Environ Sci Pollut Res DOI 10.1007/s11356-015-5081-7

ECO-AQUACULTURE, SUSTAINABLE DEVELOPMENT AND PUBLIC HEALTH

Biosorption studies on copper (II) and cadmium (II) using pretreated rice straw and rice husk W. C. Li 1 & F. Y. Law 1 & Y. H. M. Chan 1

Received: 30 April 2015 / Accepted: 15 July 2015 # Springer-Verlag Berlin Heidelberg 2015

Abstract This study investigated the adsorption and removal behaviour of copper (Cu) (II) and cadmium (Cd) (II) ions using rice husk and rice straw in aqueous solutions. Different parameters were used to investigate their adsorption performance in saline conditions and the optimal level of biosorption at different pH levels. The main parameters were pH (3, 6 and 9), initial concentration level of heavy metals (Cu (II) 5, 10, 20, 40 and 60 mg/L and Cd (II) 0.5, 1, 2, 4 and 8 mg/L, respectively), salinity (0, 50 and 100 mM NaCl) and contact time (ranging from 3 to 60 min). Langmuir and Freundlich isotherm models were applied to analyse the removal efficiency and sorption capacity of the pretreated rice husk and rice straw. The removal efficiency and adsorption capacity generally increased with the pH and reached a plateau in alkaline conditions. The percentage removal of Cu (II) by rice husk reached 97 % at pH 9 and 95 % by rice straw at pH 6. Biosorption performance increased in the absence of NaCl. Kinetic studies for both metals revealed that the biosorption of Cu (II) and Cd (II) onto rice straw and husk was pseudo-second order.

Keywords Biosorption . Rice straw . Rice husk . Isotherm . Kinetic Responsible editor: Philippe Garrigues * W. C. Li [email protected] 1

The Centre for Education in Environmental Sustainability, and Department of Science and Environmental Studies, The Hong Kong Institute of Education, Hong Kong, SAR, China

Introduction An excessive discharge of municipal and industrial wastewater with a high heavy metal content causes freshwater and marine environment contamination (Lim et al. 2012). Wastewater recycling is considered a new alternative to increasing the water supply because of the growing difficulty in obtaining freshwater in some parts of the world (Ding et al. 2012). Heavy metals in waste effluent should be treated during the recycling process before use. Moreover, because heavy metals are toxic and non-degradable, their bioaccumulations can cause disastrous environmental and ecological problems (Liu and Xu 2007). Copper (Cu) is identified as one of the most widespread heavy metal contaminants in the environment (Ho and McKay 2003). Cu (II) is found in the effluents of industries such as metal cleaning, electroplating, refineries, paper and pulp and wood preservatives (Zhang et al. 2014a, b). Excessive Cu intake by human beings can trigger kidney damage, gastrointestinal irritation, cirrhosis, central nervous disorders, hepatic or renal damage, anaemia and lung cancer (Gündoğan et al. 2004; Komy et al. 2013). Cadmium (Cd) is considered a human carcinogen by several regulatory agencies (for example, the International Agency for Research on Cancer Monographs 1993; the National Toxicology Program 2000). Cd mainly comes from the electroplating, smelting, alloy, pigment, plastic manufacturing, mining, metallurgy and refining industries (Farooq et al. 2010). Mining, industrial and agricultural activities release excessive Cd into the environment, which poses high ecological risks to groundwater and soil (Kannan and Rengasamy 2005). The bioaccumulation of Cd threatens human health through the food chain (Loganathan et al. 2012). High-volume intake can result in renal disturbances, lung insufficiency, bone lesions, anaemia, hypertension, itai-itai disease and weight loss (Qi and Aldrich 2008; Rocha et al. 2009; Farooq et al. 2010).

