2.1 Adsorbent preparation

The low-cost materials used in the present work were obtained from dried plants such as C. rhizoma as Romanian plant and W. frutescens as Moroccan plant. A recent screening (Chiban et al., 2007, 2011b) for chemical composition and surface characterization has shown that the major functional groups on W. frutescens plant are polar hydroxyl, aldehydic, and carboxylic groups. These groups have made these plants to have great potential as adsorbents for cations and anions ions in aqueous solutions. In addition, the micro-particles of these plants are also known to be non-toxic (Ahmad et al., 1990) and their selection was made in relation to their relative abundance in all the zones where they are considered as a worthless matter. The plant parts (leaves and stems) were dried under permanent air-conditioning during 1 week. These organs were chopped into small fragments, then washed several times with double distilled water and again dried in an oven at 35 °C during 24 h. After drying, the organs were pulverized with an electric grinder to get fine powders. The micro-particles of these dried plants are used as adsorbent materials in batch experiments without any pre-treatment to avoid extra expenditure.

2.2 Solution preparation

Stock solution of arsenate (1 g/l) was prepared by dissolving 4.164 g of sodium arsenate (Na₂HAsO₄·7H₂O, Sigma, product of USA) in double distilled water to make a solution volume of 1 l. The aqueous solution was diluted with double distilled water to obtain the As(V) synthetic solution of desired concentration. Other agents used, such as KNO₃, NaCl, Na₂HPO₄·12H₂O, Na₂SO₄·10H₂O, Mg(NO₃)₂·6H₂O, Cd(NO₃)₂·4H₂O, Cu(NO₃)₂·3H₂O, and Zn(NO₃)₂·6H₂O were all of analytical grade and without further purification. All experiments employed double distilled water.

2.3 Instrumentation

A UV–Vis Spectrophotometer (Jasco, model V-550, Japan) was employed to measure residual arsenate ion concentrations. The pH of the solutions was measured with Titroline (SCHOTT instruments) pH meter with a glass electrode. A mechanical shaker model Labotec was used for shaking the adsorption batches. The centrifuge HETTICH model EBA 21 was used to separate the micro-particles of plants from the solutions after complete adsorption experiments. An analytical balance Precisa model XT 220A was used for weighting the dried plants and salts samples.

2.4 Experimental procedure

3.1 Effect of adsorbent mass

Fig. 1 shows the arsenate ions removal and pH final of medium as a function of adsorbent mass using batch adsorption technique. These graphs show the increase of As(V) ions removal against the increase in the weight of both dried plants up to a weight of 1 g for 100 mg/l of arsenate initial concentration. The arsenate adsorption for an agitation time of 12 h increased from 59% (2.36 mg/g) to 91% (3.64 mg/g) and 33% (1.32 mg/g) to 72% (2.88 mg/g) when the adsorbent amount was increased from 0.25 to 1 g for C. rhizoma and W. frutescens, respectively. Increasing the amount of the adsorbent makes a large number of sites available leading to an increase in adsorption. There are many factors that can contribute to this adsorbent concentration effect. The most important factor is that adsorption sites remain unsaturated during the adsorption reaction. This is due to the fact that as the mass of adsorbent is increased, there is a less commensurate increase in adsorption resulting from the lower adsorptive capacity utilization of the adsorbent. It is also seen from Fig. 1 that pH of solution after contact with both dried plants decreased and remained nearly constant at about 6 for C. rhizoma and 7 for W. frutescens. This difference can be explained by the difference in the chemical composition of these plants. In order to achieve experiments for maximum retention of the As(V) ions, the plant weight has been chosen to be 1 g in all experiments.

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Figure 1

Effect of amount of both dried plants individually on the adsorption of As(V) at initial As(V) concentration, 100 mg/l; temperature, 22 °C; contact time, 12 h; and initial pH_i = 8.3.

