Application of laboratory prepared and commercially available biochars to adsorption of cadmium, copper and zinc ions from water

Bioresource Technology 196 (2015) 540–549

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Application of laboratory prepared and commercially available biochars to adsorption of cadmium, copper and zinc ions from water Aleksandra Bogusz a, Patryk Oleszczuk a,⇑, Ryszard Dobrowolski b a b

Department of Environmental Chemistry, Faculty of Chemistry, Maria Curie-Sklodowska University, Poland Department of Analytical Chemistry and Instrumental Analysis, Faculty of Chemistry, Maria Curie-Sklodowska University, Poland

h i g h l i g h t s
Lab-prepared biochar was better than commercially available one.
Adsorption was correlated with O-containing functional groups on the biochar.
Efficiency of metal removal was depended on initial pH of solution.
Adsorption was affected by presence of chlorides and nitrates in the solution.

a r t i c l e

i n f o

Article history: Received 28 May 2015 Received in revised form 1 August 2015 Accepted 3 August 2015 Available online 7 August 2015 Keywords: Biochar Adsorption Desorption Heavy metals XPS

a b s t r a c t The goal of the presented work was the evaluation and comparison of two biochars (produced from Sida hermaphrodita – BCSH/laboratory produced and from wheatstraw – BCS/commercial available) to adsorb heavy metal ions (Cd(II), Cu(II) and Zn(II)) from water. Kinetics of the sorption as well as sorption isotherms, the influence of solution pH and interfering ions were investigated. Different physico-chemical properties of biochars had the great influence on adsorption capacity. The greater adsorption efficiency was observed for BCSH than for BCS in the case of all investigated metals. The adsorption efficiency of BCSH was correlated with higher content of carbon and oxygen, what is equal with higher content of polar-groups on the BCSH surface e.g., –COOH. Furthermore, the molar ratio of O/C as well as polarity index (which was higher for BCSH) was also important parameters. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Increasing industrialization has exerted substantial pressure on the environment since the last century. The global emission of heavy metals, especially of Cd, Cu, and Zn, has become a serious problem because of the toxicity of these metals toward living organisms. In contrast to organic contaminants, the abovementioned heavy metals are nonbiodegradable and persist for a long time in ecosystems, especially in soils. Moreover, these substances tend to be accumulated by a variety of organisms, animals, and plants, and they are also toxic or carcinogenic when their concentrations exceed certain tolerance levels. The wide usage of heavy metals such as Cd, Cu, and Zn in various industries, e.g., in mining operations, fertilizer production, battery manufacture, and tanneries, is responsible for their direct or indirect discharge as ⇑ Corresponding author at: Department of Environmental Chemistry, University of Maria Skłodowska-Curie, pl. M. Curie-Skłodowskiej 3, 20-031 Lublin, Poland. Tel.: +48 81 5375515; fax: +48 81 5375565. E-mail address: [email protected] (P. Oleszczuk). http://dx.doi.org/10.1016/j.biortech.2015.08.006 0960-8524/Ó 2015 Elsevier Ltd. All rights reserved.

wastewaters into the environment. This industrial discharge is the major cause of groundwater and soil contamination. Because of the negative influences of these contaminants, their removal from wastewaters and soil has become a crucial issue, and a number of purifications methods have been applied. Conventional methods for the removal of heavy metal ions from wastewater or contaminated soil are based on ion exchange processes, chemical precipitation, separation through membranes, electrochemical techniques, and adsorption (Fu and Wang, 2011; Mohan et al., 2014). However, many of those methods are expensive because they require specialized reagents and apparatus, and they may also coproduce a large quantity of waste. Taking into account the above drawbacks, the removal of heavy metal ions from large volumes of polluted water or soil should be achieved using the immobilization route via adsorption onto low-cost materials (Mohan et al., 2014). Currently, a large variety of carbonaceous materials are used for the removal of heavy metal contaminants. Properties such as high surface area and large micropore and mesopore volumes make activated carbon (AC)

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the most commonly used material in adsorption processes. Nowadays, however, the costs of coal-based AC are increasing because of the depletion of commercial sources. Recently, because of the wide variety of low-cost sorbents and the potential financial rewards, adsorption has become the most frequently used method for the separation of metal ions from aqueous solutions. The most popular sorbents used are carbon-based materials and their complexes (Barczak et al., 2015; Gupta et al., 2015). These materials have great advantages such as a large sorption capacity, a developed internal pore structure, a large specific surface area, the presence of wide varieties of functional groups on the surface, and readily achieved modification. Therefore, researchers are searching for new low-cost adsorbents for the removal of heavy metal ions from wastewater. Researchers have recently focused on biochar (BC) as a novel material for the removal of heavy metals (Cao et al., 2009; Frišták et al., 2015; Mohan et al., 2014; Qiu et al., 2008). Natural materials such as plant waste are widely available in large quantities and they also have great potential as low-cost and, most importantly, environmentally friendly adsorbents. The production of biomass-derived materials also has several positive effects on environmental conditions and climate protection (Roberts et al., 2010). Nowadays, substantial attention is being paid to the application of BCs not only from the agro-environmental perspective but also in the sorption processes to remove inorganic and organic contaminants (Ahmad et al., 2014; Mohan et al., 2014). Biomassderived adsorbents have properties similar to those of AC; e.g., an extensive surface area, a high degree of porosity, and a surface carbon matrix. This similarity suggests that AC could be replaced with BCs. In BC-based materials, the sorption of metal ions depends on the material’s content of functional groups containing oxygen, including phenolic, carboxyl, and hydroxyl groups. These materials are used as low-cost sorbents of heavy metal ions and organic pollutants (Cao et al., 2009; Chen et al., 2011; Kołodyn´ska et al., 2012; Park et al., 2013; Wang et al., 2015; Xu et al., 2014). However, before the application of BCs as a soil amendment to stabilize and reduce the potential toxicity of heavy metal ions, these materials should be tested in model aqueous solutions. Therefore, the present work assesses the efficiency of two biomass-derived materials, which were obtained from Triticum straw and Sida hermaphrodita under different conditions, as sorbents for the removal of Cd(II), Cu(II), and Zn(II) ions from wastewater. Industrially and lab-produced BCs were compared. The influence of BC properties on the adsorption abilities was studied in detail, including the degree of carbonization, polarity, and presence of polar groups on the surface. Because the specific surface areas of BCs are nearly uniform, SBET was not considered. The studied BCs, which do not differ significantly in SBET but do exhibit different oxygen contents on their surfaces, were chosen based on the notion that the adsorption capacity for heavy metals is dependent on the amount of oxygen present on the surface and not on the surface area itself. During the study, batch sorption experiments were conducted to investigate the influence of operating factors such as solution pH, contact time, initial concentration of studied heavy metal ions, and the presence of interfering ions in the solution on the adsorption capacity. XPS was used to identify the forms of metal ions bonded with the BC surface and to elucidate the mechanism of the studied process. The reusability of biomass-derived materials was also investigated. The mechanisms of heavy metal removal with biochar might be attributed to electrostatic interaction, ionic exchange, chemical precipitation, and complexion with functional groups on BC surface according to recently published review papers (Tang et al., 2013; Zhang et al., 2013; Ahmad et al., 2014; Mohan et al.,