Environ Sci Pollut Res

Conventional technologies for treating heavy metals, including precipitation and sludge separation, chemical oxidation or reduction, reverse osmosis, electrochemical treatment, membrane separation, emulsion per traction, ion exchange, preconcentration, fertilisation, adsorption and evaporation (Nhapi et al. 2011) and the use of nanomaterials (Xu et al. 2012a, b), can be costly, depending on the volume, concentration of metals and salinity of wastewater (Patrón-Prado et al. 2010). In recent years, the need for reliable and cost-effective alternatives to eliminate heavy metals from contaminated waters has promoted research on biosorption. Biosorption is a new biotechnology designed to reduce chemical concentrations in an aqueous compartment using dead, inactive or readily available biomass from various origins (Volesky 2003). The removal of heavy metals using agricultural biomass, such as wheat bran (Bulut and Baysal 2006), papaya wood (Saeed et al. 2005), coffee residues (Boonamnuayvitaya et al. 2004) and cocoa shells (Meunier et al. 2004), has been widely explored. In this study, rice husk and rice straw were used as biosorbents because rice-based cropping systems are the most productive agroecosystems in China, India and Southeast Asia (Singh et al. 2008). The abundant supply of these agricultural wastes enables the biosorbents to be obtained at low cost. Rice husk is usually considered to be a waste product in rice production. According to the report of the Food and Agriculture Organization in 1995, approximately 100 million tons of rice husk is available annually in developing countries, which makes it cheap to obtain (Chuah et al. 2005). It can be used as a biosorbent material because of its high chemical stability, mechanical strength and granular structure (Wan and Hanafiah 2008). As a low-cost biosorbent, its use for removing heavy metal ions (As (V), Au, Cr (IV), Cu, Pb, Fe, Mn, Zn, Cu (II) and Cd (II)) has been reported previously (Hasfalina et al. 2012). Rice straw is another kind of low-cost biosorbent worth studying. The annual production of rice straw in Egypt is about 4.7 million tons, which makes it available at the low price of 100 L.E. per ton (Nawar et al. 2012). Traditionally, excess rice straw is burnt and used as fertiliser. It contains compounds such as cellulose, hemicellulose, lignin and silica (Ding et al. 2012) that can be used to bind heavy metals (Gao et al. 2008). Highly saline wastewater can commonly be observed in the vicinity of contaminating industries, for example textile (Ellouze et al. 2010), coal mining (Li et al. 2014), leather (Boopathy et al. 2013), chemical, pharmaceutical and petroleum (Bassin et al. 2012). Nevertheless, the application of rice husk and rice straw to remove Cu (II) and Cd (II) under saline conditions has not been studied. The objectives of this study were (1) to investigate the optimal Cu and Cd adsorption conditions and their adsorption performance with rice husk and straw in a saline water solution; (2) to explore the effects of various factors, such as initial concentration, pH values, contact time and salinity of NaCl, on the uptake and removal

of Cu (II) and Cd (II) using biosorption isotherm models; and (3) to investigate the kinetic parameters for biosorption of Cu (II) and Cd (II) by rice husk and straw that determine the order of reaction.

Materials and methods Materials The rice husk and rice straw were collected from Long Valley in Shueng Shui in the New Territories, Hong Kong. The source of rice husk and straw is the paddies of the Conservancy Association, a green non-governmental organisation promoting a sustainable farming programme in Long Valley. All rice residues that were collected were washed, dried and ground before use. Pretreatment of biosorbents Both the rice husk and rice straw were pretreated with sodium hydroxide (NaOH) to enhance their surface-adsorptive strength and ion-exchange capacities. Table 1 shows that the rice husk and straw pretreated with NaOH had better adsorption capacity than the untreated (Rocha et al. 2009; Ding et al. 2012; Toniazzo et al. 2013; Zhang et al. 2013, 2014a, b). The pretreatment of lignocellulosic materials with a NaOH solution results in the leaching out of hemicellulose and lignin (Rungrodnimitchai 2014). This enlarges the inner surface area of substrate particles by partial solubilisation and/or degradation of hemicellulose and lignin (Pandey et al. 2000). The biosorbents were maintained at an ambient temperature for 24 h in an 800-mL NaOH (0.1 mol/L) solution with a stirring speed of 200 rpm. The treated biosorbents were washed and neutralised to around pH 7 using hydrogen chloride (HCl). The rice residues were dried in an oven at 80 °C for 24 h before use. Experimental setup The biosorption experiment involved a batch of tests. First, the pH of 50 mL of distilled water was adjusted to a constant value. Metallic solutions were prepared by pipetting Cu (II) (as copper (II) sulphate, Sigma-Aldrich, USA) and Cd (II) (as cadmium sulphate, Sigma-Aldrich, USA) from a metal stock according to designated initial concentration levels. Approximately 0.2 g of pretreated biomass was then added to the metallic solution. Finally, the mixtures were agitated in an orbital shaker at 200 rpm at room temperature. Filtrates of 10 mL were obtained for further analysis. These were analysed with atomic absorption spectroscopy (AAS, Spectra AA 220, Varian, Australia) to measure the Cu and Cd levels. Wavelengths of 324.8 and 228.8 nm were applied

Environ Sci Pollut Res Table 1

Comparison of maximum adsorption capacity for different pollutants using pretreated and untreated rice husk and straw

Biosorbent

Pollutants in wastewater

Maximum adsorption capacity (mg/g)

Reference

NaOH-treated rice husk NaOH-treated rice husk NaOH-treated rice straw

Cu (II) Cd (II) Cu (II)

8.89 (pH 6)a 1.58 (pH 6) 12.22 (pH 6)

This study This study This study

NaOH-treated rice straw NaOH-treated rice husk

Cd (II) Zn (II)

9.09 (pH 6) 20.08 (pH 3.5)

This study Zhang et al. (2013)

Untreated rice husk

Zn (II)

12.41 (pH 3.5)

Zhang et al. (2013)

Untreated rice husk ash

Fe (II)

6.21 (pH 5)

Zhang et al. (2014a, b)

Untreated rice husk ash

Mn (II)

3.02 (pH 6)

Zhang et al. (2014a, b)

NaOH-treated rice husk

Basic dyes

71.40 (not provided)

