Valuation of Unmodified Rice Husk Waste as an Eco-Friendly Sorbent to Remove Mercury: a Study Using Environmental Realistic Concentrations
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- Rocha, L.S., Lopes, C.B., Borges, J.A. et al. Water Air Soil Pollut (2013) 224: 1599. doi:10.1007/s11270-013-1599-9
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The present work explores the sorption capacity of an inexpensive and highly available agricultural waste, rice husk, to remove mercury using realistic concentrations of this metal. The efficiency of the process was evaluated for two initial Hg(II) concentrations, one representing the maximum value for Hg discharges from industrial sectors (0.05 mg L−1), and the other ten times higher. A very small amount of rice husk (0.25 and 0.50 g L−1) was able to reduce the Hg(II) levels in more than 80 % for an initial concentration of 0.05 mg L−1 and in more than 90 % for 0.50 mg L−1, corresponding to residual concentrations of Hg(II) of 0.048 and 0.009 mg L−1, respectively. The biosorvent was reused in further cleaning treatments, maintaining the efficiency and high performance. The sorption kinetics of the Hg–rice husk system is well fitted by the Elovich model and the diffusion models suggested that, depending on the initial Hg(II) concentrations, the sorption process can be controlled by intraparticle diffusion or by both film and intraparticle diffusion. The equilibrium data are well described by the linear isotherm and the distribution coefficient found was 36.1 L g−1.
KeywordsMercuryRice huskWasteRemediationContaminated waters
The presence of metals in aquatic systems has become a serious threat, of environmental extreme concern, due to the non-biodegradable and persistent character of these elements (Krishnani et al. 2008). Mercury (Hg) is regarded as one of the most harmful metals found in the environment not only due to its high toxicity, volatility, and persistence both in the environment and biota, but also because Hg bioaccumulates in living organisms and bioamplifies along the food chain (Coelho et al. 2008). Moreover, according to the European Parliament and the Council of European Union, Hg (and its compounds) is considered a priority hazardous substance (EU 2008), and its emissions, discharges, or losses should be phased out.
The nature and magnitude of the environmental problems are frequently changing and with legal constraints being imposed, there is a constant need for developing newer and more appropriate technologies for water treatment. In this context, biosorption, an eco-friendly and cost-effective treatment technology presents a tremendous potential to provide the needs and holds hope for environmental protection, sustainability, and management (Krishnani et al. 2008; Juwarkar et al. 2010). In the last two decades, efficient and cost-effective approaches have been developed to remove Hg(II) and other metal ions from wastewaters based on the use of non-conventional sorbents. These materials are available in nature or are either a by-product or waste material from the industry, and since they require less prior processing and are available in large quantities, they can be classified as low-cost sorbents. As a result, these materials could represent an excellent alternative not only to expensive treatment processes (Meena et al. 2004), but also to effective but costly materials, such as activated carbon and titano- and zirconico-silicates (Krishnani et al. 2008; Lopes et al. 2008; Lopes et al. 2009).
Rice husk (RH) is a by-product of rice processing and it comprises 23 % of the rice grain, being considered a significant waste disposal problem (Jeon, 2011). This material, like all vegetable biomasses, is composed essentially of cellulose (29 to 34 %), hemicellulose (21 to 29 %), and lignin (19 to 30 %) (Ahmaruzzaman and Gupta, 2011). The presence of carbon and silica in the rice husk composition and the existence of functional groups with binding capabilities (such as carboxyl and silanol) on its surface makes the sorption processes possible, though it has been reported that this material was used to remove various pollutants from waters and wastewaters (Ahmaruzzaman and Gupta, 2011; Jeon, 2011).