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2014). To date, only a few publications have focused on the aspects of adsorption other than kinetics, isotherms, and the influence of pH. Because of the possible future applications of the examined BCs in environmental samples, the current study also addressed other parameters such as the influence of interfering ions, in the form of nitrates and chlorides, which are characterized by high abundance in environmental samples, on the adsorption ability of BCs. Additionally, the present work presents a novel desorption study. BCs are known as low-cost adsorbents with regard to their production, but the possibility of using BCs for repeated adsorption steps should be also determined. 2. Methods 2.1. Adsorbents The adsorption properties of two different biochars were studied, industrially (BCS) and lab-prepared (BCSH). The BCS produced from wheat straw was obtained from Mostostal Sp. z.o.o. (Wrocław, Poland). The limited-oxygen conditions (1–2%) and the temperature of pyrolysis range 650–700 °C (maximal combustion) were applied. The second biomass-derived material – BCSH was prepared from S. hermaphrodita in the furnace of ownconstruction at the temperature of 700 °C. The desirable temperature was kept constant for 4 h. Anaerobic conditions were obtained by the constant flow of nitrogen of 630 mL N2/min controlled by the gas flow regulator (BETA-ERG, Poland). The standard methods were used for studying the physicochemical properties of biochars. The pH of each material was measured potentiometrically using 1 mol/L potassium chloride after 24 h in the liquid/solid ratio of 10. The TOC-VCSH (SHIMADZU) with Solid Sample Moduls (SSM-5000) was applied for the total organic carbon content (TOC) determination. The total nitrogen (Nt) was measured by using the Kjeldahl’s procedure without the application of Dewarda’s alloy (Cu–Al–Zn alloy-reducer of nitrates and nitrites). FT-IR/PAS spectrum of the biochar sample was recorded by means of the Bio-Rad Excalibur 3000 MX spectrometer equipped with photoacoustic detector MTEC300 (in the helium atmosphere in a detector) at RT over the 4000–400 cm1 range at the resolution of 4 cm1 and maximum source aperture. The spectrum was normalized by computing the ratio of a sample spectrum to the spectrum of a MTEC carbon black standard. A stainless steel cup (diameter 10 mm) was filled with biochar sample (thickness < 6 mm). Interferograms of 1024 scans were averaged for the spectrum. The low-temperature (77.4 K) nitrogen adsorption–desorption isotherms were used for identify the structure of studied biochars. The data were obtained with Micromeritics ASAP 2405 N adsorption analyzer. In the base of the standard BET method the specific surface areas SBET were calculated. The carbon, hydrogen and nitrogen content was determined with the CHN Elemental Analyzer (Carlo-Erba NA1500) via high-temperature catalyzed combustion follow by infrared detection of resulting CO2, H2 and NO2. The X-ray photoelectron spectroscopy was used for surface elemental composition characterization. 2.2. Chemicals The initial standard stock solutions of Cd(II), Cu(II) and Zn(II) ions (each of 1000 mg/L) were prepared by dissolution of respectively Cd(NO3)24H2O, Cu(NO3)23H2O and Zn(NO3)26H2O powder (POCH, Gliwice, Poland) in a redistilled water. The calibration curves of determined ions were established using the standard solutions of Cd(II), Cu(II) and Zn(II) prepared in 0.5 mol/L HNO3 by dilution from stock solution each of 1000 mg/L (Merck,

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Darmstadt, Germany). Moreover, hydrochloric acid Suprapure (36%) (POCH, Gliwice, Poland), nitric acid Suprapure (65%) (POCH, Gliwice, Poland), potassium nitrate (POCH, Gliwice, Poland) and sodium chloride (POCH, Gliwice, Poland), were used. Furthermore, hydrochloric acid and sodium hydroxide solutions were used for pH adjustment of solutions. 2.3. Kinetic study In the adsorption study of Cd(II), Cu(II) and Zn(II) ions onto BCS and BCSH materials a batch equilibration technique was used. Firstly, the effect of time on metal ions adsorption onto biochars was investigated vs. time intervals up to 23, 32 and 29 h for Cd (II), Cu(II) and Zn(II) ions, respectively. The initial concentration of each ions in the single element solutions were prepared to be at the level of 100 mg/L, and the studied adsorbents masses were around 0.2 g ± 0.03 g. The optimal pH in case of each metal ions were adjusted to 6; 5.5 and 7 for Cd(II), Cu(II), Zn(II) ions respectively. Kinetics solutions were agitated on a shaker at room temperature 22 ± 2 °C at 120 rpm constant speed. Mixtures were filtered with 0.45 lm PTFE hydrophobic syringe filters (AlfaChem, Poland), then the filtrates were used for analyzing the heavy metals concentration. The last step was the same for each adsorption steps. 2.4. Adsorption isotherms Secondly, the adsorption capacities of Cd(II), Cu(II) and Zn(II) ions on BCS and BCSH were obtained according to the initial runs of adsorption isotherms of each ions. The adsorption isotherms were carried out at initial pH of 6; 5.5 and 7 for Cd(II), Cu(II), Zn(II) ions, respectively. During experiments, the heavy metal ions concentrations from 5 to 500 mg/L range was used. The single adsorption system were consist of 50 mL of solution and 0.2 g ± 0.03 g of biochar. Solutions were agitated on a shaker with temperature of 22 ± 2 °C and the equilibrium time of 24 h.