Toniazzo et al. (2013)

Untreated rice husk

Basic dyes

16.90 (not provided)

Toniazzo et al. (2013)

Untreated rice husk Untreated rice husk Untreated rice straw

Direct Orange-26 Cd (II) Cd (II)

19.96 (pH 3) 8.58 (pH 6.6–6.8) 13.89 (pH 5)

Safa and Bhatti (2011) Kumar and Bandyopadhyay (2006) Ding et al. (2012)

NaOH-treated rice straw

Cd (II)

14.95 (pH 5)

Rocha et al. (2009)

a

Data in brackets represents constant pH values for biosorption isotherm analysis

to the Cu and Cd measurements, respectively. All experiments were performed in triplicate. Effect of initial concentration Approximately 0.2 g of biomass was added to the metallic solutions in different concentrations ranging from 5, 10, 20, 40 to 60 mg/L and from 0.5, 1, 2, 4 to 8 mg/L for Cu (II) and Cd (II), respectively. The mixtures were then shaken for 24 h. Effect of pH Different pH values were used to represent acidic (pH 3), neutral (pH 6) and alkaline conditions (pH 9), respectively. Rice residues of 0.2 g were mixed with 50 mL of the prepared metallic solutions and continuously agitated for 24 h.

Characterisations of rice husk and straw The characterisations of the rice husk and straw were based on the results of previous studies. The X-ray fluorescence analysis of the rice husk showed that the element sodium (Na) disappeared after adsorbing metal ions like iron (Fe (II)) and manganese (Mn (II)) because the adsorption process caused damage to the cytoderm of the husk, leading to dissolution of the intracellular substances (Zhang et al. 2014a, b). A scanning electron micrograph of the NaOH-treated rice husk showed that the surface structure of the cell was loose, suggesting that the rice husk was conductive to the uptake of metal ions (Zhang et al. 2013). A Fourier transform infrared spectroscopy analysis of both the rice husk and straw also showed that ionisable functional groups such as amino, carboxyl and hydroxyl were present and able to interact and chelate with the metal ions (Ding et al. 2012; Zhang et al. 2013).

Effect of salinity A 50-mL metallic solution with 0.2 g of biosorbent was prepared for the salinity studies. The saline solution was prepared by adding NaCl correspondingly to the final concentrations of 0, 50 and 100 mM before mixing. The mixtures were shaken for 24 h. Effect of contact time A 50-mL metallic solution with 0.2 g of biosorbent was prepared for the kinetic sorption studies. The mixture was shaken at 200 rpm for different time periods. The time intervals were set at 3, 6, 9, 15, 20, 25, 30 and 60 min for Cd (II) and Cu (II), respectively. All experiments were conducted in triplicate.

Analysis The influence of each parameter was determined by keeping other parameters constant. For the pH studies, the salinity was maintained at 0 mM NaCl to keep the aqueous solution free from salinity. For the salinity studies, pH 6 was used to keep the aqueous solution neutral. The removal efficiency of the metal ions was calculated as follows (Haris et al. 2011): R:E: ¼

C 0 −C e  100 C0

where C0 is the initial concentration of the metal ions (mg/L), and Ce is the equilibrium concentration of the metal ions in the

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solution (mg/L), respectively. The equilibrium adsorption capacity of the biomass was evaluated by the following equation (Haris et al. 2011): q¼

ðC 0 −C Þ  V m

where q is the amount of metal ions adsorbed per unit weight of biosorbent at equilibrium (mg/g), C0 is the initial concentration of the metal ions (mg/L), C is the equilibrium concentration of the metal ions in the solution (mg/L), m is the weight of the biosorbent and V is the volume of the metallic solution, respectively.

Biosorption isotherm models The two most widely used isotherm models, Langmuir and Freundlich, were used to analyse the isotherm data. Fitting the data to the two isotherm models revealed the biosorption capacity, which was used to evaluate the properties and application of the rice husk and straw. The Langmuir model is established on several hypotheses: (1) uniformly energetic adsorption sites, (2) monolayer coverage and (3) no lateral interaction between adsorbed molecules on the assumption that sorption takes place at specific homogeneous sites within the adsorbent (Langmuir 1918). The equation is presented as follows (Wang et al. 2012): C eq C eq 1 ¼ þ qeq Qmax bQmax where Ceq is the equilibrium concentration of the metal ions (mg/L), qeq is the amount of metal ions adsorbed per unit weight of biosorbent at equilibrium (mg/g), Qmax is the maximum sorption capacity at equilibrium (mg/g) and b is the Langmuir adsorption equilibrium constant (mg/L), respectively. The Freundlich model is an empirical expression used for heterogeneous surface sorption and multilayer sorption under various non-ideal conditions (Freundlich 1906). A nonuniform distribution of heat of adsorption over the surface was assumed (Mall et al. 2005). The linear expression of the Freundlich isotherm equation was obtained as follows (Wang et al. 2012): lnqeq ¼

1 lnC eq þ lnðK F Þ n

where Ceq is the equilibrium concentration of the metal ions (mg/L), qeq is the amount of metal ions adsorbed per unit weight of biosorbent at equilibrium (mg/g), n is the Freundlich constant implying the biosorption intensity and KF is the Freundlich constant implying the sorption capacity (mg/g), respectively.