Despite the fact that the potential of rice-husk-based sorbents to remove Hg(II) from waters has already been investigated by some authors, the studies developed in this field used initial concentrations of Hg(II) considerably high, i.e., between 8 to 2,000 mg L−1 (Tiwari et al. 1995; Khalid et al. 1999; Feng et al. 2004; Krishnani et al. 2008; El-Said et al. 2010; El-Shafey 2010), values that largely exceed the values found in contaminated environments. As a result, the removal capacity of rice-husk-based materials in those studies is being overestimated and even when high removal values were achieved, the residual concentration of Hg(II) at the end of the process, largely exceeds (at least ten times) the guideline limits for effluents discharges (0.05 mg L−1, Council Directive 84/156/EEC). For those reasons, it is very important to evaluate the sorption capacity of rice husk toward Hg(II), for concentrations close to the ones found in contaminated environments, so that, the real value of this material as a cleaning agent is recognized. Besides that, it should also be highlighted that in most of the studies, the authors performed several pre-treatments to the sorbent and, frequently the physical (calcination) and/or chemical (using acidic, alkaline, among other solutions) modified forms of rice husk are used (Tiwari et al. 1995; Feng et al. 2004; Krishnani et al. 2008; El-Said et al. 2010; El-Shafey 2010). Although the modification of the rice husk usually increases the sorption efficiency of the material, it is also time consuming, increases the cost of the material and consequently of the cleaning process, and also wipes out one of the initial concepts behind biosorption, the revaluation of agricultural wastes. Additionally, in those studies, a substantial amount of rice husk per volume of solution (m/VRH) is usually used (between 2 and 100 g L−1), which may cause secondary problems of waste disposal (Tiwari et al. 1995; Feng et al. 2004; Krishnani et al. 2008; El-Said et al. 2010; El-Shafey 2010).
To answer to some of the drawbacks of the studies reported in the literature, the main goal of this work was to evaluate the efficiency of unmodified rice husk to uptake Hg(II) from aqueous solutions under more realistic conditions and to optimize the sorption process in terms of cost (type and amount of rice husk). In order to achieve this, only metal concentrations similar to those that can be possibly found in the environment and the natural/unmodified form of rice husk were used. The study comprised the assessment of the effect of some physico-chemical parameters such as contact time, initial Hg(II) concentration, and mass of rice husk, together with the kinetic and equilibrium behavior of the Hg(II)–rice husk system.
2 Materials and Methods
2.1 Material and Chemicals
All chemicals were of analytical reagent grade and solutions were prepared with ultra-pure water (18.2 MΩ cm, Milli-Q system). The standard stock solution of mercury (1001 ± 2 mg L−1) and the nitric acid 65 % (Suprapur) were purchased from Merck. Biohit Proline pipettes equipped with disposable tips were used for appropriate dilutions. Experiments were carried out at room temperature (21 ± 1 °C). All glassware material used in the experiments was acid-washed prior to use.
2.2 Biosorbent Material Preparation and Characterization
The rice husk was kindly supplied by the industries of Álvaro Alves Borges Lda. from Figueira da Foz, Portugal, whose activity is related with the peeling, bleaching, and other treatments of rice. The rice husk was washed with distilled water and dried at 60 °C. Afterwards, the material was triturated using a coffee mill (model Taurus aromatic), sieved to obtain a fraction with particles of size ≤500 μm and preserved at room temperature. The procedure used for the preparation of this sorbent was simple, less time consuming, and inexpensive.
The structural characteristics of the rice husk were determined by powder X-ray diffraction of the dried samples and at room temperature. The spectrum was obtained using a Philips X’Pert X-ray diffractometer equipped with a Cu Ka monochromatic radiation source. Intensity data were collected by the step counting method (step 0.02° and time 5 s) in the range of 2θ = 3.5°–70°. The morphological characteristics of rice husk were evaluated by scanning electron microscopy (SEM) using an S-4100 HITACHI equipment. This system was coupled with an electron dispersive spectroscopy (EDS) allowing the identification of the most abundant inorganic elements in the rice husk. Prior to the SEM analysis, the rice husk samples were covered with a thin layer of carbon and an electron acceleration voltage of 20 kV was applied.
