Plant refuses driven biochar: Application as metal adsorbent from acidic solutions

2.1 Biochars preparation and their characterization

Biomass samples from root of rose (Rosa damascena) & stem of eucalyptus (Eucalyptus citriodora) were taken for the preparation of biochar. The samples of two particle size, i.e.: 210 and 150 micron were taken for biochar preparation. The samples were pyrolyzed in a fixed bed furnace with a capacity 300 g ± 60 (depending upon bulk density), fitted with data logger for temperature, pressure and gas flow, variable heating rates with wide range, tube dia 50 mm, sample bed up to 300 mm, at 450 °C. The heating rate of furnace was 10 °C. Biochar samples were placed in a dried place for further analysis. In text, biochars were named according to plant material and particle size, i.e, rose & eucalyptus with particle size 210 micron were named as R₂₁₀ and E₂₁₀.

Physico-chemical properties of biochar were determined through proximate, ultimate and functional group analyses. Proximate analysis was done by ASTM method D3172. RSD (5) for replicates for moisture, volatile matter and ash content were 0.36%, 0.17% and 0.03%, respectively.

Carbon, hydrogen, and nitrogen contents of the biochars were determined using a CHN Elemental Analyzer (Euro-Vector) via high-temperature catalyzed combustion followed by infrared detection of resulting CO₂, H₂ and NO₂ gases. The pH of the biochar samples (Biochar: DI Water ratio 1:20) was measured before the experiment. BET-N₂ Surface area of biochar was measure by SA 3100 surface area analyzer.

2.2 Sorption of heavy metals

The sorption ability of R₂₁₀ and E₂₁₀ was initially evaluated using a mixed heavy metal solution containing Cd²⁺, Cr²⁺, Pb²⁺ and Cu²⁺. The pH of the solution was maintained at 4. About 0.5 g of the test biochar was taken into the column and the 10 ml of heavy metal solution passed through it at room temperature (30 ± 0.5 °C). After giving contact times, solution was removed. The Cd²⁺, Cr²⁺, Pb²⁺ and Cu²⁺concentrations were determined using ICP–AES. The sorbed heavy metal concentrations were calculated based on the difference between the initial and final metal concentrations. Deionised water without metals was treated as blank and processed in a similar manner as metal solution. Sorption isotherms and thermodynamic parameters were obtained with varying concentrations of each metal (10, 20, 30, 40, 50, 60 and 70 mg/ml) and reaction temperatures (303, 313, 323 and 333 K), respectively. Calculations of absorption capacity, kinetics and isotherm are shown in Supplementary materials.

2.3 Post-sorption characterizations

Scanning electron microscopy (SEM) (JEOL (Make), JSM-6100 (Model), USA) was used to examine surface morphology of both pre-sorption and post sorption (metal-laden or fully saturated biochar) biochar. Fourier transform infra-red (FTIR) (Perkin Elemr, Spectrum BX) analysis was used to characterize functional groups present on the biochar surfaces in both pre-sorption and post sorption.

2.4 Metal analysis

For the metal analysis, leahcate and biochar samples were digested in diacid (Nitric acid and perchloric acids mixture) and triacid mixture (nitric acid, sulfuric acid and perchloric acids) at 180 °C. Major inorganic elemental constituents of the biochars and metals of leachates were determined by inductively coupled plasma with atomic emission spectroscopy (ICP-AES, Perkin Elmer, and Optima 5300 V).

2.5 Breakthrough curves

0.5 g of the biochar mixed with small glass spheres in order to obtain suitable packing of the biochar was loaded on the column (details are given above section). The mixed metal solution (70 mg/ml each) was then passed through the column (flow rate 0.4 ml/min) and 10 ml of the portion of effluent was taken and analyzed by ICP.

2.6 Simulated acidic mine water

Mine rejects were collected from the coal mines of North eastern part of the India. Leaching of these mine rejects and overburden was done in zero hours with distilled water. The mine waters A (from mine reject) and B (from overburden) were characterized for pH, conductivity and metal under study.