Biosorption kinetics Biosorption kinetics illustrate the rate of metal ion uptake by rice residues, which depends on their interactions and operating conditions. The data obtained from kinetics adsorption is used to investigate the adsorption process in terms of the order of the rate constant (Kumar et al. 2012). Several kinetic models are used for understanding adsorbent behaviour and the controlling mechanism (Sari and Tuzen 2009). In this study, two kinetic models, pseudo-first order and pseudosecond order, were used to analyse the biosorption equilibrium data. The linear form of the pseudo-first-order rate equation given by Langergren and Svenska is expressed as follows (Lagergren 1898): lnðqe −qt Þ ¼ lnqe −k 1 t where qe and qt (mg/g) are the amounts of metal ions sorbed at equilibrium and time t (min), respectively, and k1 (min−1) is the rate constant for the equation. The rate constant (k1) and the calculated value of qe can be obtained experimentally by plotting ln(qe −qt) against t. The pseudo-second-order kinetic model is given in the following form (Ho and McKay 1999): t 1 1 ¼ þ t 2 qt k 2 qe qe where k2 (g/mg (min)) is the rate constant for the pseudosecond-order equation. The calculated values of qe and k2 can be obtained from the slope and intercept by plotting t/qt against t.

Quality control Each container was soaked in an acidic bath for at least 12 h before use to ensure that no residual metal was attached to the inner wall of the containers. Each experiment was repeated three times to obtain the mean values. The standard deviation and error bars were indicated whenever necessary. The correlation coefficient (R2) was used to test for the suitability of the isotherm and kinetic models for the adsorption process. The fitness of the sorption data for the kinetic models was analysed by calculating the sum of squared errors (SSE), and its value was evaluated by the following (Subbaiah et al. 2011):   X qt;1 −qt;2 2 SSE ¼ qt;1 2 where qt,1 and qt,2 are the experimental biosorption capacities of the metal ions (mg/g) at time t, and the corresponding calculated values are obtained from kinetic models.

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Results and discussion

At a low pH value, the migration rates of the hydrogen ions were higher than those of the metal ions in the solution (Zhang et al. 2014a, b). Thus, the hydrogen ions were very close to the binding sites of the biosorbents and inhibited the approach of the metal ions due to repulsive force (Deng and Ting 2005). Moreover, the functional groups on the cell surfaces of the biomass were highly protonated. Competition occurred between protons and metal cations for the same binding sites, resulting in low adsorption capacity (Nasif and Saeed 2013). When the pH increased, more negatively charged metalbinding sites were exposed on the surface of the adsorbent at a higher pH level (Ammar et al. 2012). The metallic ions with positive charges were attracted and adsorbed onto the cell surface (Taffarel and Rubio 2009). A decrease in the positive charges on the surface of the biosorbents resulted in a smaller electrostatic repulsion between the metal ions and the biosorbent surface (Haris et al. 2011). After the optimal biosorption levels for Cu (II) and Cd (II) were reached at pH 6, a further increase in pH did not significantly affect adsorption performance. The decrease in the removal efficiency of the heavy metal ions was observed at pH 9. The adsorption decrease at the higher pH level was explained by the reduced solubility and precipitation of the metal hydroxides and an increase in the negatively charged cell wall surfaces of the adsorbate (Harris and Ramelow 1990; Sari et al. 2008; Ye et al. 2010). Adsorption of the heavy metal ions and active surface sites became difficult (Zhang et al. 2013). Previous studies also found that maximum biosorption capacity for Cu (II) and Cd (II) was observed at pH 5–6 and dropped

Effect of pH The pH of metal solutions was one of the most significant variables influencing the uptake of the heavy metal ions because it affected the surface structures of the biosorbents by way of functional group dissociation and surface charge (Subbaiah et al. 2011). In the experiment, both the adsorption capacity and the removal efficiency were highly dependent on the pH value (Anayurt et al. 2009). Figure 1 shows that the uptake of the metal ions by the rice straw increased as the pH of the solution increased. The maximum equilibrium uptake of the Cu ion was 8.78 mg/g at pH 9, while the minimum uptake was 7.33 mg/g at pH 3. In the case of the rice husk, the graph shows that the adsorption capacity was similar at pH 6 and 9 but significantly lower at pH 3. The metal uptake by the rice husk had a low pH value. The maximum adsorption was 8.20 mg/g at pH 9 compared with a minimum uptake of 5.53 mg/g at pH 3. Figure 2 shows that the adsorption capacity of the rice straw rose initially with increasing pH in the solution from pH 3 to 6 but decreased slightly at pH 9. The maximum level of equilibrium uptake was 1.61 mg/g at pH 6. In contrast, the minimum uptake was 0.85 mg/g at pH 3. For the rice husk, the graph indicated that the performance of biosorption at pH 6 coincided with its performance at pH 9, compared with that at pH 3. The highest uptake level was 1.42 mg/g at pH 9, while the lowest was 0.80 mg/g at pH 3. Fig. 1 Effect of pH (3, 6, 9) on biosorption of Cu (II) onto rice straw and rice husk in the absence of NaCl