Total Hg concentration in the rice husk was evaluated by pyrolysis atomic absorption spectrometry with gold amalgamation (LECO® model AMA-254). No sample digestion was involved in this process.
2.3 Sorption Kinetic Experiments
Batch sorption kinetic experiments were performed at room temperature (21 ± 1 °C), by contact of a certain amount of the unmodified rice husk samples (particle size <500 μm) with a known concentration of Hg(II) solution (0.05 and 0.5 mg L−1). The experimental trials were carried out in volumetric flasks (2 L) under constant stirring conditions (1,400 rpm). The two initial Hg(II) concentrations selected aimed to represent the maximum value for Hg discharges from industrial sectors (0.05 mg L−1) and to simulate an eventual situation of an accidental spill (0.5 mg L−1). The amount of rice used to perform the experiments was 0.50 and 1.00 g for both initial Hg(II) concentrations, corresponding to a rice husk mass-to-volume ratio of (m/VRH) of 0.25 and 0.50 g L−1, respectively. The pH of the solution was kept between 5.7 and 6.0. According to several authors (Khalid et al. 1999; Chuah et al. 2005; El-Said et al. 2010), pH values around six favors and maximizes the sorption of Hg(II) by the rice husk. Two replicates of each experiment were performed and the results were always expressed as the mean value obtained.
Hg(II) solutions were prepared by diluting the standard stock solution to the desired concentration in ultra-pure water. Experiments started when a known mass of the biosorbent was added to Hg(II) solutions and stirring was initiated. Aliquots (10 ml) were collected at increasing times, filtered through an acid-washed 0.45-μm Millipore membrane and then the filtrate was adjusted to pH < 2 with Suprapur nitric acid. Each experiment was maintained until the Hg(II) concentration in the solution remained constant. An experiment without rice husk was always run as a control to assess the loss of Hg(II) to the glass vessels and in the sample filtration procedure.
Hg(II) concentration in the samples was evaluated by cold vapor atomic fluorescence spectroscopy (CV-AFS), on a PSA cold vapor generator, model 10.003, associated with a Merlin PSA detector, model 10.023, using SnCl2 as reducing agent.
2.4 Sorbent Reuse
Kinetic experiments were performed as described previously, and whenever the equilibrium was attained (Ce), the solution was spiked with Hg(II). Each spike (spk) corresponded to an increase of 0.50 mg L−1 in the concentration of Hg(II). This procedure was repeated every time a new equilibrium was achieved (Ce_spk1; Ce_spk2, and so on), until the amount of Hg(II) sorbed by the rice husk remained constant. Two m/VRH were used: 0.25 and 0.50 g L−1. An experiment without rice husk was run as a control, to assess the concentration of Hg(II) in the solution after each spike, and the losses of Hg(II) to the glass vessels and in the filtration procedure.
2.5 Sorption Isotherms Experiments
Sorption equilibrium experiments were conducted by maintaining the initial Hg(II) concentration constant at 0.5 mg L−1 and increasing the amount of rice husk. The m/VRH used for this study was: 0.005, 0.012, 0.025, 0.050, 0.12, 0.25, 0.50, and 1.0 g L−1. The experiments were performed at room temperature (21 ± 1 °C) and at a pH range of 5.7–6.0. The concentration of Hg(II) in the samples was quantified as described previously.
2.6 Evaluation of the Experimental Results
The C0 and Ct values were always corrected by means of a recovery factor that accounts for the Hg(II) losses from the controls.
2.7 Kinetic and Equilibrium Models
Additional information regarding the kinetic and equilibrium models is described in Appendix A.
