rice mill in Besut, Terengganu. Potassium dihydrogen phosphate (KH2PO4), ammonium molybdate, ammonium metavanadate, hydrochloric acid (HCl), NaOH, and sulfuric acid with 99% purity (H2SO4) purchased from Sigma Aldrich and Fluka were used to prepare the adsorbent and phosphate analysis. All chemicals and reagents used were of analytical grade.

Pre-treatment of rice husk

First, the impurities in the rice husk were removed. Then, the rice husk was washed thoroughly with distilled water and dried at 105 °C until a constant weight was attained (around 24 h). The dried rice husk was ground to form ash and sieved to obtain uniform particle size of 850 μm, and then stored in vacuum desiccators for experimental purposes.

Chemical and heat treatment of rice husk

For the chemically and heat treated adsorbent, the rice husk ash was then refluxed with 0.1 N of NaOH for 24 h to remove lignin-based substances and increase the specific surface area of the rice husk. After the reaction, the rice husks were washed with 0.1 N of H2SO4 for pH value adjustment (until pH 7.0) and then rinsed with distilled water (twice) to remove the acid trace. Finally, it was dried for 24 h at 105 °C and heated in the muffle furnace at three different temperatures of 450, 650, and 750 °C for 4 h and 30 min, and denoted as TRHA-450, TRHA-650, and TRHA-750, respectively.

Characterisation of adsorbent

The morphology of adsorbent was determined by SEM (Model TM-1000). The functional groups analysis of adsorbent was determined by using an FTIR spectrophotometer (Perkin-Elmer Spectrum-100). The rice husk samples were mixed with potassium bromide (KBr) crystal and were pressed into a pellet. The pellets were about 10 mm in diameter and 1 mm thickness.

Batch adsorption experiment

Phosphate stock solution (50 mg/L) was prepared by dissolving KH2PO4 in 1 L of distilled water. The stock solution was further diluted to obtain synthetic wastewater of the desired initial concentration from 2 to 10 mg/L.

Batch adsorption experiments were conducted to investigate the effects of different parameters on phosphate removal from synthetic wastewater by using the treated and untreated rice husk ash. In each experiment, the phosphate solution was prepared by adding 35 mL of phosphate stock solution to 5 mL of ammonium molybdate solution and 5 mL ammonium metavanadate solution and mixed well. The flask was immersed in a boiling water bath for 10 min. Thereafter, the flask was removed and cooled rapidly. The flask was shaken and yellow colour appeared. The addition of distilled water was needed based on the desired concentration of phosphate solution.

For the first stage of the batch experiment, the treated and untreated rice husk ash with a fixed dosage of 0.5 g/L were mixed with 100 mL of phosphate solution (8 mg/L) in a conical flask and agitated in a water bath shaker at 150 rpm for 15–120 min. The solution and residue were centrifuged at 3,000 rpm for 10 min (Model 5702 R). The suspension was filtered using a Whatman filter no. 42 and the obtained filtrate solution was analysed by using a UV-VIS spectrophotometer at 450 nm for phosphate content. All experiments were repeated three times. The mentioned techniques were also used for adsorption studies at different adsorbent dosages (0.3–1 g/L), initial phosphate concentration (2–10 mg/L), and initial pH values (3–9). The NaOH solution (0.1 N) and nitric acid solution (0.1 N) were used for pH value adjustment. The efficiency of phosphate removal was calculated by using Equation (1): 
formula
(1)
where co is the initial concentration of phosphate stock solution in mg/L, and cf is the solution concentration after adsorption (desired reaction time) in mg/L.