2.7. Desorption Desorption study in respect to HCl and HNO3 concentration was investigated using biochar materials with Cd(II), Cu(II), Zn(II) ions adsorbed on the surface. As a desorptive agents hydrochloric acid and nitric acid were used with increasing concentration, from 0 M to 9.5 M and 14.5 M ranges, respectively for HCl and HNO3. 2 mL of acidic solution was mixed with 0.008 g of biomassderived materials with metal ions adsorbed on their surface and agitated in shaker at room temperature of 22 ± 2 °C for 24 h. 2.8. Determination of heavy metals Heavy metal ions concentrations in the studied adsorption system were analyzed using the flame atomic absorption spectrometer VARIAN Spectra AA-880 (Carl Zeiss, Jena, Germany). For the measurement of Cd(II) < Cu(II) < Zn(II) ions equilibrium concentration the hollow cathode lamp (Varian) was used. The basic parameters of heavy metal ions determination are assembled in Table S1. The adsorption value in the equilibrium state on studied BCs was calculated using following equation:

a¼

ðci  cÞ  V m

ð1Þ

where ci is the initial concentration of heavy metal ions in the solution (mg/L), c is the equilibrium concentration of heavy metal ions in the solution (mg/L), V is the volume of the initial solution (L) and m is the mass of the biochar (g). 2.9. Data analysis The kinetics data were analyzed using Lagergren pseudo-firstorder equation and pseudo-second-order equation. For modeling the rate the following equations were used:

lnðaeq  at Þ ¼ ln aeq  k1 t

ð2Þ

t 1 1 ¼ þ t at k2 a2eq aet

ð3Þ

2.5. Optimization of pH Next, the impact of pH of solution on Cd(II), Cu(II) and Zn(II) ions adsorption onto examined biochars was carried out. In this step, the initial pH value of the solutions was adjusted to 1.5–7; 2–8 and 1–7.5 range in case of Cd(II), Cu(II), Zn(II) keeping the heavy metal ions concentration (100 mg/L for Cd(II) and 150 mg/L for Cu(II) and Zn(II)) and adsorbent mass at constant value (0.2 g ± 0.03 g). Solutions were agitated on a shaker with temperature of 22 ± 2 °C and the equilibrium time of 24 h. After reaching the equilibrium state by studied systems the pH was measured to detect the changes in the solutions. On the basis of obtained results the relation between initial and equilibrium pH was evaluated. 2.6. The effect of interfering ions The effect of interfering ions, nitrates and chlorides, on adsorption of studied heavy metal ions was performed with using series of solutions keeping the same concentration of cadmium, cooper and zinc ions (100 mg/L for Cd(II) and 150 mg/L for Cu(II) and Zn(II)), pH values (6; 5.5 and 7 for Cd(II), Cu(II), Zn(II)), mass of adsorbent (0.2 g ± 0.03 g) and increasing Cl and NO 3 concentration for the initial adsorption system (from 0.001 M to 1 M for KNO3 and 2 M for NaCl). Solutions were agitated on a shaker with temperature of 22 ± 2 °C and the equilibrium time of 24 h. The percentage results were calculated according to a comparison of adsorption of each ion obtained for solutions with adjusted optimal pH and without addition of the interfering anions.

where aeq is the amount adsorbed (mg/g) at equilibrium time (mg/g), at is the amount adsorbed (mg/g) at any time t, t is contact time (min), k1 (1/min) and k2 (g/mgmin) are the rate constants of the pseudo-first-order equation and pseudo-second-order equation, respectively. Two widely applied models, Freundlich and Langmuir models, were used to fit the sorption isotherms in OriginPro 8. The Langmuir model describes a monolayer adsorption onto a porous surface with finite number of homogenous centers without an interaction between sorbed ions. The linear form of this equation can be written:

C eq 1 C eq þ ¼ K L am am a

ð4Þ

where a is the heavy metal ions amount adsorbed onto biochars surface (mg/g) at the equilibrium concentration of examined ions Ceq (mg/L), am is known as maximum adsorbed amount required to form a monolayer on the material surface (sorption capacity), KL is the Langmuir constant related to sorption energy. The KL and Ceq values characterized the Langmuir adsorption model. The Freundlich adsorption model based on adsorption onto a monolayer heterogeneous surface with an interaction between adsorbed ions. The linear form is given by following equation:

ln a ¼ lnðam KÞ þ n ln C eq

ð5Þ

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where the Freundlich constant n (0 6 n 6 1) indicates the intensity of adsorption and characterizes the quasi Gaussian energetic heterogeneity of the adsorption system. 3. Results and discussion 3.1. Biochar properties The physicochemical properties of the studied BCs are listed in Table 1. Both BCs are alkaline materials and are characterized by very similar SBET values. Considerably higher pH values were noted for BCSH than BCS BC. BCSH also has a higher carbon concentration than BCS BC. A higher ash content was observed for BCS than for BCSH. This difference can be explained by higher concentrations of nutrient elements in the feedstock of BCS (Uchimiya et al., 2011). Furthermore, the higher PV value of BCS is related to its less microporous structure compared with that of BCSH. The molar H/C ratio can be used to describe the degree of carbonization or aromaticity (Xu et al., 2014). The lower H/C ratio of BCS is related to its higher carbonization level and aromatic structure. C/N provides information regarding the degradation process of BCs after further application to soil. According to the obtained results, the lower C/N ratio of BCS suggests that this material contains less labile (O-alkyl) components than BCSH. Thus, BCSH applied to the soil should be favorably degraded (Qiu and Ling, 2006). In comparison, considering the opposite trend of the O/C and (O + N)/C ratios, BCSH is likely to present a relatively hydrophilic and polar structure. The structures of BCS and BCSH were analyzed with FT-IR/PAS, and the obtained spectra are presented in Fig. S1 (electronic annex). The spectra of the two examined BCs include several bands in the 4000–2500 cm1 range (Fig. S1A), which are mostly related to the presence of –OH and –C–H groups on the surface. However, within this region, BCSH exhibits additional peaks. The broad bands centered at approximately 3515 cm1 and 3450 cm1 may be attributed to the stretching vibration of hydrogen-bonded hydroxyl groups of water or to phenolic C–OH stretching (Socrates, 2004). Additionally, the low intensity bands at 2898 cm1 in the BCSH spectrum may be attributed to aliphatic C–H stretching. In the FT-IR/PAS spectra of BCSH and BCS samples in the 2000– 600 cm1 range (Fig. 1B), the bands are mostly associated with the presence of carbonyl and carboxyl groups (C@O in COO groups, which are present in BC at high pH values), indicating the presence of oxygen-containing functional groups on the surface. In this region, the BCS spectra exhibit more peaks. The bands centered at approximately 1750, 1685, and 1630 cm1 are characteristic of C@O vibrations. The bands at 1400, 1211, 1157, 1117, 1083, and 1055 cm1 may indicate the presence of inorganic compounds such as carbonates, sulfates, phosphates, and silica compounds (Nyquist et al., 1997). 3.2. Sorption kinetics The adsorption of the investigated metals was studied as a function of time, and the results are presented in Fig. 1. The percentages

of Cd(II), Cu(II), and Zn(II) adsorption increased with time. The adsorption process of metal ions on the investigated BCs was biphasic, which is in accordance with other research (Thirumavalavan et al., 2011). The first rapid step is attributed to the surface adsorption of ions on the adsorbent surface. The following slow step is related to the distribution of the adsorbed ions from the outer to inner surfaces of the adsorbent (Thirumavalavan et al., 2011). The time required to reach the equilibrium state was different for the examined BCs. The sorption kinetics of Cd(II), Cu(II), and Zn(II) ions was more rapid on BCS than on BCSH. This difference can be explained by the surface properties of BCs. Sorption kinetics strongly depends on the physical or/and chemical characteristic of materials, which also influence the adsorption mechanism. The elemental composition of BCSH suggests that the contents of carbon and oxygen are higher than those in BCS, and therefore, the surface of BCSH exhibits a greater amount of polar groups responsible for binding the heavy metal ions. On the BCS surface, the content of hydroxyl groups is higher than on the BCSH surface, which mostly contains carboxyl groups. Adsorption proceeds faster on BCS because hydroxyl groups are characterized by lower binding energy than carboxyl groups (Bansal and Goyal, 2005), and this allows metal ions to bind more readily to hydroxyl groups. Another reason could be the availability of surface groups. The higher content of carboxyl groups on the BCSH surface, its greater microporous structure (resulting in a lower pore volume in BCSH), and the larger size of carboxyl groups compared with hydroxyl groups hinders the adsorption of metal ions on BCSH. For these reasons, the equilibrium state is reached more rapidly with BCS than with BCSH. The time required to reach the equilibrium state was also different for the studied heavy metals (Fig. 1). The adsorption process of Cu(II) ions was the most rapid of all the ions studied for both BCs. For Cu(II), 3 and 4 h were sufficient to reach the equilibrium state using BCS and BCSH, respectively. Accordingly, Fig. 1 shows that the adsorption kinetics of Cd(II) is the slowest of the metals studied. To summarize, to reach the equilibrium state, contact times with the BCS and BCSH surfaces of 4 and 6 h were required for Cd(II), 3 and 4 h were required for Cu(II), and 3.5 and 5 h were required for Zn(II) ions. The differences between the investigated ions may be explained by differences in the solubility of M(OH)2 and by the ionic radii of these ions (Table S2). Cu(II) ions reached the equilibrium state most rapidly because they exhibit the smallest affinity for the solvent and are less reactive with hydroxide. Cd(II) has a greater affinity for the solvent and a smaller affinity for the surface of BCs (Guzel et al., 2008). Thus, the adsorption process was longer for Cd(II) ions than for the other investigated ions. The ionic radii of the examined metals, which varied in the following order Cu(II) < Zn(II) < Cd(II), may have also contributed to the different adsorption behaviors. It was observed (Guzel et al., 2008) that when the ionic radius decreases, the adsorption increases; this trend was confirmed by the results presented here. However, Cu ions react readily with functional groups containing elements with unshared pairs of electrons, such as S, N, O, and P, and have great affinity to carboxyl groups (Chen et al., 2011). It

Table 1 Physicochemical properties of BCS and BCSH materials. Biochar

BCS BCSH

pH

9.9 12.51

Elemental composition C

H

N

O

54 72

1.8 1.3

0.9 0.95

2.3 15.5

Ash

H/C

(O + N)/C

O/C

SBET

PV

41.1 10.1

0.033 0.22

0.06 0.17

0.043 0.16

26.3 27.1

0.026 0.011

pH in KCl, CHNO – the contribution (%) of carbon, hydrogen, nitrogen and oxygen, Ash – ash content (%), H/C – ratio of hydrogen to carbon, (O + N)/C – polarity index, O/C – ratio of oxygen to carbon, SBET – specific surface area (m2/g), PV – pore volume (cm3/g).