2.8 Error Analysis
The parameters of the kinetic and equilibrium models here considered were obtained by nonlinear regression analysis using GraphPad Prism 5 program (trial version), which uses the least-squares as fitting method and the method of Marquardt and Levenberg, which blends two other methods, the method of linear descent and the method of Gauss-Newton for adjusting the variables. The goodness of the fittings of the kinetic and equilibrium models to the experimental data was confirmed by calculating the R square (R2) and the sum of squares (SS). Additionally, the Akaike’s information criteria (AIC) and the evidence ratio were used to compare equilibrium models. Lower AIC values (on a scale form –∞ to +∞) suggests that the corresponding model is more likely to occur than the alternative models and the evidence ratio is a numerical value representative of the number of times that the model with a lower AIC is more likely to be correct (Malash and El-Khaiary 2010).
The analyses of the experimental sorption dynamic results and the determination of parameters from the diffusion models (film and intraparticle diffusion) were obtained by a Microsoft® ExcelTM worksheet developed by Malash and El-Khaiary (2010). The use of this method allows the definition of the linear segments and provides a much more accurate prediction of the diffusion coefficients and the diffusion mechanisms involved. The definition of the numbers of linear segments that described the diffusion process (two, three, or four steps) was assessed by comparing the AIC obtained for each model and the evidence ratio. The goodness of the fittings was also evaluated in terms of the correlation coefficient (r) of linear segments. The standard deviations (SD) of all fitted parameters are presented.
3 Results and Discussion
3.1 Rice Husk Characterization
The analysis of the total concentration of mercury in the rice husk indicates that this material has a very low Hg(II) content (3.9 ng g−1, mean value of three replicates, with a relative standard deviation less than 8 %).
3.2 Sorption Studies
3.2.1 Influence of the Initial Hg(II) Concentrations and Rice Husk Amount on the Sorption Kinetic Process
Removal of Hg(II) (Rt, %), residual concentration of Hg(II) in solution (CHg,, mg/L), and amount of Hg(II) sorbed per gram of rice husk qt (mg g−1), at different times: 6 and 24 h and at equilibrium. The values are expressed as the mean of two replicate experiments
C0 Hg (mg L−1)
m/VRH (g L−1)
Rt (%) (%)
CHg (×10−3 mg L−1)
qt (mg g−1)
For the highest initial concentration and for the entire range of time, no relevant differences were obtained on the removal of Hg(II) when different rice husk amounts were used (m/VRH of 0.25 and 0.50 g L−1). However, when starting with C0 Hg of 0.05 mg L−1 the time removal profiles are quite distinct for the initial hours but tend to converge for the same removal value (ca. 80 %), at equilibrium. In terms of the amount of Hg(II) sorbed at equilibrium per gram of material, the values varied between 0.16 and 1.80 mg g−1 (m/VRH of 0.25 g L−1) and 0.083 and 0.92 mg g−1 (m/VRH of 0.50 g L−1), for an initial Hg(II) concentration of 0.05 and 0.50 mg L−1, respectively (Table 1), increasing with the increasing of initial Hg concentration and with the decreasing of m/VRH. Considering the levels of Hg(II) remaining in solution after the sorption process with unmodified rice husk, it must be highlighted that for an initial concentration of 0.5 mg L−1 and for both m/VRH, the residual concentrations achieved were lower than the guideline limits established in the Portuguese legislation for Hg discharges from industrial sectors (CHg < 0.05 mg L−1, Council Directive 84/156/EEC). When the initial Hg(II) concentration was the actual maximum value for Hg discharges from industrial sectors, itself, the equilibrium concentrations achieved for both m/V ratios (0.25 and 0.50 g L−1) were lower than 9 × 10−3 mg L−1, a value relatively close to the guideline value for drinking water quality (1 × 10−3 mg L−1, Council Directive 98/83/EC).