3.1 Biochar characteristics

Elemental compositions of the bio chars are shown in Table 1. Nitrogen distribution in the bio chars varied in two plant species and was unaffected by particle size of parent material and residence pyrolysis time. It indicates the persistency of nitrogenous compounds throughout pyrolysis at this temperature. It is also reported that most of nitrogenous compounds released at higher temperature during thermal conversion of solid fuels. Biochars prepared from barks of eucalyptus have more nitrogen content as compared to that from rose root, attributable to the nitrogen-fixing capabilities of the Eucalyptus. In contrast to nitrogen, the distribution of hydrogen in the bio chars was similar across plant materials and particle size. However, oxygen distribution in biochars was unaffected by particle size and varied with the parent material. It is also evident by the O/C and H/C ratios (Table 1). It is due to the difference in lignin and cellulosic content of plant materials. The specific surface areas of biochar E₁₅₀ and R₁₅₀ were 34.99 and 38.99 (m²/g), respectively. The values are similar to the biochar prepared from wood (Keiluweit et al., 2010).

3.2 Optimization

The optimization of the process was done by changing metal concentrations and flow rate of the metal solution at a fixed biochar amount of 0.5 g. Experiments were set up for flow rates 0.5, 1.0, 1.5 and 2.0 ml/min. The optimized flow rate on R₁₅₀ biochar at a metal concentration of 60 mg/L is shown in Supplementary material. Hence, this low rate was taken for further experimentation. The Maximum adsorption was observed at 60 ppm at this optimized flow rate (Fig. 1). Hence, biochar prepared from plant refuses has maximum adsorption capacity of the individual metal at 60 ppm. After that, equilibrium was achieved due to saturation of active sites. It is evident from the figure that bio-sorption did not vary with particle size except for Pb, which showed no linearity in adsorption vs concentration for 210 micron particle size. Hence, R₁₅₀ and E₁₅₀ were taken for morphological and FTIR study and only R₁₅₀ was taken for the kinetic, isotherm and thermodynamic study.

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Figure 1

Effect of initial concentration of metal ions on two bio-char with different particle sizes.

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3.3 Effect of biochar morphology and functionality on metal adsorption

Effect of surface texture porosity of biochar on metal adsorption was done by morphological study using SEM (Fig. 2). The surface texture of biochars is characterized by thinner walls and channels with particle sizes of the order of 1−10 μm. These biochar have pores with varying sizes within the particles. Because of their size, the large pores are more easily accessible to metals. Porosity of the biochar is given in Table 1. However, SEM analysis after metal loading, shows different morphology for E₁₅₀ and R₁₅₀. These macropores are responsible for the metal saturation in R₁₅₀ (Fig. 2), where the metal laden surface of biochar is non porous. In case of E₁₅₀, metal laden biochar is showing a micropore on its surface. It might be possible for some other processes to dominate here.

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Figure 2

SEM analysis of bio-char under study; (a) and (b). before and after metal adsorption on E₁₅₀, respectively; (c) and (d) before and after metal adsorption on R_150,v.

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The FTIR study provides the change in functional groups of biochar before and after the metal adsorption (Fig. 3). The FTIR spectrum of native biochar revealed a large number of absorption peaks within the interval of 4000–400 cm⁻¹, which is only a sign of the complex chemical nature of this bio-material. Aromatic compounds such as PAHs showed characteristic absorption peaks at 873 and 781 cm⁻¹ (Khare and Baruah, 2010). The bands corresponding to OH and COOH were detected at 2278.79, 1607.84, 1432.02, 1230, and 1080 cm⁻¹. The absorption band at 3396.41 cm⁻¹ was due to O–H groups. The absorption peaks at 2924.76 and 2858.00 cm⁻¹ were attributable to alkyl radicals and other saturated aliphatic groups. The peaks at 2376 and 1319 cm⁻¹ were assigned to –PH stretch and NO₂ symmetric stretch, respectively. The absorption spectrum of biochar treated with metals showed evident changes with respect to that of native biochar (Fig. 3). Among these changes were the broadening of the absorption bands between 3396.41 and 1607 cm⁻¹, which suggests the superposition of numerous peaks that appeared in these regions. In addition, peaks appeared at 1645, 1417, and 1395 cm⁻¹ which have been previously reported as characteristic of the complexation of metal cations with carboxyl groups (Chen and Yang, 2006; Sawalha et al., 2007). The band at 1645 cm⁻¹ is associated with the complexation of the carbonyl from the carboxyl functional group, while the bands around 1400 cm⁻¹ are due to the complexation of the oxygen from the carboxyl C–O bond (Fourest and Volesky, 1996). With the loading of metals ions, the shifting of the peaks is seen from about 3396, 2974, 1607, 1432, 873 and 781 cm⁻¹. It is an indication of the active participation of –CO–, –OH, –SiOH and –C–OH group in metal binding (Srivastava et al., 2006).