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Fig. 1. Adsorption kinetics of Cd(II) (1), Cu(II) (2) and Zn(II) (3) ions onto BCS and BCSH, m = 0.2 g ± 0.03 g, V = 50 mL, Cions = 100 mg/L, pH: Cd 6, Cu 5.5 and Zn 7, T = 22 ± 2 °C.

Fig. 2. Initial runs of adsorption isotherms of Cd(II) (1), Cu(II) (2) and Zn(II) (3) ions onto BCS and BCSH, m = 0.2 g ± 0.03 g, V = 50 mL, pH: Cd 6, Cu 5.5 and Zn 7, T = 22 ± 2 °C, t = 24 h.

can be suggested that the pair of electrons present on the oxygen atoms of BC surface groups was donated to the oxygen-metal bond. The values of k2 for each adsorption system revealed that the adsorption process occurred in the following order (from rapid to slow): Cu(II) > Zn(II) > Cd(II). The highest k2 value was obtained for Cu(II) as an adsorbate and BCS as an adsorbent. The kinetic fitting plots constructed with these equations were used to elucidate the mechanisms underlying the adsorption process (Fig. S2). Table 2 presents the kinetic parameters obtained for the studied adsorption systems. Only the pseudo-second-order model provided the best fitting for all of the experimental data. The plots show regression coefficients higher than 0.997 for all ions and BCs (Table 2). It was suggested (Kołodyn´ska et al., 2012) that the kinetics of metal ions following the pseudo-second-order model is controlled by a chemisorption process. In this case, it can be observed that the sorption of heavy metal ions involved precipitation and inner-sphere complexation without electrostatic interaction, which, in this case, was used to achieve ion exchange (Kołodyn´ska et al., 2012). Taking into account the previous studies investigating the adsorption of heavy metal ions, the obtained results suggested that the second-order sorption kinetics are in good agreement with data reported by other authors (Dubey et al., 2014; Park et al., 2013; Pyrzynska, 2010). According to the available data regarding the adsorption of Cu(II) and Zn(II), the previously obtained k2

Table 2 Parameters for kinetics simulated by different equations. Metal

Biochar

Kinetic model Pseudo-first order

ak Pseudo-second order

k1

R2

k2

R2

aeq

Cd

BCS BCSH

0.0024 0.0044

0.1637 0.7217

0.044 0.040

1.000 1.000

17.92 20.61

17.92 22.89

Cu

BCS BCSH

0.0035 0.0019

0.7994 0.9773

0.056 0.045

1.000 1.000

25.79 20.81

19.48 20.98

Zn

BCS BCSH

0.0018 0.0031

0.6789 0.8865

0.045 0.043

1.000 0.9997

22.09 24.87

22.09 24.97

at – the amount adsorbed (mg/g) at time t, aeq – the amount adsorbed at equilibrium time (mg/g), k1 (1/min) and k2 (g/mgmin) – the rate constants of the pseudo-firstorder equation and pseudo-second-order equation, respectively, R – regression coefficient, ak – maximal adsorption capacity achieved in the kinetic study (mg/g).

values were lower than those found in the current study. The adsorption of metal ions on BC produced from corn straw at 600 °C was characterized by k2 values of approximately 0.003 for Cu(II) and 0.006 for Zn(II). In the case of BC obtained via hardwood pyrolysis at 450 °C, the adsorption of the studied metal ions was described by k2 values of 0.011 and 0.009 for Cu(II) and Zn(II), respectively. These lower values correlate with longer durations

A. Bogusz et al. / Bioresource Technology 196 (2015) 540–549

needed to reach the equilibrium state in the systems studied by Chen et al., and these durations were as long as 23 h (Chen et al., 2011). Melo et al. reported that k2 of the adsorption of Cd(II) onto BC prepared from Manicaria saccifera was 0.09. This higher value relative to that listed in Table 2 is correlated with a more rapid adsorption process requiring approximately 15 min. The differences between k2 values are primarily directly related to the rate of the adsorption process, which mostly depends on the physicochemical properties of the adsorbent. Many previous studies have reported that the time needed to reach the equilibrium state was 24 h or less (Chun et al., 2004; Pyrzynska, 2010; Weber and Chakravorti, 1974). In addition to the equilibrium times discussed above, Frišták et al. (2015) reported the adsorption kinetics of Cd(II), Cu(II), and Zn(II) over a 24-h period on two different wood-derived BCs. Furthermore, Kołodyn´ska et al. (2012) studied the adsorption of these metal ions and found that the equilibrium state on BCs obtained from pig and cow manure was achieved after 30–60 min by Cu(II) and Zn(II) and after 120–180 min by Cd(II). This agreement can also be seen when the experimentally determined equilibrium adsorbed amount (ak) and that calculated according to Eq. (3) (aeq) are compared. There is no significant difference between these values for any of the studied metal ions. In conclusion, sorption capacities ak were consistent with the aeq values, which confirms that the adsorption processes proceeded under optimal pH conditions and reached undisturbed equilibrium states. 3.3. Sorption isotherms According to the experimental data, BCSH had a higher adsorption capacity for the studied heavy metal ions than with BCS (Fig. 2). The adsorption capacities determined experimentally on the basis the maximal experimental amount adsorbed at equilibrium time for BCS and BCSH were 30.88 and 34.77 mg/g for Cd(II) ions, 25.51 and 31.73 mg/g for Cu(II), and 40.18 and 45.62 mg/g for Zn(II), respectively. A possible reason for the different capacities could be related to the normal potentials of the studied metals. When the value of the normal potential is more negative (Table S2), the process of removing a hydrogen from a surface group such as a carboxyl group will be more effective (Guzel et al., 2008). Zn(II) ions were characterized by the most negative normal potential among the ions studied, and therefore, Zn(II) had the greatest ability to remove hydrogen and bind with the dissociated surface groups. In contrast, the normal potentials of Cd(II) and Cu(II) are closer to 0, and thus, their ability to bind with carboxyl groups is weaker and the observed adsorption capacity is lower. The differences in the sorption capacities for the investigated metals of different BCs can be explained on the basis of their surface properties. As we can see from FT-IR analysis, BCSH contains a larger amount of surface polar groups, which are involved in the adsorption process and the binding of heavy metals, compared with BCS. Thus, BCSH is characterized by a higher polarity than BCS. The adsorption capacities of the tested heavy metals and BCs obtained in the present study are higher than or very similar to the results reported by other authors (Table S3). The differences between the results are probably correlated with the different structures and chemical compositions of the tested materials. Two nonlinear isotherm models (Freundlich and Langmuir) were tested to fit the experimental data. The Langmuir model (LM) was found to give a better fit than the Freundlich model (Fig. S3). The LM had the highest coefficient of determination (R2) for both BCS and BCSH (Table 3). The obtained results are in accordance with previous reports, indicating that the LM resulted in better fitting of the experimental data of heavy metal adsorption by BCs than the Freundlich model (Dubey et al., 2014; Park et al.,