Adsorption isotherm

For the adsorption isotherm, experiments were carried out until all the adsorption had equal adsorbate affinity and the concentration was in the equilibrium stage. The adsorption capacity in mg/g was determined by using Equation (2): 
formula
(2)
where q is the adsorption capacity in mg/g, co is the initial concentration in mg/L of phosphate in the sample, ce is the equilibrium concentration in mg/L of phosphate in the sample, v is the volume of adsorbate solution in L, and w is the mass of adsorbent in g.
Two models, Langmuir and Freundlich, were used to correlate the experimental equilibrium adsorption data of phosphate adsorption onto the treated and untreated rice husk ash. The Langmuir equation assumes that the solid surface presents a finite number of identical sites that have consistent and homogeneous energy. There are also no interactions among adsorbed species, meaning that the amount adsorbed has no influence on the rate of adsorption and a monolayer is created whereby only one layer of molecule attaches to the surface and when the solid surface achieves saturation (Elham et al. 2011). The Langmuir isotherm was evaluated by using Equation (3): 
formula
(3)
where qm is the adsorption capacity in mg/g, KL is the rate of adsorption in L/mg, and Ce is the equilibrium concentration of adsorbate in mg/L. The values of Langmuir constant can be obtained from the plots between Ce/qe and Ce. The important features of the Langmuir isotherm were expressed in terms of equilibrium parameter (RL) whereby a dimensionless constant is referred to as the separation factor or equilibrium parameter by using Equation (4): 
formula
(4)
where co is the highest initial concentration and RL is the adsorption nature.
The Freundlich adsorption isotherm is commonly used to describe the adsorption characteristics for the heterogeneous surface and was evaluated by using Equation (5): 
formula
(5)

The value of Freundlich constant (Kf) is related to the adsorption capacity and its intensity respectively, which was calculated from the linear plots of log qe versus log ce (Gulipalli et al. 2011), where Kf is the adsorption capacity, 1/n is a function of the strength of adsorption in the adsorption process, and n is the intensity of adsorption. If n= 1, then the partition between the two phases is independent of the concentration. If the value of 1/n is below unity, it refers to a normal adsorption. On the other hand, if 1/n is above unity it refers to cooperative adsorption.

Statistical analysis

In this study, statistical analysis was carried out to determine the optimum isotherm model that best fits the experimental data obtained in the adsorption study. The first analysis was error analysis (sum of squares of errors and sum of absolute errors) and the second analysis was called chi-square analysis, as displayed in Table 1. The lower numerical values of an error function represent the better fit for the adsorption isotherm model. The determination of the optimum adsorption isotherm model by linear regression may be erroneous since linear regression is influenced by various axis settings of the linearised equation of the adsorption isotherm model (Chowdhury et al. 2009). Thus, chi-square analysis was used to evaluate the optimum adsorption isotherm model of phosphate for the present system. On the other hand, the non-linear chi-square test is free from such errors as it compares all isotherm models on the same abscissa and ordinate (Ho 2004).

Table 1

Error function and chi-square test

 Abbreviation Formula 
The sum of squares of error SSE  
The sum of absolute error SAE  
Chi-square x2  
 Abbreviation Formula 
The sum of squares of error SSE  
The sum of absolute error SAE  
Chi-square x2  

RESULTS AND DISCUSSION

Characterisation of adsorbent

The surface morphology of treated and untreated rice husk ash was determined using SEM at 400× magnifications and the images obtained are depicted in Figures 1(a)–1(d). The SEM image of rice husk ash in Figure 1(a) shows the outer epidermis of rice husk which has a corrugated structure (Sarangi et al. 2009). The varying form of silica particles on the organic matrix were mainly localised in the tough interlayer (epidermis) of the rice husk and consisted of cellulose, hemicellulose, and lignin. These silica particles also concentrated mainly in protuberances and hair trichomes on the outer epidermis and adjacent to the rice kernel (Krishnarao & Godkhindi 1992). The image also shows that rice husk ash was compact and with very few pores and this result was in agreement with the findings of Kudaibergenov et al. (2012).

Figure 1

Surface morphology of (a) untreated rice husk ash (RHA), (b) TRHA-450, (c) TRHA-650, and (d) TRHA-750.

Figure 1

Surface morphology of (a) untreated rice husk ash (RHA), (b) TRHA-450, (c) TRHA-650, and (d) TRHA-750.

Thermally treated rice husk ash was activated by soaking the rice husk ash in NaOH to remove inorganic compounds such as silica and carbonate. After the pre-treatment with NaOH, the silica reacted with NaOH to form sodium silicate (NaSiO3) which is soluble in water and is removed by adequate water-washing (Daffalla et al. 2010), hence the pore structure developed on the outer epidermis of rice husk. Furthermore, soaking the rice husk ash with NaOH also resulted in the development of a rough texture, making the surface of the adsorbent more suitable for attachment of reactive functional groups. According to Figures 1(b)–1(d), it was observed that the thermally treated rice husk ash shows the occurrence of button-like structures with small pores, which were not found in the untreated rice husk ash. This button-like structure increased with the increase in temperature used. The emergence of pores and button-like structures may be caused by the fast removal of volatile organic components from the particle (Bharadwaj et al. 2004).

