The removal of chromium(III) (Cr(III)) from industrial wastewater by various low-cost methods has been widely investigated. In this paper, a type of bio-adsorbent was prepared using rice straw modified by fermentation and simple chemical treatment. The aim is to detect the adsorption mechanism and characteristics on Cr(III) ions. The analysis shows that the bio-adsorbent possesses four modified characteristics for Cr(III) adsorption. The first one is the acquired physical adsorption involving concave and convex structures. The second one is the effects of the hydrogen bonding surface hydroxyl groups and the metal chromium ion with complexation. The third one is mainly caused by hydrophilic active groups that possess carboxyl and hydroxyl groups during microbial degradation to combine with ions. The final one is the bio-adsorbent had high adsorption for low concentration of Cr(III) ions. The highest removal of around 97.45% was obtained at pH 5.0, bio-adsorption dosage of 0.5 g L−1, and initial Cr(III) concentration of 20 mg L−1. The adsorption process followed the pseudo second-order model (R2 > 0.99), while the isotherms were fitted to the Freundlich equation (68.1926 mg g−1), mainly by chemical adsorption. This study demonstrates the potential of using this biosorbent to remove Cr(III) from both synthetic and industrial wastewater.

INTRODUCTION

Nowadays, one of the key environmental problems facing humanity is the increasing worldwide contamination of water systems with heavy metals (Zhang et al. 2011). Chromium is used in various industrial applications such as tanneries, electroplating, textile dying, wood preservation, the petroleum industry and metal finishing, accompanied by terrible harm (Kanmani et al. 2012). Conventional methods to remove heavy metals are generally effective at metal concentrations greater than 100 mg L−1, so near-zero emissions are very important. Various techniques have been employed for removal of metals from wastewaters. These methods include adsorption (Serrano et al. 2011), flocculation (Cao et al. 2010), and oxidation (Subba Rao & Venkatarangaiah 2014). Compared with other techniques, adsorption is not only efficient but also economical for elimination of metals (Serrano et al. 2011). Among all adsorbents, activated carbon is the most popular material; however, its high cost restricts its large-scale use. Thus, due attention has been paid to search for effective, low-cost, and renewable adsorbents.

Recently, special attention has been focused on the use of natural materials that are available in large quantities, or certain waste products from industrial or agricultural operations. Materials such as spent grains (Ferraz et al. 2014), leaves (Gebrehawaria et al. 2014), fruit waste, and algal biomass (Pakshirajan et al. 2013) have been applied as adsorbents. In China, there are extensive plantations of rice in the southern area of the country. The annual production of rice in China is approximately 204 million tons. Rice straw (RS), an agriculture residue, represents 45% of the volume of the production of rice and in China this residue is still directly combusted in the fields, which is not only destruction of the environment, but also a waste of resources (Liguori et al. 2013). It is imperative to find effective ways to take advantage of the excess straw (Binod et al. 2010; Sindhu et al. 2012). One valuable way is to produce straw-based adsorbents as they are characterized by natural polymer materials, such as cellulose and lignin (Ai et al. 2013; Ang et al. 2013). However, the adsorption capacity of natural straw is insignificant; the uptake capacity of oat straw and sorghum straw studied for Cr(III) removal was only 12.97 and 6.96 mg g−1 (Pakshirajan et al. 2013), since straw materials are deficient in free ionic groups. Therefore, there is still a need to develop cheap and easy methods to improve straw adsorption capacity. The existing methods suffer, however, from one or more drawbacks, such as inefficient removal, high reagent or energy requirements, and high operation and maintenance costs. In recent years, some species of fungi, especially Trichoderma, have been widely studied to improve the adsorption performance of straw in biological renovation and modification (Gharieb et al. 2014; Panda & Sarkar 2015). Their advantages include the production of appropriate groups, like amine, amide, aldehyde group, hydroxyl and mercaptan, and the manufacture of porous structures. Therefore, the adsorption capacity of straw-based adsorbents could be improved by introducing some ionic functional groups through microbial fermentation (Solisio et al. 2013; Gautam et al. 2014; Lee et al. 2014).