Effect of pH on Cu Adsorpon

10 9

Adsorpon capacity (mg/g)

8 7 6 5 4 3 pH3, 0 mM NaCl (straw) pH6, 0 mM NaCl (straw)

2

pH9, 0 mM NaCl (straw) pH3, 0 mM NaCl (husk)

1 0

pH6, 0 mM NaCl (husk) pH9, 0 mM NaCl (husk) 0

10

20

30

40

Inial concentraon (mg/L)

50

60

70

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Fig. 2 Effect of pH (3, 6, 9) on biosorption of Cd (II) onto rice straw and rice husk in the absence of NaCl

Effect of pH on Cd Adsorpon

1.6

Adsorpon capacity (mg/g)

1.4

1.2

1

0.8

0.6 pH3, 0 mM NaCl (straw) 0.4

pH6, 0 mM NaCl (straw) pH9, 0 mM NaCl (straw) pH3, 0 mM NaCl (husk)

0.2

0

pH6, 0 mM NaCl (husk) pH9, 0 mM NaCl (husk) 0

1

2

3

4

5

6

7

8

9

Inial concentraon (mg/L)

and 6 (effects of pH and salinity) revealed that the metal uptake increased, while the percentage of biosorption decreased with the increase in the initial metal concentration. This effect indicated an increase in the driving force of the concentration gradient (Hasfalina et al. 2012). Similar results have been reported in sorption studies of Cd by loquat leaves, which stated that the rise in Cd concentration would increase the driving force to overcome mass transfer resistances between the solid and aqueous phases, causing an increase in the metal uptake (Mohammad

significantly beyond pH 7 (Khomaei et al. 2007; Hasan and Srivastava 2009; Ertugay and Bayhan 2010; Ammar et al. 2012). Effect of initial concentration In both the Cu (II) studies (at an initial concentration level ranging from 5 to 60 mg/L) and the Cd (II) studies (at an initial concentration level ranging from 0.5 to 8 mg/L), Figs. 1, 2, 5

Effect of Contact Time on Cu Adsorpon in 0 mM and 100 mM NaCl

Fig. 3 Effect of contact time (60 min) on biosorption of Cu (II) onto rice straw and rice husk in 0 and 100 mM NaCl

2.5 2.25

Adsorpon capacity (mg/g)

2 1.75 1.5 1.25 1 0.75 0.5 pH6, 0 mM NaCl (straw) pH6, 100 mM NaCl (straw) pH6, 0 mM NaCl (husk) pH6, 100 mM NaCl (husk)

0.25 0 0

10

20

30

40

Contact me (min)

50

60

70

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affected by the saline aqueous solutions. Only 77 and 59 % of Cu (II) ions were removed within 20 min by the rice straw and rice husk, respectively. After 30 min, the adsorption rates only reached 80 and 88 %, respectively. The removal efficiency of Cd (II) by the rice straw and rice husk was 96 % in the absence of NaCl within 20 min in the initial stage. No significant increase in adsorption rate was observed beyond 30 min. In the saline aqueous solutions, the adsorption performance of Cd (II) was significantly affected. In the initial stage, only 31 and 50 % of Cd (II) were removed within 20 min by the rice straw and rice husk, respectively, and after 30 min of adsorption, the removal rate only increased to 40 and 58 %, respectively. From 60 min onwards, 45 and 65 % adsorption were found, which reflected that the processes of adsorption were still functioning with capacity. As the biosorption reached equilibrium within 30 min, this was used as the cutoff time for the kinetic study. The initial rapid uptake of metal could be explained by more available vacant surface sites. The high adsorption rate was related to the binding potential from bare surfaces and, therefore, to rapid binding (Kshama and Varsha 2011). The slowdown in the adsorption rate was due to the saturation of active sites. More vacant active sites were occupied by the metal ions as the contact time increased because of the repulsive forces between the solute molecules on the adsorbent surface in the bulk phase (Wongjunda and Saueprasearsit 2010). Before diffusing from the boundary layer film onto the surface of the adsorbent and, consequently, into the porous adsorbent, a longer contact time was required for the metal ions to overcome the boundary layer effect (Haris et al. 2011), and this resulted in slower intracellular diffusion (Kahraman et al. 2005). The biosorption rate of the rice straw was relatively faster than that of the rice husk, which may be explained