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3.2 Effect of contact time and initial concentration

The contact time of adsorbate and adsorbent has a great importance in adsorption, because it depends on the nature of the system used. Figs. 2 and 3 show the effect of contact time on batch adsorption of different initial arsenate concentrations ranging between 0.1 and 1 g/l at ambient temperature for W. frutescens and C. rhizoma, respectively. The pH of the solutions was not fixed but measured before and after each experiment. The amount of As(V) adsorbed increases very rapidly up to about 120 min and slowly reaches saturation at about 180 min. The adsorption of As(V) remained constant after 180 min implying that equilibrium had been reached. The plots also show that the time of equilibrium as well as time required to achieve a definite fraction of equilibrium adsorption is independent of the initial concentration. With the increase in the initial concentration of As(V) from 0.1 to 1 g/l, the amount adsorbed increased from 3.04 mg/g (73%) to 16.88 mg/g (42%) for W. frutescens and from 3.64 mg/g (91%) to 22.04 mg/g (55%) for C. rhizoma. At higher initial concentration the available sites of adsorption become fewer and hence the percentage adsorption depends on the initial concentration. These values depend to some extent on the type of plant. For a fixed adsorbent dose, the total available adsorption sites are limited, there by adsorbing almost the same amount of the adsorbate, thus resulting in a decrease in percentage adsorption corresponding to an increase in initial adsorbate concentration. The increase of loading capacities of adsorbents with the increase of arsenate ion concentration is probably due to higher interaction between arsenate ions and adsorbent. The fast adsorption kinetics observed for both adsorbents represents a key advantage for designing water treatment systems based on this principle. In comparison, the equilibrium time for the adsorption of As(V) by some other adsorbents in the literature is acceptable. For example, Ranjan et al. (2009), using agricultural residue ‘rice polish' for biosorption of arsenic from aqueous solution, found that the equilibrium time needed for As(III) and As(V) sorption on rice polish was 60 and 40 min, respectively. Gimenez et al. (2007) using different natural iron oxides for the removal of both As(III) and As(V) oxyanions, found that equilibrium was reached in less than 2 days and the kinetics of sorption seems to be faster for goethite and magnetite than for hematite. However, the results from Jeon et al. (2009) were similar to those found in this investigation.

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Figure 2

Effect of contact time and initial arsenate ion concentration on the removal of As(V) by W. frutescens: m/v = 25 g/l, pH_i = 8.3, and temperature ∼22 °C.

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Figure 3

Effect of contact time and initial arsenate ion concentration on the removal of As(V) by C. rhizoma: m/v = 25 g/l, pH_i = 8.3, and temperature ∼22 °C.

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3.3 Effect of solution pH

One of the most critical parameters in the adsorption process of arsenic ions from aqueous solutions is the pH of the medium (Gimenez et al., 2007). The initial pH of adsorption medium is related to the adsorption mechanisms onto the adsorbent surface from water and reflects the nature of the physicochemical interaction of the species in solution and the adsorptive sites of adsorbents (Sari and Tuzen, 2009). The standard range of pH in drinking water varies from 6.5 to 8.5 (Menhage-Bena et al., 2004) and therefore the investigation of the effect of a wide range of pH on arsenate removal is needed. Hence, the effect of initial pH on the uptake capacities of As(V) on both dried plants was studied at room temperature. The pH range studied was between 2.0 and 12.0 for 100 mg/l of initial As(V) concentration. Equilibrium pH (pH_final) values as well as the retained arsenate concentration, C_r (mg/l), are measured and the results are presented in Fig. 4. It is clear that the pH of the medium affected the adsorption of arsenate by the adsorbents and the micro-particles of the plants used as adsorbents shown to be effective for the quantitative removal of As(V) at pH ranging from 6 to 9. It was found that in the lower pH range (below 9.0) the adsorption of arsenate was good, but above pH 9.0, the adsorptivity decreases sharply. This can be due to the fact that at the higher pH range there is a competition between the hydroxyl ions present at higher pH and arsenic species, for the adsorption sites. In addition, the carboxyl, hydroxyl, and amide groups of the plants will be negatively charged in alkaline conditions. Therefore, there exists a repulsive force between the negatively charged adsorbent and anionic species of arsenic, resulting in reduced adsorption efficiency (Rahaman et al., 2008). For both adsorbents, the pH_final increases smoothly with the increase of pH_initial (Fig. 4). However, in case of C. rhizome adsorbent, a plateau is found up to pH 6 and then again starts to increase with initial pH (Fig. 4). In both cases, arsenate uptakes increase up to near pH 6 and afterward decrease with the increase of pH. At pH 6, maximum arsenate removal percent of 70% and 89% is obtained for W. frutescens and C. rhizoma, respectively. As(V) speciation is affected by the solution pH through the following equilibrium (Yusof and Malek, 2009):