Accordingly, the present study aimed to develop a suitable and low-cost method of bio-adsorbent production, which is beneficial for selective removal of Cr(III) ions via adsorption from industrial wastewater. In this paper, fermented and chemically treated RS was used for the preparation of bio-adsorbents, which were applied to adsorb Cr(III) ions. The corresponding structure and the fundamental adsorption behaviors, including the pH effect, adsorption isotherms, kinetics, and adsorption mechanisms, of the bio-adsorbent were investigated, respectively.

METHODS

Materials

Straw was obtained from fresh RS from Jiaxing's suburbs. Trichoderma viride strain 1285 was kept in our laboratory. Industrial effluent samples were obtained from electroplating enterprises in Jiaxing. The stock Cr(III) solution (1,000 mg L−1) was prepared by dissolving 5.125 g of chromium chloride (CrCl3 · 6H2O) (analytical reagent grade) in 1,000 mL of MilliQplus water. All the simulated wastewater solutions were prepared by diluting the stock solution in MilliQplus water. All chemical reagents used were of analytical reagent grade, purchased from Aladdin.

Methods

Preparation of bio-adsorbent

The natural RS was washed three times in water and dried at 60 °C. Soon afterwards the RS was triturated and the particles were sieved to obtain a fraction with particle size between 0.25 and 0.42 mm which was then used in this work.

Two hundred grams of triturated RS, 600 mL water, and 0.2 g NH4NO3 were mixed in a fermenter to culture a Trichoderma viride strain in this fermenter medium. The incubation temperature was maintained at a constant 28 °C by a constant temperature box for 28 days. The fermented rice straw was dried at 80 °C. The material obtained in this process is termed FRS (fermented rice straw).

FRS was immersed in 0.1 mol L−1 NaOH standard solution at 1:100, and heated in a water bath at 60 °C for 2 hours. Constant stirring in the process of heating is necessary. Alkali treatment at high temperature conditions was optimized to improve the hydrolysis of RS (Rocha et al. 2009). Samples were filtered by suction filtration and then the solution was collected in glass bottles. A volume of 0.1 mol L−1 HCl was added to the solution bottles until pH was 2.0, and then the bottles were kept for 24 hours. Finally, residues of acid treated solution were baked in an oven at 105 °C. The dry matter was used as bio-adsorbent (termed BSA hereafter).

Characterization methods of BSA

Powder X-ray diffraction (XRD) patterns were recorded operating at 30 kV and 20 mA on a DX-2600 diffractometer with Cu Kα radiation (λ = 1.5418 Å). The morphologies of products were examined with an acceleration voltage of 1.0 kV by Hitachi S-4800 electron microscope. Fourier transform infrared (FTIR) spectra of the samples were recorded in the range of 400–4,000 cm−1 using NEXUS470 FTIR spectroscopy. The samples were prepared in pellet form with spectroscopic-grade KBr. The energy dispersive spectroscopy (EDS) images of samples were obtained with an acceleration voltage of 20.0 kV by HORIBA EX-250. N2 physical adsorption was used to determine the Brunauer–Emmett–Teller (BET) specific surface area; the pore volume and pore size of the samples were calculated using the Barrett–Joyner–Halenda (BJH) method. The pH value of the solutions was measured with a Mettler toledo digital pH-meter, model PHS-3C.

Adsorption tests

The simulated wastewater solutions used in the study were prepared by appropriate dilution in 500 mL conical flasks containing 300 mL of the test solutions shaken (120 rpm) in an electronic thermostated shaker. To investigate the effect of initial solution pH on adsorption performance, experiments were carried out at Cr(III) concentration of 20 mg L−1 and BSA dosage of 0.5 g L−1 with different initial pH from 2.0 to 7.0 at 30 °C. When pH > 7, the system cannot be interpreted. For adsorbent dosage studies, 20 mg L−1 Cr(III) concentration was added to various concentrations of BSA solution for 24 hours adsorption, pH 5.0 at 30 °C. For reaction time studies, experiments were carried out at BSA dosage of 0.5 g L−1, pH 5.0, and 20 mg L−1 Cr(III) concentration, shaken for the required contact time at 30 °C. Experimental assays with industrial wastewater were carried out at BSA dosage of 0.5 g L−1, Cr(III) ion concentration of 20 mg L−1, pH 5.0, for 24 hours adsorption at 30 °C. The dilution ratio is determined by Cr(III).