3.2.2 Sorbent Reuse
According to Fig. 4a, the amount of Hg(II) sorbed per gram of rice husk at equilibrium is directly proportional to the increase of Hg(II) concentration in solution for a narrow range of concentrations. The strong correlation, given by the linearity between qe_spk and C0_spk, for the range of concentrations 0–1.0 mg L−1 in the case of 0.25 g L−1 of m/VRH (r = 0.999) and 0–2.0 mg L−1 for m/VRH of 0.50 g L−1 (r = 0.997), is indicative of the unsaturation of the rice husk and of the existence of the available binding sites on the material. Although qe_spk increases continuously with C0_spk after several spikes, the deviation from linearity started to occur for concentrations of Hg(II) (C0_spk) higher than 1.0 and 2.0 mg L−1, respectively, for m/VRH of 0.25 and 0.50 g L−1, suggesting the beginning of the saturation of this sorbent. The “plateau” value on qe_spk was only reached for Hg(II) concentrations higher than 3.2 mg L−1 (Fig. 4a). Moreover, the results suggest that with the same rice husk, it was still possible to achieve concentrations lower than the maximum value for Hg discharges from industrial sectors (0.05 mg L−1, value represented by a dashed line in Fig. 4b) after two and three consecutive spikes, respectively, for m/VRH of 0.25 and 0.50 g L−1, which corresponded to Hg(II) concentrations in the range of 0.5–1.6 mg L−1 (Fig. 4b). For concentrations of Hg(II) higher than 1.0 mg L−1 (third spike) for m/VRH = 0.25 g L−1 and higher than 1.6 (fourth spike) for m/VRH = 0.50 g L−1, the removal process continues to run with removal percentages that varied between 69 and 64 % for a m/VRH of 0.25 g L−1 and between 87 and 77 % for a m/VRH of 0.50 g L−1.
3.2.3 Kinetic Modeling of the Results
Experimental qeexp and data obtained from the fitting of the experimental results with: first-order (qefit and k1), pseudo-second-order (qefit and k2) and Elovich (a and α) reaction models, and Boyd’s film diffusion (intercept and its corresponding interval, Di and r) and Weber’s pore diffusion (breakpoint, kid and r) models
qeexp ± SDa
qe1fit ± SDb
k1 ± SDb
qe2fit ± SDb
k2 ± SDb
(g mg−1 h−1)
(g mg−1 h−1)
0.163 ± 0.003
0.130 ± 0.008
1.27 ± 0.37
0.135 ± 0.007
11.4 ± 3.9
57.2 ± 3.8
0.669 ± 0.241
0.083 ± 0.003
0.074 ± 0.003
2.50 ± 0.43
0.076 ± 0.002
48.3 ± 7.2
128 ± 9
2.72 ± 1.38
1.80 ± 0.01
1.55 ± 0.07
1.26 ± 0.30
1.61 ± 0.06
1.13 ± 0.26
5.51 ± 0.20
22.6 ± 5.0
0.921 ± 0.004
0.808 ± 0.039
2.67 ± 0.61
0.825 ± 0.028
5.29 ± 1.15
12.8 ± 0.6
61.0 ± 20.8
Boyd’s film diffusion
Webber’s diffusion model
kid ± SD
(×10−2 mg g−1 h−1/2)
5.77 ± 6.31
0.967 (n = 4)
0.66 ± 0.09
0.994 (n = 8)
4.00 ± 4.53
0.952 (n = 4)
0.17 ± 0.02
0.992 (n = 8)
5.13 × 10−4
53.2 ± 6.33
0.996 (n = 6)
4.41 ± 0.62
0.993 (n = 7)
1.14 × 10−3
35.0 ± 9.6
0.988 (n = 5)
2.05 ± 0.16
0.997 (n = 8)
The works reported in the literature for rice-husk-based materials, generally uses the pseudo-first-order and pseudo-second-order models to describe the sorption kinetic behavior of Hg(II)–rice husk systems, and in most of these studies, the pseudo-second-order model was better adjusted to the experimental data (Wong et al. 2003; Al-Degs et al. 2006; Kumar and Bandyopadhyay, 2006; El-Said et al. 2010; El-Shafey, 2010; Feng et al. 2004). In the present work, besides the pseudo-first-order and pseudo-second-order, the Elovich model was used, and the results obtained indicate that this model provides a better adjustment to experimental data than the other models.