FTIR analysis was carried out to determine the functional groups on the surface of the samples. The spectra of adsorbents were measured within the range of 400–4,000 cm−1 wave number. Table 1 shows the fundamental peaks of the untreated and thermally treated rice husk. The infrared spectrum of untreated rice husk ash contained intensive absorption bands at 3,394, 2,924, 1,636, 1,080, 797, and 468 cm−1. The adsorption peak around 3,494 cm−1 indicates the presence of free hydroxyl groups (Bansal et al. 2009). The characteristic of absorption bands at 2,924 cm−1 was related to the –C–H stretching vibrations of methylene groups (Genieva et al. 2008) and the C = C stretching vibration at 1,637 cm−1 indicates the aromatic functional groups (Daffalla et al. 2010). The pronounced peak at 1,080 cm−1 with high intensity was attributed to the stretching vibrations of the siloxane groups (Chen et al. 2011). The peaks around 797 and 468 cm−1 corresponded to hydrogenated amorphous silicon (Si–H) (Dey et al. 2013). After thermal treatment, the effect was observed on the FTIR spectrum of all treated rice husk ash samples when the spectrum at 2,924 cm−1 disappeared which indicates the evolution of CO2 at higher temperatures and possible decomposition of the residual methylene group (Kudaibergenov et al. 2012). Meanwhile, other fundamental peaks of rice husk still existed with a slight shift of wavelength. The presence of polar groups on the surface increased the cation exchange capacity of the adsorbents.

Surface characteristics such as specific surface area, pore volume, and average pore diameter are very important parameters that should be studied since these parameters greatly influence the absorption capacity of an adsorbent. The results for surface characteristics of the prepared adsorbent are presented in Table 2. The rice husk ash treated with NaOH followed by thermal treatment shows an improvement of surface area of 34.6, 204.1, and 337.9 mg/g for TRHA-450, TRHA-650, and TRHA-750, respectively, compared to the untreated rice husk ash at 4.29 mg/g. The NaOH developed the porosity based on dehydration and degradation (Nor et al. 2013) and has become a popular activation agent for lignocellulosic materials activation. In addition to the results, an increase in temperature increased the surface area of adsorbent. Fu et al. (2019) concluded that the surface area of active biochar (rice husk) increased with the increase of temperature and amount of potassium hydroxide. Foo & Hameed (2011) investigated the impact of rice husk activated carbon using potassium hydroxide and potassium carbonate, where the surface area of treated samples was larger (752 and 1,165 m2/g) than those of the untreated sample (164 m2/g). The high surface area of thermally treated rice husk ash was hypothesised to enhance microporosity as well as narrowing the pore size distribution. Increasing the temperature results in a decrease in average pore diameter from 32.2 (TRHA-450) to 26.2 Å (TRHA-750). A similar trend was also observed by Liou & Wu (2009) when their findings determined that the micropore structure of rice husk heated above 500 °C was destroyed by collapsing or combining together (Table 3).

Table 2

IR bands of RHA and TRHA

Sample Band position (cm–1Assignment 
Rice husk ash 3,394 O-H 
2,924 C-H 
1,637 Aromatic compound 
1,080 Si-O-Si 
797, 468 Hydrogenated amorphous silicon (Si-H) 
TRHA- 450, TRHA- 650 , TRHA-750 3,420.69 , 3,421.90, 3,467.93 O-H 
1,609.48, 1,623.18, 1,650.72 Aromatic compound 
1,091.92, 1,079.46, 1,087.89 C = O 
800–400 Si-H 
Sample Band position (cm–1Assignment 
Rice husk ash 3,394 O-H 
2,924 C-H 
1,637 Aromatic compound 
1,080 Si-O-Si 
797, 468 Hydrogenated amorphous silicon (Si-H) 
TRHA- 450, TRHA- 650 , TRHA-750 3,420.69 , 3,421.90, 3,467.93 O-H 
1,609.48, 1,623.18, 1,650.72 Aromatic compound 
1,091.92, 1,079.46, 1,087.89 C = O 
800–400 Si-H 
Table 3

Structure properties of adsorbent

 BET surface area (mg/g) Average pore diameter (Å) 
Rice husk ash 4.29 60.2 
TRHA − 450 34.6 32.2 
TRHA − 650 204.1 29.2 
TRHA − 750 337.9 26.2 
 BET surface area (mg/g) Average pore diameter (Å) 
Rice husk ash 4.29 60.2 
TRHA − 450 34.6 32.2 
TRHA − 650 204.1 29.2 
TRHA − 750 337.9 26.2 