Concentration of Cr(III) ions in the solution was measured by inductively coupled plasma emission spectrometry (ICP) analysis. The equipment used was a DIONEX ICS-2000 spectrometer. Three repetitions were prepared for each assay, and the average values are presented in the ‘Results and discussion’ section. 
formula
1
 
formula
2
where q is adsorption capacity (mg g−1); C0 is initial adsorbate concentration (mg L−1); Ct is adsorbate concentration at time t (mg L−1); V is the volume of solution (L); m is the quantity of the adsorbent (g).

Adsorption kinetics

Adsorbent solution (0.5 g L−1, pH 5.0) was added into 300 mL ionic liquid, with Cr(III) at 20 mg L−1 concentration. Concentration of metal ions in the solution was measured at indicated times by ICP analysis. Adsorption kinetics curves were drawn.

The first-order kinetics fitting equation is 
formula
3
The second-order dynamics equation is 
formula
4
where qe is the adsorption equilibrium; qt is the adsorbing capacity at time t; k1 and k2 are the coefficients of quasi-one and - two level, respectively; t is time.

Adsorption isotherm

Adsorbent solution (0.5 g L−1) was added into 300 mL ionic liquid, with Cr(III) at various concentrations: 5 mg L−1, 10 mg L−1, 20 mg L−1, 30 mg L−1, 40 mg L−1, 50 mg L−1, 60 mg L−1, 70 mg L−1, and 80 mg L−1, respectively. The pH value was adjusted to 5.0 at 15 °C and 30 °C, respectively. Adsorption isotherm curves were drawn.

The Langmuir adsorption isotherm is 
formula
5
The Freundlich adsorption isotherm is 
formula
6
where qe is adsorption capacity of adsorption equilibrium (mg g−1); b is the Langmuir constant, which is related to the affinity between metal and adsorbent; qmax is the maximum adsorption capacity (mg g−1); Ce is adsorbate concentration of adsorption equilibrium (mg L−1); kf represents the Freundlich constant, which is related to the biomass sorption uptake; and 1/n is an indication of the sorption strength.

RESULTS AND DISCUSSION

XRD analysis

As shown in Figure 1, both RS (line (a)) and FRS (line (b)) possess two peaks at 22 ° and 16 °, respectively. The main peak represents cellulose with a highly ordered crystalline structure, and the secondary peak represents polysaccharide with a low degree of order structure. The results suggest cellulose was degraded during the biological modification (Tiwari et al. 2013), which is beneficial for metal ions adsorption. The XRD curve showed FRS and BSA (Figure 1, lines (b) and (c)) possess a new peak at 26.6 ° which represents SiO2. The new peak may be due to microbial degradation of organic silicon compound into inorganic silicon oxide (Farooq et al. 2010), consistent with the results of infrared spectrum and energy spectrum analysis. Silicon is adsorbed by plants in the form of H4SiO4 and mainly accumulated as SiOn(OH)4-2n. Also, the peak of cellulose disappeared in BSA, indicating that the highly ordered crystalline structure was fully degraded during the modification. As shown in line (d) (Figure 1), Cr(III) may be adsorbed during adsorption reaction and form coordination compounds.

Figure 1

XRD analysis on RS (a), FRS (b), BSA (c) and BSA after Cr(III) adsorption (d).

Figure 1

XRD analysis on RS (a), FRS (b), BSA (c) and BSA after Cr(III) adsorption (d).

Microstructure and morphology of BSA

The scanning electron microscopy (SEM) results showed that irregular superficial layers of protective silica and natural resins are present in RS (Figures 2(a) and 2(b)). It can be observed that the surface of RS was destroyed during the biological fermentation process. It is possible to observe free fragments of FRS (Figure 2(d)), the FRS surface was porous, wax materials were destroyed, internal organizations such as active groups were exposed, and porosity increased (Figure 2(d)). The uneven surface of FRS with an enriched and developed honeycomb porous structure will provide sufficient adsorption space and sites for metal ions to be absorbed (Figures 2(c) and 2(d)). The appearance of perforations in the material indicates the leaching of structural substances that might have generated or exposed the active sites in FRS and BSA. BSA (Figures 2(e) and 2(f)) possesses a high internal surface area and exposed internal structure, which is much more conducive to combining with metal ions. The existence of the pore structure plays a key role in the stability of the metal binders.