et al. 2011). The percentage of uptake declined gradually, indicating that the biosorption processes were reaching a plateau. The plateau represented a surface area that was insufficient to accommodate more metal from the solution (Ammar et al. 2012). The percentage of biosorption at higher metal concentration levels decreased, while the metal uptake capacity showed the opposite trend with an increase in the metal initial concentration. This effect could be explained by the number of exchangeable sites in the adsorbent structure and the metal to adsorbent ratios (Kavak 2009). At low metal concentrations, adsorption sites on the biomass remain unsaturated, whereas at high metal concentrations, the number of ions competing for the available binding sites on the biomass increases, and thus, the binding sites for the complexation of Cu (II) and Cd (II) ions become saturated (Kshama and Varsha 2011). Aggregation of adsorbent particles at higher concentrations causes a decrease in the total surface area of adsorbent particles available for adsorption and an increase in the length of the diffusion path (Kumar et al. 2009). Effect of contact time Experiments conducted at different contact time intervals revealed that the uptake of metal ions increased remarkably in the first 20 min. No further increment in adsorption capacity indicated that the metal adsorption attained equilibrium level after about 30 min. Figure 3 shows that in the absence of NaCl, 83 and 93 % of Cu (II) were removed by the rice straw and rice husk, respectively, within 20 min in the initial stage. The adsorption rate reached 88 and 98 %, respectively, after 30 min and became saturated. The metal ion uptake attained equilibrium and reached a plateau after 20 min. In contrast, Fig. 4 shows that the removal efficiencies were greatly

Effect of Contact Time on Cd Adsorpon in 0 mM and 100 mM NaCl

Fig. 4 Effect of contact time (60 min) on biosorption of Cd (II) onto rice straw and rice husk in 0 and 100 mM NaCl

0.5 0.45

Adsorpon capacity (mg/g)

0.4 0.35 0.3 0.25 0.2 0.15 0.1

pH6, 0 mM NaCl (straw) pH6, 100 mM NaCl (straw) pH6, 0 mM NaCl (husk) pH6, 100 mM NaCl (husk)

0.05 0

0

10

20

30

40

Contact me (min)

50

60

70

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by different function groups and the fibrous composition of the biomass surface. Biosorption kinetic study The kinetic data were fitted with pseudo-first-order and pseudo-second-order models, and the comparison was based on the correlation coefficient (R2) and the SSE. The data for the biosorption of Cu (II) and Cd (II) onto the rice straw and husk are shown in Table 2. For both Cu (II) and Cd (II), the obtained R2 values for the pseudo-first-order model were much lower than for the pseudo-second-order model. The R2 values for the pseudo-second-order model were in the range of 0.9922–1.0000. The SSE values for the two kinetic models are summarised in Table 2. The lower values of SSE demonstrate a better fit with the biosorption data (Ho et al. 2000; Sundaram et al. 2008). Table 2 shows that the SSE values of the pseudosecond-order model are surprisingly smaller than those of the pseudo-first-order model. Based on the values of R2 and SSE, it is therefore suggested that the pseudo-second-order model, but not the pseudo-first-order, best fits the biosorption data and that this indicates the biosorption of Cu (II) and Cd (II) onto the rice straw and husk chemisorption mechanism. Effect of salinity Salinity was an important factor affecting the adsorption of heavy metals. Both the adsorption capacity and the removal efficiency decreased with increased salinity. Figure 5 shows that the performance of the rice straw was slightly affected by the saline solution. The removal efficiency of Cu (II) decreased from 62 % in the absence of salt to 58 % in the presence of salt. In the case of the rice husk, the removal efficiency also varied greatly according to the salinity level: it decreased from 60 % in the absence of salt to 43 % in the presence of salt. The results showed that the ability of the rice Table 2 Kinetic parameters evaluated from pseudo-first order and pseudo-second order for biosorption of Cu (II) and Cd (II) onto the rice straw and husk

husk and rice straw to remove Cu (II) was affected by increased salinity slightly, ranging from a 4 to 17 % decrease in the removal rate. These results are consistent with a previous study in which the biosorption capacities of Cu (II) by treated rubber (Hevea brasiliensis) leaves powder decreased as the salinity of NaCl increased (Ngah and Hanafiah 2008). According to Fig. 6, the removal rate of Cd (II) by the rice straw depended largely on salinity. The removal efficiency decreased from 81 to 54 % with an increase in salinity, whereas for the rice husk, the removal efficiency was much diminished from 70 to 53 % in the presence of NaCl. These results are similar to those of Javier (2012), who reported that the removal of Cd (II) by grapefruit biomass (Citrus paradisi L.) decreased by about 70 % in the presence of Na+. The decrease in the biosorption capacities of Cu (II) and Cd (II) is attributable to several factors. First, there is a competitive effect of Na+ on binding sites. At a high NaCl concentration, Na+ tends to saturate the binding sites of biosorbents and, thus, inhibit the metal ions from approaching the biosorbent surface (Zouboulis et al. 1998). Therefore, more metal ions remain in the solution. Second, there is an electrostatic attraction between the metal ions and the binding sites. The presence of NaCl increases or expands the electrical diffused double layer and prevents the biosorbents and metal ions from approaching each other more closely; this decreases the electrostatic attraction (Wang et al. 2007). Moreover, the decrease in the biosorption capacities of Cu (II) and Cd (II) may be related to the formation of metal chloro-complexes (Dönmez and Aksu 2002). An increase in chloride ion concentration may therefore cause a decrease in free metal ions and an increase in chloro-complexes formation, affecting the adsorption process significantly.