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Figure 4

The effect of initial pH_i on adsorption of As(V) onto C. rhizoma and W. frutescens: initial concentration of As(V) was 100 mg/l, temperature was 22 °C, dose of dried plants was 25 g/l, and mixing time was 12 h.

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From Eqs. (1)–(3), the As(V) species occurs mainly in the form of ${\text{H}}_2{\text{AsO}}_4^{-}$ in the pH range between 3 and 6, while a divalent anion ${\text{HAsO}}_4^{2-}$ dominates at higher pH values (such as between pH 8 and 11). In the intermediate region which is in the pH range between 6 and 8, both species coexist with one another (Kim et al., 2004). Thus, it is evident that the adsorption of As(V) by dried plants is pH dependent.

3.4 Adsorption isotherms

Adsorption isotherms are mathematical models that describe the distribution of the adsorbate species among the solid and liquid phases, and are important data to understand the mechanism of adsorption. Several models have been published in the literature to describe experimental data of adsorption isotherms. The Langmuir, Freundlich and Temkin models are the most frequently employed models. In this study, these three models were used to describe the relationship between the adsorbed amount of arsenate on dried plants and its equilibrium concentration in solution.

3.4.1

3.4.1 Langmuir adsorption isotherm

The data obtained were then fitted to the Langmuir adsorption isotherm applied to equilibrium adsorption assuming monolayer adsorption onto a surface with a finite number of identical sites and is expressed by the following equation (Langmuir, 1918):

(4)

$$q_{\text{e}}=q_{\max }\frac{K_{\text{L}}{C}_{\text{e}}}{1+K_{\text{L}}{C}_{\text{e}}}$$

It can be expressed in the linear form in Eq. (5)

(5)

$$\frac{1}{q_{\text{e}}}=\frac{1}{q_{\max }}+\frac{1}{q_{\max }{K}_{\text{L}}{C}_{\text{e}}}$$ where q_e is the equilibrium arsenate concentration on the adsorbent (mg/g), C_e is the equilibrium As(V) concentration in the solution (mg/l), q_max is the maximum adsorption capacity of the adsorbent (mg/g), and K_L is the Langmuir adsorption constant (l/mg) and is related to the free energy of adsorption (Chen et al., 2008). The plot of 1/q_e versus 1/C_e for both dried plants gives a straight line of slope 1/q_maxK_L and intercepts 1/q_max (Figs. 5 and 6). The values of the adsorption capacity (q_max), the Langmuir constant (K_L), and the coefficient of determination (r²) were presented in Table 1. The values of the coefficient of determination for both dried plants were 99% and 98% for W. frutescens and C. rhizoma, respectively, indicating a good fit of the Langmuir model to the adsorption of arsenate on the two plants. The Langmuir model is an indication of surface homogeneity of the adsorbent. The basic assumption of Langmuir adsorption isotherm is also based on monolayer coverage of the adsorbate on the surface of adsorbent. The adsorption capacity of the adsorbent increased on increasing the temperature. The highest value of q_max obtained at 40 °C was 29.06 mg/g for W. frutescens and 34.72 mg/g for C. rhizoma. It appears to be the highest in comparison with the uptake obtained at the other temperatures (Table 1). The essential feature of the Langmuir isotherm can be expressed by means of ‘R_L', a dimensionless constant referred to as separation factor or equilibrium parameter to predicting whether an adsorption system is favorable or unfavorable. R_L is calculated using the following equation (Weber and Chakkravorti, 1974):