Figure 2

Microstructure and morphology of biosorbent BSA, RS, and FRS: (a) and (b) the natural straw RS is waxed and tightly structured; (c) and (d) FRS is porous, wax is destroyed, and content organization exposed; and (e) and (f) the surface of BSA is porous and the surface area increased.

Figure 2

Microstructure and morphology of biosorbent BSA, RS, and FRS: (a) and (b) the natural straw RS is waxed and tightly structured; (c) and (d) FRS is porous, wax is destroyed, and content organization exposed; and (e) and (f) the surface of BSA is porous and the surface area increased.

BET analysis of BSA

Figure 3 shows the nitrogen adsorption–desorption isotherm and the BJH pore size distribution curve of the as-prepared BSA. The isotherm (Figure 3(a)) exhibits a type IV isotherm according to International Union of Pure and Applied Chemistry standard and indicates the existence of a pore size range from micropores to macropores, which is in good agreement with the pore size distribution curve and SEM characterization. The BET specific surface area of BSA is 5.014 m2 g−1 (Figure 3(a)); however, the BET specific surface area of FRS is 2.0392 m2 g−1 (Supporting information, Figure S1, available online at http://www.iwaponline.com/wst/072/237.pdf), and the values of RS were negligible. The calculation from the desorption branch of the nitrogen isotherm with the BJH method demonstrates that the average diameter of the pores is 29.02 nm (Figure 3(b)).

Figure 3

(a) N2 adsorption desorption isotherms and (b) pore size distribution curve of the as-prepared bio-adsorbent.

Figure 3

(a) N2 adsorption desorption isotherms and (b) pore size distribution curve of the as-prepared bio-adsorbent.

EDS analysis

The EDS images are shown in Figure 4. Elements of FRS (Figure 4(b)) were different from RS (Figure 4(a)) indicating the leaching of structural substances in FRS. EDS analysis of BSA after adsorption shows that the main element chromium exists in the materials (Figure 4(c)).

Figure 4

EDS results of RS (a), BSA (b), and BSA after Cr(III) adsorption (c).

Figure 4

EDS results of RS (a), BSA (b), and BSA after Cr(III) adsorption (c).

Analysis by FTIR spectroscopy

The FTIR spectra analysis showed that natural RS (Figure 5(a)) contains two characteristic peaks. The peak at 2,917.78 cm−1 represents –CH3 and –CH2 in carbohydrate, lignin and the aliphatic compounds. The stretching vibration peak at 1,100–1,120 cm−1 is for silicate minerals and silica with Si–O. The results indicate that RS contains more crystalline lignin and silicate deposition on the leaf epidermis than does BSA, because these substances were degraded in the fermentation process.

Figure 5

FTIR spectrogram of RS (a), BSA (b), and BSA after adsorption (c).

Figure 5

FTIR spectrogram of RS (a), BSA (b), and BSA after adsorption (c).

The FTIR spectra analysis of BSA (Figure 5(b)) was different to RS. BSA contains four characteristic peaks. The first one peaking at 2,400–2,200 cm−1 belongs to –NH4+ in protein, amino acid, and ammonium salt compounds, which were significantly enhanced. The multiple composite bands were formed during biological modification with the glycoprotein, mucopolysaccharide, and macromolecular compounds such as proteins and DNA. The second one peaking at 1,100–970 cm−1 belongs to polysaccharide –C–O stretching vibration absorption. The third stretching vibration absorption that peaked at 1,375–1,385 cm−1 belongs to aliphatic compounds with –CH3. The last one peaked at 3,500–3,300 cm−1 and belongs to –OH in –COOH, alcohol, and phenol (Binod et al. 2010). After adsorption (Figure 5(c)) the characteristic peaks of polysaccharide and protein changed. The results indicated that acyl amino, carboxyl, and hydroxyl groups are the key compositions that do significant work for Cr(III) adsorption performance improvement.