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Figure 3

FTIR spectra of Biochar (R₁₅₀) before and after metal adsorption.

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3.4 Sorption kinetics

In order to describe the kinetic profiles bio sorption of metals at different temperatures, four different mathematical models were used: pseudo first order model, pseudo second-order, Ritchie’s second order and intra-particle diffusion (For detail calculation, please see the Supplementary materials). To quantitatively compare the applicability of each model, a normalized standard deviation (Δq) is calculated as reported by Kara and Demirbel, 2012.

Pseudo first order model, Ritchie’s second order and intra-particle diffusion were not able to describe the kinetics of metal adsorption effectively. Hence, results of these models are shown in Supplementary material.

Fig. 4 shows the pseudo second-order model for all metal adsorption. The parameters deduced from the pseudo first and second order kinetics are shown in Table 2. It is evident from the table that adsorption of metal ions can be approximated more appropriately by the Pseudo second order kinetics model than the first order kinetic model. It suggests that rate limiting step may be the chemical adsorption. This might be because of the tremendous surface area provided by small-sized biochar, containing plentiful hydroxyl and carboxyl groups at the surface to serve as functional groups to bond with the large amount of metals.

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Figure 4

Pseudo second order adsorption kinetics of adsorption of Cd, Cr and Cu.

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The values of the pseudo-second-order rate constant, k₂ were found to increase with an increase in the solution temperature of 303–333 K. There is a linear relationship between k₂ and temperature (Fig. 5). The adsorption rate constant is usually expressed as a function of solution temperature by the following Arrherius type relationship (Hoa and Ofomaja, 2006).

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Figure 5

Arrhenius plot of metal adsorption on biochar at different temperatures.

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It is reported that if the value of activation energy ranged between 8 and 16 kJ mol⁻¹, the adsorption process can be assumed to involve chemical sorption (Table 3). There are two types of chemisorptions activated (E_a > 7) and non activated (E_a = 0) (Kara and Demirbel, 2012). In the present case, the activation energies for all the metal ions are >7 kJ mol⁻¹ suggesting the activated chemisorption process. However, the activation energy for Pb is lower than that of other metal ions. It may be due to similar ionic radii of Pb, Ca and Na (1.80 Å), which ease the replacement of these ions. It is also interesting that activation energy for Cr is quite high that for other metals. It might be due to some other interactions that were involved in the Cr adsorption.

3.5 Adsorption isotherm

The adsorption models taken for the present investigation are the Langmuir and Freundlich. Detail calculation methods are reported by Kara et al., 2006 and Tekin et al., 2006; Tassist et al., 2010 and are given in Supplementary methods.

Table 3 shows the constants derived from the isotherm. The adsorption isotherms of metals by biochars are shown in Supplementary materials. Freundlich isotherm fits quite well with the experimental data correlation coefficients. However, the low correlation coefficients of the Langmuir isotherm show poor agreement to experimental data. The fact that the Freundlich isotherm fits the experimental data very well may be due to heterogeneous nature of the surface sites involved in the metal uptake. These results were also in agreement with the previous kinetic data, which suggested the chemisorption.