(6)

$$R_{\text{L}}=\frac{1}{1+K_{\text{L}}{C}_{\text{i}}}$$ where C_i is the initial As(V) concentration (mg/l). The parameter R_L indicates the shape of the isotherm and nature of the adsorption process. According to the value of R_L the isotherm shape may be interpreted as follows (Bulut et al., 2007) (Table 2):

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Figure 5

Langmuir adsorption isotherm for As(V) on C. rhizoma at various temperatures: m/v = 25 g/l, pH_i = 8.3, T_c = 12 h.

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Figure 6

Freundlich adsorption isotherm for As(V) on C. rhizoma at various temperatures: m/v = 25 g/l, pH_i = 8.3, T_c = 12 h.

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Table 1 Isotherm constants for As(V) adsorption onto both dried plants at different temperatures: m/v = 25 g/l, pH_i = 8.4, T_c = 12 h.

	C. rhizoma				W. frutescens
	25 °C	30 °C	35 °C	40 °C	25 °C	30 °C	35 °C	40 °C
Langmuir
Qmax	35.97	33.33	35.97	34.72	23.53	21.98	28.57	29.07
KL	0.004	0.005	0.005	0.007	0.004	0.005	0.007	0.011
r2	0.979	0.981	0.992	0.983	0.992	0.990	0.982	0.997
RL	0.198–0.553	0.159–0.486	0.157–0.482	0.13–0.427	0.198–0.553	0.158–0.484	0.123–0.413	0.079–0.300
Freundlich
1/nf	0.71	0.66	0.65	0.63	0.57	0.55	0.58	0.53
Kf	0.36	0.50	0.58	0.70	0.47	0.58	0.73	1.19
r2	0.988	0.991	0.997	0.990	0.981	0.987	0.997	0.988
Temkin
B	9.33	8.86	8.91	8.95	5.70	5.71	7.65	7.60
KT	0.032	0.039	0.044	0.051	0.033	0.038	0.048	0.07
r2	0.900	0.910	0.948	0.944	0.963	0.961	0.946	0.911

Table 2 Type of isotherm for different values of R_L.

Value of RL	Type of isotherm
RL > 1	Unfavorable
RL = 1	Linear
0 < RL < 1	Favorable
RL = 0	Irreversible

In this study, the values of R_L calculated from Eq. (6) were incorporated in Table 1. Further, for C₀ from 0.2 to 1 g/l used in the present study, the values of R_L at all temperatures studied range from 0.079 to 0.553 for W. frutescens and 0.129 to 0.553 for C. rhizoma, therefore, the adsorption is favorable.

3.4.2

3.4.2 Freundlich adsorption isotherm

The adsorption data obtained were then fitted to the Freundlich adsorption isotherm (Freundlich, 1906), which is the earliest relationship known describing the adsorption equilibrium and is expressed by Eq. (7) and its linear form in Eq. (8):

(7)

$$q_{\text{e}}=K_{\text{f}}{C}_{\text{e}}^{1/n}$$

(8)

$$\text{Ln}{q}_{\text{e}}=\text{Ln}{K}_{\text{f}}+\frac{1}{n}\text{Ln}{C}_{\text{e}}$$

The Freundlich isotherm constants K_f and n are constants incorporating all factors affecting the adsorption process such as of adsorption capacity and intensity of adsorption. The constants K_f and n were calculated from Eq. (8) and Freundlich plots (Figs. 7 and 8). The amount of absorbent required to reduce any initial concentration to predetermined final concentration can be calculated. The values for Freundlich constants and correlation coefficients (r²) for the two adsorbents used during the study are also presented in Table 1. The values of n between 1 and 10 (i.e., 1/n less than 1) represent a favorable adsorption (Faust and Aly, 1987). The values of n, reflects the intensity of adsorption. The numerical values of n at all temperatures lies between 1.792 and 2.177 and is greater than unity, indicating that arsenate ions are favorably adsorbed by both dried plants at all the temperatures studied. In general, both Freundlich and Langmuir models had a good agreement with the data for arsenate adsorption, evidenced by the high r² values (all greater than 0.97) (Table 1). These experiments confirm the efficiency of the micro-particles of dried plants to remove As(V) from aqueous solution.