Effect of initial concentration of Cr(III) ions on adsorption performance

The adsorption performance of BSA for low concentration of Cr(III) ions is high. As shown in Figure 6(a), when the initial concentration of Cr(III) is below 30 mg L−1, the adsorption removal percentage was higher than 80%, and the remaining Cr(III) concentration is less than 5 mg L−1, which meets the requirements of the discharge standard.

Figure 6

Factor conditions of adsorbent: (a) the effect of initial concentration of Cr(III) ions on the adsorption performance; (b) the effect of reaction time on the BSA adsorption performance; (c) the adsorption removal percentage of BSA, RS, and FRS on Cr(III); and (d) the adsorption removal percentage on 20 mg L−1 Cr(III) at various pH values.

Figure 6

Factor conditions of adsorbent: (a) the effect of initial concentration of Cr(III) ions on the adsorption performance; (b) the effect of reaction time on the BSA adsorption performance; (c) the adsorption removal percentage of BSA, RS, and FRS on Cr(III); and (d) the adsorption removal percentage on 20 mg L−1 Cr(III) at various pH values.

Effect of reaction time on adsorption performance

As shown in Figure 6(b), the removal percentage of the adsorption reaction almost reached a peak after 6 hours' reaction. Therefore, 6 hours will be appropriate for Cr(III) ions to react with BSA.

Effect of adsorbent dosage on Cr(III) ions removal

In the Cr(III) ion system, BSA possesses strong adsorption. As shown in Figure 6(c), the removal percentage at BSA dosage 0.5 g L−1 is best. The removal percentage at this dosage of Cr(III) is 97.45%, nearly two times higher than FRS and four times higher than RS adsorbent. In conclusion, the adsorbent dosage and concentration of metal ions at the proper ratio will maintain the effective adsorption. At a constant pH and concentration of metal ions, adsorbent dosage is the decision factor for the adsorption reaction. As a result, the suitable dosage of BSA is 0.5 g L−1 and FRS is 0.6 g L−1.

Effect of pH value on Cr(III) ions adsorption

The removal percentage was more than 50% with a broad range of pH from 4.0 to 7.0 (Figure 6(d)). Meanwhile, the reaction condition is common, i.e., room temperature for 6 hours' reaction. The removal percentage was 97.45% at pH 5.0 and 92.09% at pH 7.0. According to Cr(III) speciation with pH, above pH 5.0 precipitation occurs; therefore, the best pH value would be 5.0.

Adsorption isotherm

The adsorption capacity of BSA increases in a certain concentration range. This is due to the BSA adsorption sites being gradually occupied by Cr(III). At 30 °C, the BSA adsorption was saturated at 30 mg L−1 of Cr(III), and the adsorbing capacity reached 68.1926 mg g−1. The results indicated that BSA is suitable for the adsorption of low concentrations of Cr(III) ions.

The Langmuir and Freundlich equations were used for fitting the adsorption isotherm data. The dynamic balance can be observed from Figure 7, and we can see the fitting effect by the Freundlich equation is good. The results demonstrate that the process of adsorption is multilayer adsorption, and the surface of BSA is non-uniform. Comparison of the adsorption isotherm was made based on the Freundlich equation; the kf and n values are increased when the temperature rises (Table 1). It suggests that temperature is more advantageous to the adsorption when temperature is within a certain range. Furthermore, the actual adsorption capacity (68.1926 mg g−1) almost reached that of the theoretical adsorption capacity (68.4932 mg g−1).

Table 1

Parameters at 15 °C and 30 °C by Langmuir equation and Freundlich equation

Freundlich
Langmuir
Temp.kfnR2Fitting equationqmax (mg g−1)bT (L mg−1)R2Fitting equation
30 °C 37.5435 5.8651 0.9986  68.4932 0.5681 0.9477  
15 °C 8.0145 1.8345 0.9910  85.4956 0.0500 0.9688  
Freundlich
Langmuir
Temp.kfnR2Fitting equationqmax (mg g−1)bT (L mg−1)R2Fitting equation
30 °C 37.5435 5.8651 0.9986  68.4932 0.5681 0.9477  
15 °C 8.0145 1.8345 0.9910  85.4956 0.0500 0.9688  
Figure 7

The adsorption isotherm of Cr(III) at 30 °C and 15 °C: (a) the adsorption isotherm; (b) the fitting diagram by Langmuir equation; (c) the fitting diagram by Freundlich equation.