In order to gain insight into the mechanism and rate-controlling steps affecting the kinetic of the sorption process of Hg(II) onto unmodified rice husk, the Webber’s pore-diffusion and Boyd’s film-diffusion models were used. The piecewise linear regression (PLR) was applied to the experimental data and the numerical results are presented in Table 2. By applying the PLR treatment to the data obtained using a C0 Hg of 0.05 mg L−1 and for both m/VRH, it was found that the first linear segment in the Boyd’s plot (data not showed) is linear and that the confidence interval of the intercept, for a 95 % confidence level, includes zero. This strongly suggests that film diffusion is not the rate controlling during this stage and that intraparticle diffusion controls the entire sorption process (Hameed and El-Khaiary 2008; Malash and El-Khaiary, 2010). The Weber Morris plots (Fig. 5e and f) indicate the existence of multi-stages for both m/VRH. The definition of the numbers of linear segments was assessed by comparing the Akaike’s information criteria obtained for each model and by calculating the evidence ratio. Lower AIC were obtained considering two linear segments and the evidence ratio calculated, indicate that the two stages approach is 145 (m/VRH of 0.25 g L−1) and 413 (m/VRH of 0.50 g L−1) times better than the three stages. As result, the Weber Morris plots for both m/VRH were defined by two operational stages, with a break point (time corresponding to point where the two linear segments meet) of 1.66 h. The first stage corresponds to the steep-sloped portion (from 0.08 to 1.66 h) and the second stage (from 1.66 to 144 h) corresponds to the linear gentle-sloped portion of the qt vs t1/2 plot. The regression results from the first linear segment of Weber Morris plots estimated intercept values that are not significantly different from zero, corroborating the conclusion obtained from the Boyd plots, i.e., that pore diffusion controls the overall rate sorption process, even in the early stage. The intraparticle diffusion rate constants kid were calculated from the slope of the two corresponding lines (Table 2) and the values obtained were higher for the first than for the second stage. According to some authors, this fact can be explained by the diffusion of Hg(II) in pores of rice husk of distinct sizes and progressively smaller (macropores and mesopores) (Hameed and El-Khaiary 2008; El-Khaiary and Malash, 2011). Since the available path for the diffusion of Hg(II) in pores of smaller sizes is reduced, this explains the lower kid values obtained in the second linear segment. For a C0 Hg of 0.05 mg L−1, the amount of sorbent used had a little effect on the diffusion of Hg(II) through macropores of rice husk (kid for the lowest m/VRH was 1.4 times higher than kid for the highest one), however the diffusion through mesopores was more affected by the m/VRH, resulting in a decrease in kid ca. four times by doubling the m/VRH.
Starting with an initial concentration of Hg(II) of 0.50 mg L−1 and for both m/VRH, the Boyd’s plots are linear (correlation r ≥ 0.989) in the initial sorption period and presents an intercept, for a 95 % confidence level, statistically different from zero. These results strongly suggest that under these experimental conditions, film diffusion is the rate-controlling process in the period between 0.08 and 1 h. The Webber’s pore-diffusion plots to the same set of experimental data (Fig. 5g and h) indicate the existence of multi-linearity, and according to the AIC and the evidence ratio, two linear segments can be defined with a break point of 3.70 and 1.66 h, for m/VRH of 0.25 and 0.50 g L−1, respectively. The information obtained from Boyd’s plots and the intercept values from the Weber–Morris plots (statistically different from zero for a 95 % confidence level), indicate that for the period corresponded to the first segment, the sorption process is controlled by film diffusion of Hg(II) from the external surface into the pores (Hameed and El-Khaiary 2008; Malash and El-Khaiary, 2010). The period that corresponds to the second segment is controlled by the diffusion of Hg(II) on the internal surface of the pores. The kid calculated from the second segment and for a m/VRH of 0.50 g L−1 was ca. 2.4 times higher than the one obtained for 0.25 g L−1. The modeling of the kinetic process by the intraparticle diffusion model also indicates that kid (from the second stage) increased ca. one order of magnitude, increasing the initial Hg(II) concentration in solution. According to Ofomaja (2010)), this phenomenon can be explained by the fact that a high initial concentration will give rise to a higher concentration gradient, being responsible to eventually cause a faster diffusion and a rapid sorption process.