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Figure 7

Temkin adsorption isotherm for As(V) on C. rhizoma at various temperatures: m/v = 25 g/l, pH_i = 8.3, T_c = 12 h.

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Figure 8

Langmuir adsorption isotherm for As(V) on W. frutescens at various temperatures: m/v = 25 g/l, pH_i = 8.3, T_c = 12 h.

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3.4.3

3.4.3 Temkin Isotherm

The Temkin adsorption isotherm model based on the heat of adsorption of the ions, which is due to the adsorbate and adsorbent interactions taken (Temkin and Pyzhev, 1940). The Temkin isotherm equation is given as:

(9)

$$q_{\text{e}}=\frac{\mathit{RT}}{b}\text{Ln}(K_{\text{T}}·C_{\text{e}})\text{where}\mathit{RT}/b=B$$

A linear form of the Temkin isotherm can be expressed as:

(10)

$$q_{\text{e}}=B\text{Ln}(K_{\text{T}})+B\text{Ln}(C_{\text{e}})$$ where T is absolute temperature in Kelvin and R the universal gas constant, 8.314 J/(mol K). b (J/mol) is Temkin isotherm constant and constant B (dimensionless) is related to the heat of adsorption (Bulut et al., 2008). K_T (l/mg) is the equilibrium binding constant (l/g) corresponding to the maximum binding energy. The Temkin isotherm plot for both plants at four temperatures are presented in Figs. 9 and 10 and the isotherm parameters are given in Table 1. The Temkin constant (B) related to the heat of adsorption for the two dried plants was between 5.7 and 7.6 kJ/mol for W. frutescens and from 8.86 to 9.33 kJ/mol at studied temperature ranges. It has been reported (Ho et al., 1996) that the typical range of bonding energy for ion-exchange mechanism is 8–16 kJ/mol. The low values in this study in case of W. frutescens indicate a weak interaction between adsorbate and adsorbent, supporting an ion-exchange mechanism for the present study. From linear regression of the data points, the r² values range from 0.91 to 0.96 for W. frutescens and 0.90 to 0.94 for C. rhizoma, respectively, are rather low indicating that the adsorption of As(V) did not follow the Temkin isotherm closely.

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Figure 9

Freundlich adsorption isotherm for As(V) on W. frutescens at various temperatures: m/v = 25 g/l, pH_i = 8.3, T_c = 12 h.

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Figure 10

Temkin adsorption isotherm for As(V) W. frutescens at various temperatures: m/v = 25 g/l, pH_i = 8.3, T_c = 12 h.

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3.5 Thermodynamic studies

The thermodynamic parameters such as standard Gibbs free energy change (ΔG⁰), enthalpy change (ΔH⁰), and entropy change (ΔS⁰) were estimated to evaluate the feasibility and nature of the adsorption process. The values of standard Gibbs free energy change for the adsorption process were evaluated by using K_L values obtained from the Langmuir model at different temperatures and thermodynamic parameters are presented in Table 3. The Gibbs free energy change, of the process is related to equilibrium constant by following the equation (Parfitt and Rochester, 1983; Bulut and Tez, 2007):

According to thermodynamics, the Gibbs free energy change is also related to the enthalpy change (ΔH⁰) and entropy change (ΔS⁰) at constant temperature by the following equation,

Now using Eqs. (12) and (11) can be transformed to Eq. (13).