The specific surface area of the thermally treated rice husk increased with an increase in temperature and can be explained by the formation of new micro- and mesopores during the thermal treatment (Kalderis et al. 2008). The average particle size was in good agreement with the specific surface area, as material with smaller particles had a higher specific surface area. The increase in specific surface with the increase of thermal treatment was due to the tar release from a cross-linked framework generated by NaOH activation and the volatile organic compounds released from the precursor material (Demiral et al. 2007; Li et al. 2008). Prahas et al. (2008) also reported that by increasing the activation temperature, the amount of acidic functional groups decreased while the basic surface groups of the carbon increased.

The effect of contact time and types of adsorbent for phosphate removal

Both treated and untreated rice husk ash with a particle size of 850 μm were tested for phosphate removal via batch adsorption experiment at different contact times (15–120 min, with 15 min time interval). The results obtained are plotted in Figure 2. It is observed that all adsorbents recorded the increase of phosphate removal from 15 to 120 min of contact time. TRHA-750 achieved its saturation time at 90 min (84.1%) and was selected as the best adsorbent since it has the highest capability for phosphate removal compared to other adsorbents. After achieving maximum removal, adsorbent–adsorbate interaction reached its saturation site, desorption started and hence reduced the removal efficiency. In agreement with the current work, several studies reported that the adsorption of phosphate increased with contact time and reduced after the binding sites of adsorbent became saturated (Mor et al. 2016). In addition to this finding, the treated rice husk ashes have a better capability for phosphate removal compared to the untreated rice husk ashes due to their higher surface area, as discussed in the previous section.

Figure 2

Phosphate removal of treated and untreated rice husk ash at different contact times (phosphate concentration: 8 mg/L, adsorbent dosage: 0.5 g/L, temperature: 25 °C).

Figure 2

Phosphate removal of treated and untreated rice husk ash at different contact times (phosphate concentration: 8 mg/L, adsorbent dosage: 0.5 g/L, temperature: 25 °C).

The effects of adsorbent dosage on the phosphate removal

The dependence of phosphate sorption on the adsorbent dosage used was studied at various amounts of adsorbent from 0.3 to 1 g/L and the result obtained is shown in Figure 3. It is observed that 0.3 g/L adsorbent was able to remove 59% phosphate. Increasing the adsorbent dosage by around 0.5 g/L made the removal efficiency improve to 84%. These findings can be explained by the availability of larger surface area and more adsorption sites for phosphate when a higher dose of adsorbent is used. However, the continuous increment of dosage (more than 0.5 g/L) caused the aggregation of adsorbent and overlapped the active sites and hence reduced the removal efficiency. The results obtained were in agreement with the recent findings by Nomanbhay & Palanisamy (2005), who studied the removal of heavy metal from industrial wastewater using chitosan coated oil palm shell charcoal. The dosage used was in the range of 13–30 g/L and the experiment was carried out at 25 °C. This study determined that the higher the dosage of adsorbent in the solution, the greater the availability of exchangeable sites for the heavy metal to take place; and the NH3+ group on the chitosan was responsible for the adsorption process. After a certain dose of adsorbent, maximum adsorption set in and hence the number of ions bound to the adsorbent and the number of free ions remained constant, even with further addition of the dose of adsorbent.

Figure 3

Phosphate removal of TRHA-750 at various dosages (phosphate concentration: 8 mg/L, temperature: 25 °C).

Figure 3

Phosphate removal of TRHA-750 at various dosages (phosphate concentration: 8 mg/L, temperature: 25 °C).

The effects of initial concentration on phosphate removal

The initial concentration gives an essential driving force to overcome the resistance to the transfer of phosphate ion in the solid–liquid interface. To determine the effect of initial concentration on phosphate removal, the experiment was conducted using five series of phosphate concentration (2–10 mg/L) with a fixed amount of TRHA-750 dosage; 0.5 g/L. The result obtained is shown in Figure 4. In general, the percentage of phosphate removal in wastewater decreased with increased initial phosphate concentration. At the lowest phosphate concentration used (2 mg/L), phosphate removal achieved up to 95%. However, when the initial concentration was increased up to 10 mg/L, the removal decreased by 38%, making the phosphate removal only 69%. According to Yadav et al. (2015), at the higher concentration of phosphate solution, more ions are left un-adsorbed in the solution to achieve site saturation. In addition, at the higher concentration, the sites vacancies of adsorption decreased and consequently the percentage of phosphate removal was decreased.