545

2013; Pyrzynska, 2010). Thus, the better correlation with the LM suggests that the adsorption of Cu(II), Zn(II), and Cd(II) ions onto BCSH and BCS is a monolayer process. The calculated parameters for the LM (Table 3) indicate that BCSH has a higher sorption capacity for the investigated heavy metals than BCS. Moreover, on the basis of the am values (Table 3), it can be stated that the examined BCs have the highest affinity and sorption capacity for Zn(II) ions, followed by Cd(II) and Cu(II) ions. The increasing KL value between the BCs for a single metal is correlated with increasing am. Both KL and am exhibited higher values for BCSH than for BCS, and these relatively high values are correlated with the higher experimental adsorption capacity of BCSH. A comparison of the experimentally obtained adsorption values a (Eq. (1)) with those calculated from Eq. (4), am, indicates that the experimental adsorption is lower than am. Thus, it should be noted that even though the studied systems reached the equilibrium state, monolayer adsorption was not accomplished by either material (am is higher than a). A possible reason underlying this observation could be the filling up of adsorption centers on the surfaces of the BCs by different, interfering substances present in the solution. Furthermore, a spherical blockade that makes the adsorption centers unavailable for heavy metal ions may also explain the observed differences. The dimensionless separation parameter RL can be used to describe the characteristics of the adsorption isotherm of the LM (Lin et al., 2014). To determine the favorability of heavy metal ion sorption onto BCs, the data were tested using Eq. (6):

RL ¼ 1=ð1 þ K L C 0 Þ

ð6Þ

where KL is a Langmuir constant and C0 is the initial concentration of each heavy metal ion in the solution (mg/L). Using the numerical value of the RL parameter, the shapes of the isotherm curves can be expressed as follows: when RL > 1, the conditions are unfavorable; when RL = 1, linear conditions prevail; when RL is between 0 and 1, the conditions are favorable; and when RL = 0, the reaction is irreversible. The lowest and highest concentrations were used as C0 in the calculations. For the highest heavy metal ion concentrations, the RL value remains in the 0–1 range (Table 3), which indicates that sorption of the studied ions is a favorable process with the tested BCs and that these BCs may be suitable for the removal of Cd(II), Cu(II), and Zn(II) ions from contaminated water and for the immobilization of metals in soils. 3.4. Effect of solution pH The effect of the initial solution pH on the adsorption of Cd(II), Cu(II), and Zn(II) is presented in Fig. 3. The adsorption capacities of all the investigated metals and BCs were recorded from pH 1 to pH 8 and were found to increase as the solution pH increased. This phenomenon may be explained by the changing surface character of the BC. Under acidic conditions, the groups present on the surface of the BCs are protonated, while ‘‘free” heavy metal ions exist in the solution. As the pH increases, the surface group character, e.g., that of carboxyl groups, changes. Carboxyl groups become deprotonated, and the metal ions then bind to the BC structure. Because the metals precipitate as hydroxides under basic conditions, the relationship between the initial pH and the percentage adsorption of each element was studied within a restricted range of pH values. The results indicate that the adsorption capacities of the studied BCs increase as the pH increases (Fig. 3). The amount of negatively charged surface groups also increases as the pH increases, which allows for covalent bonding between these groups and heavy metal cations. Based on the obtained results (Fig. 3), it may be assumed that the optimal pH values to achieve the most efficient removal of metal ions by both investigated BCs are 6, 5.5, and 7 for Cd(II), Cu(II), and Zn(II), respectively. The changes in the sorption capacity

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Table 3 Parameters of heavy metal ions sorption isotherms fitted with Langmuir and Freundlich models. Metal

Biochar

Langmuir

Freundlich 2

RL 2

KL

am

R

K

1/n

R

The lowest concentration

The highest concentration

Cd

BCS BCSH

0.043 0.2

32.57 35.71

0.9896 0.9986

0.064 0.12

0.84 1.88

0.8354 0.7702

0.992 0.989

0.067 0.019

Cu

BCS BCSH

0.037 0.11

29.41 33.33

0.9555 0.9983

0.037 0.11

1.37 1.76

0.7521 0.8595

0.980 0.991

0.099 0.027

Zn

BCS BCSH

0.112 0.114

41.84 48.08

0.9974 0.9802

0.12 0.15

2.102 2.168

0.9442 0.5537

0.996 0.944

0.044 0.041

am – the maximal theoretical adsorbed amount (sorption capacity) (mg/g), KL – the Langmuir constant – the quasi Gaussian energetic heterogeneity of the adsorption system, R – regression coefficient, K, n – empirical constants indicative of sorption capacity and sorption intensity, RL – The dimensionless separation parameter RL.