Biosorption isotherms The relationship between sorption capacity and equilibrium concentration was analysed by two typical biosorption

Pseudo-first order

Rice straw pH 6, S0 pH 6, S100 Rice husk pH 6, S0 pH 6, S100

Pseudo-second order

k1 (min−1)

R2

SSE

Cu (II)

0.097

0.9511

3.708

Cd (II) Cu (II) Cd (II)

0.112 0.215 4.567

0.8390 0.9991 0.9871

4.346 2.553 4.900

Cu (II) Cd (II) Cu (II) Cd (II)

0.206 0.129 0.082 0.107

0.9405 0.7671 0.9853 0.9402

2.437 4.129 0.538 0.322

0.628 7.050 0.164 1.031

R2

SSE

1.216

0.9996

0.0215

11.16 0.888 0.822

0.9998 1.0000 0.9965

0.00427 0.0622 3.952

0.9997 0.9998 0.9932 0.9959

0.0744 0.0215 0.2121 0.1711

k2 ((g/mg) min)

Environ Sci Pollut Res 10

Fig. 5 Effect of salinity (0, 50, 100 mM NaCl) on biosorption of Cu (II) onto rice straw and rice husk at pH 6

Effect of Salinity on Cu Adsorpon

9

8

Adsorpon capacity (mg/g)

7

6

5

4

3 pH6, 0 mM NaCl (straw)

2

pH6, 50 mM NaCl (straw) pH6, 100 mM NaCl (straw) pH6, 0 mM NaCl (husk)

1

0

pH6, 50 mM NaCl (husk) pH6, 100 mM NaCl (husk) 0

10

20

30

40

50

60

70

Inial concentraon (mg/L)

isotherm models, known as the Langmuir and Freundlich models. These describe the interaction between adsorbates and absorbents and are crucial in optimising the use of adsorbates (Kuamr et al. 2012). This study showed that the amount of Cu (II) and Cd (II) adsorbed by a given quantity of rice residues rose with the increase in metal ion concentrations. The correlation coefficient (R2) indicated the compliance of

the isotherm models with the experimental data. Sorption isotherm studies on Cu (II) and Cd (II) by rice straw at three pH levels and three different salinity levels were obtained and are shown in Table 3. The correlation coefficient (R2) reflected that the experimental data of this study were better fitted with the Freundlich isotherm model. As the Freundlich isotherm was valid for heterogeneous surface studies, it suggested a

1.8

Fig. 6 Effect of salinity (0, 50, 100 mM NaCl) on biosorption of Cd (II) onto rice straw and rice husk at pH 6

Effect of Salinity on Cd Adsorpon

1.6

Adsorpon capacity (mg/g)

1.4 1.2 1 0.8 0.6 pH6, 0 mM NaCl (straw) 0.4

pH6, 50 mM NaCl (straw) pH6, 100 mM NaCl (straw) pH6, 0 mM NaCl (husk)

0.2 0

pH6, 50 mM NaCl (husk) pH6, 100 mM NaCl (husk) 0

1

2

3

4

5

Inial concentraon (mg/L)

6

7

8

9

Environ Sci Pollut Res Table 3 Langmuir and Freundlich constants obtained for the biosorption of Cu (II) and Cd (II) onto rice straw and rice husk

Langmuir model

Cu (II)

Cd (II)

Freundlich model

Qmax (mg/g)