Figure 4

Phosphate removal by TRHA-750 with different initial phosphate concentration.

Figure 4

Phosphate removal by TRHA-750 with different initial phosphate concentration.

Adsorption isotherm

Adsorption isotherms are mathematical models that explain the distribution of the type of adsorbate species between adsorbate and adsorbent. The assumptions usually link to the heterogeneous or homogenous adsorbents, the type of coverage, and possibility of interaction between the adsorbate species. In this study, Langmuir and Freundlich isotherms were used to evaluate the adsorption mechanism of phosphate onto a selected adsorbent which has the best properties and performance; TRHA-750. The graphs of Langmuir and Freundlich isotherms are illustrated in Figures 5(a) and 5(b), respectively.

Figure 5

Adsorption isotherm of phosphate onto TRHA-750. (a) Langmuir isotherm and (b) Freundlich isotherm.

Figure 5

Adsorption isotherm of phosphate onto TRHA-750. (a) Langmuir isotherm and (b) Freundlich isotherm.

To determine the isotherm parameters, the linearised forms of Langmuir and Freundlich models were used. Correlation coefficients of linearised equations and isotherm parameters are described in Tables 4 and 5, while the results of calculations on error functions and chi-square are shown in Table 6. It can be seen from Table 4 that the best fit linear equation was the Langmuir isotherm model since the correlation coefficients obtained is higher (0.991) compared to the Freundlich isotherm (0.948). The calculated value of qm was 1.7822 mg/g. In addition, the feasibility of isotherm was also studied to characterise the adsorption beyond determining the optimum isotherm alone. The calculated value of RL obtained for the adsorption of phosphate was 0.0112, indicating that adsorption was favourable for phosphate. For Freundlich adsorption isotherm, the calculated value of N was at 1.979. According to Dawodu et al. (2012), the value of N between 1 and 10 represents a positive adsorption process and the adsorption is a physical process.

Table 4

Correlation coefficient and calculated values of isotherm parameters

Langmuir
 
Freundlich
 
Correlation coefficient y = 7,905x + 0.5611 Correlation coefficient y = 0.5052x − 1.1541 
R2 0.991 R2 0.948 
qm (mg/g) 1.7822 1/n 0.505 
KL (L/mg) 0.710 1.979 
RL 0.112 Kf (mg/g) 0.857 
Langmuir
 
Freundlich
 
Correlation coefficient y = 7,905x + 0.5611 Correlation coefficient y = 0.5052x − 1.1541 
R2 0.991 R2 0.948 
qm (mg/g) 1.7822 1/n 0.505 
KL (L/mg) 0.710 1.979 
RL 0.112 Kf (mg/g) 0.857 
Table 5

Feasibility of adsorption isotherm

Values of RL Type of isotherm 
RL > 1 Unfavourable 
RL = 1 Linear 
0 < RL < 1 Favourable 
RL = 0 Irreversible 
Values of RL Type of isotherm 
RL > 1 Unfavourable 
RL = 1 Linear 
0 < RL < 1 Favourable 
RL = 0 Irreversible 
Table 6

Calculated values of error function and chi-square values of different isotherm model

Isotherm model SSE SAE x2 
Langmuir 1.786 2.280 1.351 
Freundlich 0.288 0.554 6.642 × 10–4 
Isotherm model SSE SAE x2 
Langmuir 1.786 2.280 1.351 
Freundlich 0.288 0.554 6.642 × 10–4 

On the other hand, the error analysis revealed an interesting aspect of adsorption. The minimum values of the error functions were used, followed the Freundlich isotherm, as can be seen from Table 6. In order to avoid uncertainty in claiming a specific isotherm model, it is imperative to carry out a more normalised error analysis, as elucidated earlier in the form of a chi-square test (Chowdhury et al. 2009). The calculated values of chi-square for both isotherm models are given in Table 6. It can be seen from the table that the adsorption of phosphate followed the Langmuir isotherm with a chi-square value of 1.351. The calculated chi-square for the Freundlich isotherm was less than 0.05 which is not significant for this adsorption study. Thus, Langmuir isotherm models could govern the respective adsorptions in the present study which describes the formation of a monolayer of adsorbate on the outer surface of the adsorbent in the whole process quantitatively; however, there was no further adsorption taken afterwards.