Fig. 3. The initial pH influence on Cd(II) (1), Cu(II) (2) and Zn(II) (3) ions adsorption onto BCS and BCSH; m = 0.2 g, V = 50 mL, Cions = 100 mg/L, t = 24 h, T = 22 ± 2 °C.

that occur as the pH increases are very similar for both BCs (Fig. 3). At low pH (pH < 3), the adsorption of all tested ions is very low and does not exceed 30%. Above pH 3, the adsorption increases rapidly to the maximum value for each studied system. The pH of the studied systems changed because of the adsorption process that occurred between carbonaceous materials and the heavy metal ions used to enrich the solution. After the equilibrium state was reached, the pH was measured again to fully elucidate the abovementioned changes, and the relationship between the initial pH and the equilibrium pH is shown in Fig. S4. According to the recorded data during the sorption process, all the examined solutions became increasingly basic until the equilibrium pH values of 8, 7.2, and 7.5 for Cd(II), Cu(II), and Zn(II) ions, respectively, were reached. These differing equilibrium pH values are probably related to the presence of mineral ash that originated from pyrolysis, which was desorbed from the BC surface and caused the pH of the solution in the equilibrium state to increase. 3.5. Desorption study The efficiency of Cd(II), Cu(II), and Zn(II) removal from BCs after adsorption was determined using different eluents. This experiment was performed to evaluate the possibility of either reusing the examined BCs for water treatment or using BCs for the immobilization of contaminants in contaminated soils. Furthermore, this desorption study provides valuable information about the irreversibility of the adsorption process. The application of nitric acid in the desorption step for the studied BCs resulted in greater reversibility of the adsorption process than hydrochloric acid (Fig. 4). The highest removal efficiency of metal ions was observed for BCSH. It is worth noting that even when a nitric acid concentration as low as 3.5 M was used, the removal percentages of Cd(II), Cu(II), and Zn(II) ions were equal to 91%, 95%, and 93%,

respectively. When hydrochloric acid was used, the maximal desorption efficiencies were obtained with higher concentrations; e.g., 6 M for Zn(II) and 9.5 M for Cd(II) and Cu(II), which gave desorption values of 90%, 90%, and 85%, respectively. When the ions were desorbed from the BCS surface with nitric acid, the maximal removal values were 78% for Cd(II) and 95% for Cu(II) with 3.5-M nitric acid and 88% for Zn(II) with 1.5-M nitric acid. When higher concentrations of desorptive agents were applied, no further increases in the removal percentages of metal ions from the surfaces of BCs were observed. However, the total removal of adsorbed ions can be accomplished by consecutive washings of the material. Based on the obtained results, it can be concluded that BCS bound the metal ions on its surface more strongly than BCSH did. The incomplete desorption in the case of hydrochloric acid may have been caused by the formation of complexes with chloride ions and subsequent readsorption of the complexes. During the adsorption process, Cu(II) is reduced to Cu(I) (XPS: 3.7). When hydrochloric acid is introduced into the solution, Cu is desorbed from the BC surface as a cation, but in the presence of chloride ion, it forms anionic complexes. The stability constant of Cu2+/Cl (pKa = 0.95) is lower than that of Cu+/Cl (pKa = 7.7), which indicates that Cu(I) complexes form more readily than Cu(II) complexes. Thus, the higher content of Cu(I) in the solution is correlated with a higher content of chloride complexes, which can be readsorbed on active centers that are not dedicated to Cu cations. For Cd and Zn, this reduction process does not occur, but these elements can also form complexes with chloride ions. It should be noted that the stability constants of complexes involving Cd(II) (pKa = 2.0) and Zn(II) (pKa = 1.0) are lower than that of Cu(I). Consequently, their complex-forming abilities are lower. The readsorption of complexes can be considered as the main reason underlying the incomplete desorption of the studied heavy metal ions. The increase in

A. Bogusz et al. / Bioresource Technology 196 (2015) 540–549

547

Fig. 4. Desorption of Cd(II), Cu(II) and Zn(II) from BCS and BCSH in respect to nitric acid (1,2) and hydrochloric acid (3,4) concentration; m = 0.008 g, V = 2 mL, ACd(BCS) = 18.26 mg/g, ACd(BCSH) = 18.81 mg/g, ACu(BCS) = 26.07 mg/g, ACu(BCSH) = 31.31 mg/g, AZn(BCS) = 20.83 mg/g, AZn(BCSH) = 25.44 mg/g, t = 24 h, T = 25 °C.

the desorption can be explained by the competitive interaction between chloride complexes and chloride ions. Another reason for this behavior is the high pH of the studied BCs, and this effect is the most obvious for BCSH. High pH values result in the surface precipitation of metal ions, which, in combination with the lower pore volume of BCSH, prevents complete desorption. In the case of desorption with hydrochloric acid desorption, both explanations are likely and may actually complement each other, whereas when nitric acid is used, the surface precipitation of metal ions is the more likely reason underlying the incomplete desorption. Zn(II) ions were observed to be the most difficult ions to remove from the surfaces of the BCs. The differences in the removal of each metal ion under the same conditions may be explained based on ionic radii. Thus, the high desorption efficiency observed for Cu ions is related to the fact that they were the smallest ions studied and exhibited the easiest diffusion from the bulk structure of the porous material to the acidic solution. Regarding the recovery of the studied BCs, in some cases, adsorption was a reversible process, and the subsequent application of the recovered material is possible after a single desorptive process; e.g., when Cu(II) ions were removed from BCS and BCSH surfaces by nitric acid. However, irreversible adsorption processes were also observed, and a single washing step was not sufficient to achieve total recovery; e.g., when Zn(II) ions were removed from BCS and BCSH surfaces by hydrochloric acid. This irreversibility may be the result of the

surface precipitation of the studied metal ions as hydroxides. Consequently, more than one washing step should be applied to accomplish the total recovery of porous materials. 3.6. Effect of potentially interfering ions The effect of chlorides and nitrates, as an examples of interfering anions being present in the environmental samples, on adsorption of Cd(II), Cu(II) and Zn(II) ions was investigated. In the most of cases, the amounts of adsorbed heavy metal ions decreased at low   concentration of Cl or NO 3 and with increasing of Cl and NO3 concentration no further changes were observed. Considering the data (Fig. 5), it can be also noticed that the decrease of the adsorption of studied ions is higher for chlorides than nitrates for the corresponding biochars. The highest decrease of adsorption was observed in case of zinc adsorption, for both nitrates and chlorides. Chlorides decreased the most adsorption of Zn(II) to 73% and nitrates to 83%. Compering with BCSH adsorption of Zn(II), chlorides were responsible for decreasing to 85% and nitrates to 93%. The effects of the interfering ions were more obvious for BCS than for BCSH. In the presence of chlorides and nitrates, the sorption capacity of BCS for metal ions substantially decreased. Adsorption on BCSH did not exhibit as drastic a decrease as that on BCS. This difference can be explained by the higher content of oxygen-containing functional groups on the BCSH surface than