Figure 7

The adsorption isotherm of Cr(III) at 30 °C and 15 °C: (a) the adsorption isotherm; (b) the fitting diagram by Langmuir equation; (c) the fitting diagram by Freundlich equation.

Adsorption kinetics

The kinetics of the adsorption process is mainly used to describe the speed of solute adsorbent adsorption. The underlying adsorption mechanism on the data fitted by two kinetic models is as follows.

As shown in Figure 8(a), the greatest adsorption process was the initial phase from 0 to 2 hours; the adsorption quantity still increased after 2 hours' reaction and reached a peak gradually and achieved equilibrium value after 6 hours. The presented results of a rapid initial step followed by a slow process indicate that BSA The k possesses the characteristics not only of typical biosorption processes but also physical adsorption. On the basis of the first-order kinetics, we fitted R2, the correlation coefficient, as 0.7923 (Figure 8, Table 2). This result is not consistent with the data calculated by the dynamic model, k1 = 0.2751 h–1, qe, cal1 = 3.7333 mg g−1.

Table 2

Fitting parameters by adsorption dynamics models

ValuePseudo first-order
Pseudo second-order
qexpk1 (h−1)qe (mg g−1)R2Fitting equationk2 (h−1)qe (mg g−1)R2Fitting equation
38.9452 0.2751 3.7333 0.7923  0.1344 39.3700 0.9995  
ValuePseudo first-order
Pseudo second-order
qexpk1 (h−1)qe (mg g−1)R2Fitting equationk2 (h−1)qe (mg g−1)R2Fitting equation
38.9452 0.2751 3.7333 0.7923  0.1344 39.3700 0.9995  
Figure 8

Adsorption kinetics: (a) the effect of adsorption time on the adsorption quantity; (b) fitting diagram by second-order kinetics equation; (c) fitting diagram by first-order kinetics equation.

Figure 8

Adsorption kinetics: (a) the effect of adsorption time on the adsorption quantity; (b) fitting diagram by second-order kinetics equation; (c) fitting diagram by first-order kinetics equation.

Theoretically, the experimental value of equilibrium absorption capacity (qe, exp) should equal calculated values (qe, cal). In fact, the data we calculated upon the secondary dynamic model (qe, cal2 = 39.3700) are very close to the experimental value (qe, exp = 38.9452) (R2 = 0.9995). Thus, the adsorption process of Cr(III) is in accordance with the secondary dynamics equation. It is also stated that external liquid membrane diffusion, surface adsorption, particle internal diffusion, etc. can more truly reflect the adsorption mechanism of metal ions on the adsorbent (Willis et al. 2014).

Analysis of the BSA adsorption mechanism

BSA is an attractive adsorbent for Cr(III) removal from simulated industrial effluent in a very rapid biosorption process and a slow process of physical adsorption. Both physical and biological agents play an important role in the adsorption processes. The adsorbent BSA, straw modified by microbial fermentation and alkali extraction and acid precipitation, not only possesses natural organic polymer reactive groups but also has the characteristics of the microbial biosorption. During the process of Cr(III) ion adsorption, the adsorbent plays the role of the organic matter of the bridge and the effect of biological and physical adsorption simultaneously (Ibrahim et al. 2010). Electronegatively larger groups, such as N, P, and O from amine, protein, and carbohydrates, can chelate with metal ions or complexation, especially those containing hydroxy and protein N ligand on metals, having a strong binding ability.

Physical adsorption

After microbial modification, the uneven surface has a great deal of microchannels, an enriched and developed honeycomb porous structure, and sufficient adsorption space. As well, increased and neat arrangement of porosity, darker gaps and exposed useful groups are all favorable factors. All of these will ultimately be beneficial for the adsorbent to adsorb more metal ions.