3.6 Thermodynamics

The Gibbs free energy of activation (ΔG), enthalpy of activation (ΔH), entropy of activation (ΔS), and activation energy (E_a) for the metal adsorption were calculated to assess the feasibility (Table 4). Detail calculations are shown in the Supplementary. The negative values for the Gibb’s free energy for all three metals represent the non-spontaneous nature of the adsorption process. The values of enthalpies were found to be positive, which indicates that the reaction proceeds with the absorption of heat (endothermic nature). The positive values of the entropy of activation in the system indicate that the reaction is thermodynamically favorable. This occurs as a result of redistribution of energy between the adsorbate and the adsorbent. Before adsorption occurs, the heavy metal ions near the surface of the adsorbent will be more ordered than in the subsequent adsorbed state and the ratio of free heavy metal ions to ions interacting with the adsorbent will be higher than in the adsorbed state. As a result, the distribution of rotational and translational energy among a small number of molecules will increase with increasing adsorption by producing a positive value of ΔS and randomness will increase at the solid–solution interface during the process of adsorption. Adsorption is thus likely to occur spontaneously at normal and high temperatures because ΔH > 0 and ΔS > 0 (Reddad et al., 2002).

3.7 Breakthrough curves

Breakthrough curves for Cd, Cr, Cu and Pb for biochar are shown in Fig. 6. It is evident from the curves that after elution of 70 ml of water, the metal adsorption reached maximum. All the metals except Pb showed the similar elution pattern. The breakthrough volume (VB) was 10 ml and calculated from backthrough curves, when the C_o/C_in was 0.05, (where, C_o is the adsorbed concentration of metal and C_in is the initial concentration of metal). Retention time was at 55 ml, when the C_o/C_in was 0.5. Equilibrium volume was 70 ml, when the C_o/C_in was 0.95. The calculated numbers of theoretic plates were 24.75 and capacity factor was 4.5. The formula of the calculation is given in Fig. 6. However, Pb, the breakthrough volume (VB) was lower (5 ml) and retention time was at 50 ml. According to data obtained by the isotherm and kinetic study the adsorption of metal on biochar surface follows chemisorption (Freundlich isotherm) and inter particle diffusion plays an insignificant role. Sheindorf et al. (1983) also reported that multi-solute system on charcoal follows the Freundlich isotherm for organic adrorbate. They have reported the different behaviors of various solutes. However, all metals under study followed similar adsorption pattern on biochar. It might be due to no diffusion effect, similar chelating effect of biochar and similarity in ionic charge of the metals. The backthrough test confirms that these metals have a similar type of interaction with biochar. The FTIR results reveal the possibility of ion exchange and chelation of metal on biochar surface. To confirm the mechanism, analyses of Na, Ca, Mg and pH of water released after the adsorption were done (Fig. 7). Ca ions showed the maximum release followed by Mg and Na. It confirms the ion-exchange phenomena. However, the total cation released was quite low as compared to the total metal loading on the column. It may be due to the release of hydrogen ion by the metal which was also evident in the FTIR spectra. It is interesting to note that at lower metal loadings, hydrogen ion concentration in released water was high, while reverse is true for other cations. It implies that initially metal ions replace H ions from C–O–OH and OH groups, and after saturation exchange of other cations occurs. The possible reaction of the metal exchange is given below

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Figure 6

Back through curves of metals on biochar.

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Figure 7

Release of Ca²⁺, Mg²⁺, Na⁺, H⁺ and total cation ions in water after metal adsorption on bio-char.

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3.8 Future prospects

To examine the use of biochar for natural acidic water system i.e. AMD, simulated AMD water was passed through the biochar (R₁₅₀) and leaching was performed. Two types of simulated acidic mine drainage water (AMD) were taken. These simulated AMD water was prepared by the column leaching of mine reject (A) and overburden (B). Results are shown in Table 5. Simulated water was highly acidic with high conductance. It had high concentration of Cr and Cu. While, Pb and Cd were also present in the simulated acidic water. After treatment, biochar not only removed these elements efficiently, but it also enhances the pH. The levels of the metals were below the permissible limits. We have observed in these experiments that biochar was also capable to remove other major ions like iron and sulfate (data is not included here). However, more detail investigation is needed for the simulated water which covers the interaction of other cations and anions and dissolved solid compounds.