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Fig. 5. The influence of chlorides (1,2) and nitrates (3,4) on Cd(II), Cu(II) and Zn(II) adsorption onto BCS and BCSH; m = 0.2 g, V = 50 mL, C = 100 mg/L, t = 24 h, T = 25 °C.

on the BCS surface, which was correlated with the higher adsorption capacity of BCSH. Heavy metal ions bound to the BCSH surface are tightly distributed, thus creating spherical blockade against interfering ions. For comparison, on the BCS surface, the content of oxygen-containing groups responsible for the adsorption process is lower, and thus, the distribution of metal ions was not as tight as that on BCSH. Therefore, the BCS surface is more open to attack by chlorides and nitrates. Considering the high pH of the studied BCs, surface precipitation may be a possible sorption mechanism underlying the deactivation of the BC surface because of the formation of hydroxides, which block active centers and thus weaken the competitive interactions between heavy metal ions and interfering ions. This mechanism is particularly important in the case of BCSH, which has a higher pH and which allows precipitation to play a more important role in this material than in BCS. Competitive interactions between chlorides or nitrates and the studied heavy metal ions with regard to the active centers are responsible for reducing the loading of Cd(II), Cu(II), and Zn(II) ions onto the BC surface. This effect was intensified in the presence of chlorides compared with nitrates because chlorides have a higher affinity for the aliphatic carbonaceous groups on the surfaces of BCs. Moreover, the formation of complexes with chlorides is also an important issue. The decrease in the adsorption could be related to the competitive interaction between anionic chloride

complexes, which can be readsorbed from solution, and the chloride ions was introduced to the adsorption system. Increasing the concentration of chloride ions in the solution results in increased competition for local adsorption centers and the displacement of chloride complexes from the surface to the solution. The ions investigated here compete for the same active centers because they have the same charge. This reason could also explain the fact that above certain concentrations of chlorides and nitrates, these ions do not further decrease the adsorption of heavy metals: chloride ions do not compete with heavy metal ions for the same active centers, while chloride complexes do. 3.7. XPS study Based on the XPS results, possible mechanisms for the adsorption of Cd(II), Cu(II), and Zn(II) ions were proposed. In the Fig. S5 the XPS spectra of Cd3d, Cu2p, and Zn2p are shown. Peaks identified in the specified regions suggest that the main species of each metal present on the surfaces of BCS and BCSH were as follows: Cd(II) in the form of CdO, mixed species of Cu(II) as carbonate dihydroxide and Cu(I) as Cu2O, and Zn(II) in the form of ZnO. CdO was observed at a binding energy of approximately 406 eV in the Cd3d5/2 region. However, for BCSH, the second peak from Cd(II) also appeared at 408.4 eV (22.9%). This was likely the result of the surface

A. Bogusz et al. / Bioresource Technology 196 (2015) 540–549 Table 4 The comparison of XPS data for Cd(II), Cu(II) and Zn(II) analysis.

References

Metal

Biochar

Region

Position

% At concentration

Cd

BCS

Cd3d5/2

406.2 408.4 406.1

77.1% Cd(II) 22.9% 100 % Cd(II)

Cu2p3/2

933 934.5 933.1 934.7

77.8% 22.2% 65.2% 34.8%

Zn2p3/2

1022.3 1022.9

100% Zn(II) 100% Zn(II)

BCSH Cu

BCS BCSH

Zn

BCS BCSH

549

Cu(I) Cu(II) Cu(I) Cu(II)

heterogeneity of the BCSH sample. During Cd(II) adsorption, the surface charge was irregularly distributed. Therefore, the X-ray radiation interacted differently with the BCSH surface, resulting in the detection of photoelectrons in an atypical region. The heterogeneity of the studied materials is also responsible for the atypical position of CdO in the XPS spectrum. Furthermore, XPS revealed the reduction of Cu(II) to Cu(I) in different proportions for each BC (Table 4). Cu peaks were detected in the Cu2p3/2 region at binding energies of approximately 933 eV for Cu(I) and 934.5 eV for Cu(II). The positive normal potential of Cu(II) and the presence of large amounts of C@O groups are responsible for the reduction process. When Cu(II) is reduced to Cu(I), carbonyl groups are oxidized to carboxyl groups. The greater amount of Cu(I) species present on the BCSH surface correlates with the greater amount of reducing C@O groups. The reduction of Cd(II) and Zn(II) was not observed because of their negative normal potentials, which make these species difficult to reduce. In the XPS spectra of Zn(II), peaks were observed at a binding energy of 1022 eV in Zn2p3/2 in both materials.

4. Conclusions The maximal adsorbed amount for all metal ions was observed in basic solutions. The adsorption process proceed the quickest for Cu(II), followed by Zn(II) and Cd(II) ions. Moreover, the influence of interfering ions, in form of nitrates and chlorides, shows that chlorides are more undesirable anions than nitrates in the solution, because of their negative impact on adsorption capacity. Simple methods of preparation, cheap feedstock and their availability in a large scale, fast heavy metal removal from the solution and great adsorption capacity make the biochars a useful and economically viable sorbents for removal of tested metals from wastewater.

Acknowledgement The project was funded by the National Science Centre granted on the basis of the decision number DEC-2012/07/E/ST10/00572.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.biortech.2015.08. 006.

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