The effect of initial pH value on phosphate removal

The pH value of the phosphate solution is an important monitoring parameter in the process of adsorption since the degree of ionisation and speciation of adsorbate is mainly affected by the pH value of solution. Therefore, the effect of pH value was studied by varying the phosphate solution within the pH values ranging from 3.0 to 9.0. This stage of the study helps to optimise the appropriate pH value of effluent for achieving maximum phosphate removal efficiency (Yadav et al. 2015).

From Figure 6, it was proven that the phosphate removal by TRHA-750 was highly pH dependent. An increase in the pH value from acidic to alkaline increased the phosphate removal efficiency. The highest phosphate removal occurred at pH 9.0 (97%) while the lowest occurred at pH 3 (59%). This improvement was due to the protonation of TRHA-750, which formed a buffer in the aqueous solution. When adsorption occurred, H+ was released from the solution and led to a decrease in the pH value of the solution (Huang et al. 2011). Furthermore, as the pH value increased, competition occurred between the hydroxide ions (OH) and phosphate ions in the aqueous solution. At lower pH values, the phosphate ion removal was inhibited, possibly as a result of the competition between the hydrogen and phosphorus ions on the sorption sites. When the pH value of the solution increased, the ligand functional groups in the TRHA-750 were exposed, increasing the negative charge density on the adsorbent surface, thus increasing the attraction of the phosphate ions by the positive charge and allowing sorption onto the TRHA-750 surface (Abbas et al. 2014).

Figure 6

Phosphate removal by TRHA-750 with different initial pH of phosphate solution.

Figure 6

Phosphate removal by TRHA-750 with different initial pH of phosphate solution.

The findings of this study can be a platform in designing an eco-friendly adsorbent for water and wastewater treatments, mainly in poor countries since the chemical treatment of wastewater requires very high costs. It provides ideas on ways to fully utilise the waste for value-added products that can also reduce the cost for waste treatment and transportation. Wang & Jan (2018) studied the fluidised bed fast pyrolysis of biomass from laboratory to a pilot scale. The commercialisation of bio-oil produced from pyrolysis will generate a large amount of carbon (char) as its by-products (Kan et al. 2016). Increased efforts towards bio-oil production from biomass such as rice husk mean that it is likely to become a readily available absorbent. Studies on adsorbent to remove heavy metal (e.g. cadmium, copper, lead, mercury, nickel, zinc) and nutrients (e.g. phosphate, ammonia, nitrate, and nitrite) from wastewater are broad (Tiwari et al. 2008). For example, carbon nanotube (Ahmad et al. 2016) and nanoscale materials including metal-containing nanoparticles, carbonaceous nanomaterials, zeolites and dendrimers (Tiwari et al. 2008) were employed to fit this purpose. The adsorption properties of thermally treated rice husk ash prepared herein were studied corresponding to phosphate only. This will remain an ongoing area of research in which the development of thermally treated rice husk ash with higher surface area is crucial and the ability of this absorbent to absorb other metals will be investigated. It would be ideal if an alternative affordable adsorbent such as thermally treated rice husk ash can be employed together with the carbon nanotube or/and nanoscale material (material with high porosity and surface area); thus resulting in high adsorption of phosphorus and other metals at a low cost. Research on the performance improvement of adsorbent can be continued by using a coating method (e.g. polymer or TiO2 coated rice husk), and other parameters that influence the adsorption capacity should be explored extensively.

CONCLUSIONS

The overall findings in this present study show that a low-cost adsorbent which appears to be viable for phosphate removal was successfully synthesised by chemical activation followed by thermal treatment of rice husk. Batch adsorption experiments revealed that the maximum phosphate removal achieved was about 97% at an initial phosphate concentration of 2 mg/L and at pH 9. Data analysis of the adsorption mechanism found that phosphate adsorption followed the Langmuir adsorption model which expressed the monolayer adsorption of phosphate onto TRHA-750. Based on this study, it may be concluded that rice husk has high potential to be used as a low-cost, natural, and eco-friendly adsorbent for the removal of phosphate and may also be effective in removing other harmful species such as heavy metal ions that are present in the effluents.

ACKNOWLEDGEMENTS

The authors gratefully acknowledge the School Of Ocean Engineering, University Malaysia Terengganu for the funding to carry out this research. The SEM data presented was acquired with the support of the Akuatrop Laboratory, while the FTIR analysis was carried out at the Organic Chemistry Laboratory, Universiti Malaysia Terengganu.

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