Hydrogen bonding

Results from FTIR indicate that adsorbent containing groups such as –COOH, hydroxyl, amino and other reactive groups can react with metal ions. Then, adsorbents with significant –OH and –COOH groups become negatively charged, forming a favorable condition to adsorb positively charged metal ions by electrostatic attraction.

Removal application for real samples of industrial effluent

The adsorption effect of BSA for Cr(III) is better than for other ions in the dilution of industrial effluent, which may be due to selective characteristics of BSA for different metal ions (Table 3). When the Cr(III) proportion was larger, the removal ratio of total Cr was higher. According to reports in the literature, a microbial adsorbent of Acinetobacter haemolyticus was capable of removing 79.87 mg g−1 Cr(III) from raw leather tanning wastewater (Yahya et al. 2012). BSA also possesses good performance.

Table 3

Quality parameters of wastewater and adsorption results by BSA

WastewaterpHMetal ionCinitial (mg L−1)Cfinal (mg L−1)Removal ratio (%)
4.12 Total Cr 41 4.33 89.44 
Cr(III) 26 0.55 97.88 
Zn(II) 17 1.69 90.06 
Cu(II) 12 0.78 93.50 
II 5.10 Total Cr 171 16.90 90.12 
Cr(III) 152 4.44 97.08 
Zn(II) 18 1.71 90.50 
Cu(II) 21 1.51 92.81 
III 4.62 Total Cr 121 6.00 95.04 
Cr(III) 116 2.45 97.89 
Zn(II) 14 1.30 90.71 
Cu(II) 29 1.84 93.65 
WastewaterpHMetal ionCinitial (mg L−1)Cfinal (mg L−1)Removal ratio (%)
4.12 Total Cr 41 4.33 89.44 
Cr(III) 26 0.55 97.88 
Zn(II) 17 1.69 90.06 
Cu(II) 12 0.78 93.50 
II 5.10 Total Cr 171 16.90 90.12 
Cr(III) 152 4.44 97.08 
Zn(II) 18 1.71 90.50 
Cu(II) 21 1.51 92.81 
III 4.62 Total Cr 121 6.00 95.04 
Cr(III) 116 2.45 97.89 
Zn(II) 14 1.30 90.71 
Cu(II) 29 1.84 93.65 

CONCLUSIONS

BSA can effectively adsorb Cr(III) at low concentrations from aqueous solutions. Adsorption capacities vary with pH and adsorbent dosage, being higher at pH 5.0 and adsorbent dosage of 0.5 g L−1, and the equilibrium absorption capacity was 38.9452 mg g−1, which was experimentally fixed at 30 °C for 20 mg L−1 Cr(III) adsorption. The removal percentage obtained was 97.45% for 20 mg L−1 Cr(III), using batch adsorption system.

Chemically modified lignocellulosic materials may have enhanced biosorption capacity, but the cost of chemicals and technologies used has to be taken into consideration in order to produce low-cost adsorbents. In that regard, the better performance of BSA may be assumed as an additional advantage when used to treat wastewaters contaminated with Cr(III). Furthermore, comparing BSA maximum uptake capacity (68.1926 mg g−1) for Cr(III) with other biosorbents, it is possible to consider this low-cost residual biosorbent as a promising biosorbent: qmax (carrot residues) = 45.09 mg g−1, qmax (orange wastes) = 74.83 mg g−1, qmax (sorghum straw) = 6.96 mg g−1, qmax (oats straw) = 12.97 mg g−1 (Ferraz et al. 2014).

Use of the biosorbent in columns for the removal of metallic ions demonstrated high efficiency for industrial effluents contaminated with Cu, Zn, and Cr. From a kinetics point of view the adsorption process was associated with the secondary dynamics equation. Physical factors, hydrogen bonding, and biological adsorption are three main underlying mechanisms for BSA to remove metal ions from wastewater.

ACKNOWLEDGEMENTS

This work was financially supported by the Science & Technology Project of Jiaxing City (no. 2014AY21007), the Program for Science and Technology of Zhejiang Province (no. 2015C37010), the Scientific Research Fund of Zhejiang Provincial Education Department (no. Y201330035) and the National Science Foundation of Zhejiang Province (no. LQ13B010003).

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Supplementary data