b

R2

KF

n

R2

1/n

pH 3, S0 pH 6, S0

9.940 12.225

0.077 0.088

0.864 0.929

0.939 1.108

1.646 1.487

0.991 0.984

0.608 0.672

pH 9, S0 pH 6, S50 pH 6, S100 Rice husk pH 3, S0

12.658 11.390 10.341

0.101 0.078 0.091

0.959 0.904 0.876

1.271 0.977 1.043

1.510 1.511 1.604

0.988 0.981 0.987

0.662 0.662 0.624

5.924

0.352

0.998

1.581

2.519

0.950

0.397

pH 6, S0 pH 9, S0 pH 6, S50 pH 6, S100

8.889 8.584 7.396 6.211

0.430 1.021 0.305 0.321

0.984 0.999 0.965 0.991

2.503 3.117 1.843 1.614

2.416 2.511 2.417 2.516

0.905 0.927 0.971 0.966

0.414 0.398 0.414 0.397

Rice straw pH 3, S0 pH 6, S0 pH 9, S0

1.237 9.091 2.131

0.521 0.141 0.539

0.934 0.597 0.826

0.375 1.114 0.714

1.384 1.070 1.253

0.925 0.997 0.915

0.723 0.935 0.798

pH 6, S50

2.821

0.270

0.908

0.560

1.188

0.982

0.842

pH 6, S100 Rice husk pH 3, S0

3.766

0.112

0.870

0.362

1.105

0.994

0.905

0.801

1.358

0.836

0.387

2.519

0.867

0.395

pH 6, S0 pH 9, S0

1.581 1.717

3.066 2.299

0.996 0.994

1.117 1.111

1.815 1.688

0.892 0.918

0.551 0.592

pH 6, S50 pH 6, S100

2.623 1.456

0.289 0.366

0.589 0.701

0.537 2.893

1.289 1.467

0.973 0.973

0.776 0.682

Rice straw

biosorption mechanism of Cu (II) and Cd (II) ions onto the rice straw. In the Freundlich model, the higher KF value represented the higher adsorption capacity of the adsorbent (Öztürk and Kavak 2008). The value of 1/n reflected the measure of adsorption intensity or surface heterogeneity and became more heterogeneous when its value was close to 0 (Haghseresht and Lu 1998). Table 3 shows that the values of 1/n ranged between 0 and 1, indicating that the biosorption of Cu (II) and Cd (II) onto the rice straw and husk was favourable in the studied conditions (Subbaiah et al. 2011). In contrast, Table 3 shows that the Langmuir theory would be more suited to describe the biosorption phenomena between Cu (II), Cd (II) and the rice husk. The maximum sorption capacities (Qmax) of Cu (II) and Cd (II) by the rice husk achieved at pH 6 and in the absence of NaCl were 8.89 and 1.58 mg/g, respectively. The Qmax values obtained for Cu (II) sorption were larger than those in previous studies using Pycnoporus sanguineus (Qmax =2.76 mg/g) (Yahaya et al. 2009) and potato peels charcoal (Qmax =0.388 mg/g) (Aman et al. 2008) as biosorbents. However, the Qmax values obtained for Cd (II) sorption were lower than the previously reported values using Ulmus leaves (Qmax =6.94 mg/g) (Mahvi et al. 2008) and sugarcane bagasse (Qmax =6.97 mg/g) (Ibrahim

et al. 2006) as biosorbents. Table 1 also shows that for both the rice husk and straw, the maximum adsorption capacities (Qmax) for other metal ions or organic pollutants were generally higher when they were pretreated with NaOH (Rocha et al. 2009; Ding et al. 2012; Toniazzo et al. 2013; Zhang et al. 2013, 2014a, b). These results suggest that NaOHpretreated rice husk would be a better biosorbent for the sorption of Cu (II) and that Cd (II) could be removed better by rice straw at pH 6 and in the absence of NaCl.

Further studies In reality, different heavy metals are usually discharged as industrial effluent at the same time. Therefore, combined tests on various heavy metals are worth studying because the interaction of heavy metals may affect the binding potential and adsorption capacity of one metal in the presence of another due to an antagonistic effect (Cataldo et al. 1983). Competitions in metal ion uptake are expected that would lead to a decrease in the active sites available on biomass surfaces (Patrón-Prado et al. 2010). Therefore, a real wastewater sample was not analysed in this study to avoid an antagonistic

Environ Sci Pollut Res

effect that might affect the optimisation of results. Further research is needed to find the maximum sorption capacity of rice husk and straw on wastewater using the optimum conditions in this study. The temperature in this experiment was held constant at room temperature (20 °C). Some studies have shown that the adsorption of Cd (II) decreases with a rise in temperature due to the increasing tendency to desorb from the interface to the solution (Huang et al. 2010). Hence, the influences of temperature on aqueous solutions could be investigated.

Conclusion The adsorption performance of Cu (II) and Cd (II) by NaOHtreated rice straw and rice husk was examined in this experiment. In pH studies for the biosorption of Cu (II) and Cd (II), an increase in removal efficiency or adsorption capacity was observed with an initial pH increase from 3 to 6, and sorption ability was better at pH 6. The salinity studies showed that the presence of NaCl discouraged the biosorption. Furthermore, the rapid uptake of heavy metal ions by rice residues was observed in the first 20 min and reached equilibrium after 30 min. The Freundlich isotherm model was better at describing the heterolayer biosorption of Cu (II) and Cd (II) onto the rice straw, while the Langmuir model was better at describing the monolayer biosorption of Cu (II) and Cd (II) onto the rice husk with maximum sorption capacities (Qmax) of 8.89 and 1.58 mg/g, respectively. The R2 and SSE suggested that the biosorption of Cu (II) and Cd (II) onto the rice straw and husk was best fitted with the pseudo-second-order kinetic model. This study demonstrated the feasibility of using NaOH-treated rice straw and rice husk as stable, efficient and low-cost biosorbents for removing Cu (II) and Cd (II) ions in wastewater. Acknowledgments The work described in the article are supported by the Research Grants Council of the Hong Kong Special Administrative Region, China (HKIEd 28100014) and Faculty of Liberal Arts and Social Sciences (Dean’s Research Fund (activity code: 04021) of the Hong Kong Institute of Education.

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