3.3 Sorption Isotherms Studies
The distribution coefficient Kl obtained from the linear isotherm, which is indicative of the distribution of Hg ions between the liquid and the solid phase, was 36.1 ± 1.0 L g−1.
3.4 Sorption Efficiency of Rice Husk to Remove Hg(II)
Sorption capacity found in the literature (qe, mg g−1) for some sorbents used in the removal of Hg(II). The sorbent mass-to-volume ratio (m/Vsorbent, g L−1), pH, and initial Hg(II) concentration (C0Hg(II), mg L−1) are also mentioned
m/Vsorbent (g L−1)
C0Hg(II) (mg L−1)
qe (mg g−1)
Microorganism d (Bacillus sp.)
White rot fungus (Lentinus edodes)
Bayramoğlu and Arica 2008
Fraxinus tree leaves
Zolgharnein and Shahmoradi 2010
Ghodbane and Hamdaoui 2008
Leaves of castor tree (Ricinus communis L.)
Shaban et al. 2008
Plant (Coriandrum sativum)
Karunasagar et al. 2005
Banerjee et al. 2004
Rubber from tyre wastes (residual rubber)
Manchón-Vizuete et al. 2005
Rice husk ash
El-Said et al. 2010
Rice husk ash
Feng et al. 2004
Rice husk treated with H2SO4
Biomatrix from rice husk
Krishnani et al. 2008
Unmodified rice husk
Khalid et al. 1999
Unmodified rice husk
The removal of Hg(II) by unmodified rice husk was investigated in batch mode, and for the first time realistic environmental concentration levels of Hg(II) (μg L−1 range) were used. The performed experiments revealed that small amounts of rice husk (0.25 and 0.50 g L−1) were able to reduce Hg(II) levels in ca. 80 % when the initial concentration (C0Hg) was the maximum value allowed for Hg discharges (0.05 mg L−1) and ca. 90 % when the initial concentration was ten times higher (0.50 mg L−1). The saturation capacity of the rice husk was not reached, which allowed the reuse of the same material in some additional treatments, maintaining the efficiency in the removal process (>90 %) and the residual concentrations of Hg(II) with levels lower than the established value for Hg discharges from industrial sectors (0.05 mg L−1).
The kinetics of the Hg(II)/rice husk system was studied in this work and both reaction and diffusion models were used. The experimental data was well predicted by the Elovich reaction model and the diffusion models suggested the occurrence of two operational stages in the sorption process of Hg(II) onto the rice husk. The results obtained from the statistical piecewise linear regression indicate that intraparticle diffusion controlled the entire sorption process for the lowest levels of Hg(II) (C0Hg = 0.05 mg L−1), and for the highest levels (C0Hg = 0.50 mg L−1), both film diffusion and pore diffusion mechanisms had an important role in controlling the sorption process. The equilibrium results indicate that under the experimental conditions used in this work, the experimental data was better described by the linear isotherm model.
As a main conclusion of this work, the unmodified rice husk material proved to be very efficient for the removal of Hg(II), even from aqueous solution with relative low levels of contamination (0.05 and 0.5 mg L−1). Although the removal of pollutants from water is an abundant field in literature, the capacity of biosorbents to remove metals under realistic conditions is still largely unknown, and so, this study is an important contribution to understand and to evaluate the effective capacity of the unmodified rice husk, under realistic conditions.
Thanks are due to University of Aveiro/CESAM and Fundacão para a Ciência e a Tecnologia (FCT). Luciana Rocha and Cláudia B. Lopes acknowledge their Post-doc grants to FCT (SFRH/BPD/47166/2008 and SFRH/BPD/45156/2008, respectively).