The values of enthalpy change (ΔH⁰) and entropy change (ΔS⁰) were calculated from the slope and intercept of the plot of Ln K_L versus 1/T (Fig. 11) (Bulut and Tez, 2007). The calculated values of these parameters are given in Table 3. It can be seen in Table 3 that ΔG⁰ values were found to be negative for all temperature conditions studied, verifying that the adsorption process was spontaneous in nature (Bulut and Tez, 2007). However, the decreasing value of ΔG⁰ with increasing temperature implies that the spontaneity degree of the adsorption process increases with increasing of the temperature. The results for ΔH⁰ and ΔS⁰ were found to be 53.67 and 227.19 kJ/mol of W. frutescens and 23.27 and 125.91 for C. rhizoma, respectively. Unlu and Ersoz (2006) stated that although there are no certain criteria that are related to the ΔH⁰ values defining the adsorption, heats of adsorption between 20.9 and 418.4 kJ/mol, which are heats of chemical reactions, are frequently assumed to be comparable to those associated with the chemisorption processes. Based on the result of ΔH⁰ value, this study suggests that the adsorption of As(V) onto dried plants proceeded via chemisorption and that the adsorption process was endothermic in nature. This result is also supported by the increase in the value of uptake capacity of adsorbents with the rise in temperature. Han et al. (2006) stated that the increasing sorption capacity of the sorbent with temperature is attributable to the enlargement of pores and/or the activation of the sorbent surface. In addition to that, the positive value of entropy (ΔS⁰) revealed that the degrees of free active sites increased at the solid–liquid interface during the adsorption of As(V) onto dried plants.

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Figure 11

Plot of ln K_L vs. (1/T) for estimation of thermodynamic parameters for both dried plants.

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3.6 Desorption and readsorption capacity

Desorption studies will help to elucidate the nature of adsorption process and to recover the As(V) from both adsorbents individually. Moreover, it also will help to regenerate the adsorbents so that it can be used again to adsorb arsenate ions, and to develop the successful adsorption process. The desorption experiments were carried out at 25 °C by using various NaOH solutions and the effect of initial concentration of NaOH on desorption of As(V) from the adsorbents is shown in Table 4. It was expected that increased concentration of OH^- ions should compete greatly with the already adsorbed arsenate on both dried plants, hence sodium hydroxide solution was used to desorb arsenic from the adsorbent. Generally, desorption of the arsenate ions was more favored in strong alkaline solution. With 0.03 M NaOH solution, 78% of the adsorbed As(V) from W. frutescens plant was desorbed, and desorption percent could arrive at 91% if the concentration of NaOH was higher than 0.07 M. With regard to C. rhizoma plant, satisfactory desorption (40%) of the arsenate adsorbed ions achieved with relatively higher concentration of NaOH (0.07 M NaOH) (Table 4). A noticeable amount of arsenate can be readsorbed in a second procedure by W. frutescens plant treated with 0.7 M of NaOH at 12 h, although it does not reach the amounts retained in the first contact, it is probably due to a residual adsorption level. The readsorption percentage of arsenate onto both dried plants was found to be 52% (2.08 mg/g) for W. frutescens and 30% (1.2 mg/g) for C. rhizoma plant.

3.7 Effect of the coexisting ions

Till now all the adsorption results discussed above were obtained by taking the arsenate anions as adsorbate (As(V) alone). However, in reality the contaminated water contains several other ions (both cations and anions) which can affect the adsorption of arsenate. In a representative test, the effects of the presence of individual and simultaneous anions and cations such as: sulfate, chloride, nitrate, phosphate, magnesium, cadmium, copper, and zinc on the adsorption capacity of both crushed dried plants for As(V) were investigated. The amount of arsenate removal values in the absence and the presence of these anions and cations are given in Fig. 12. Generally, no interference was observed between arsenate and studied cations. The adsorption capacity of arsenate on C. rhizoma decreased by 42.6% and 8.1% in the presence of phosphate and sulfate anions separately; while no evidently competitive effects on the adsorption of As(V) onto the two plants separately were observed with other coexisting anions such as chloride and nitrate (Fig. 12). The significant interference on As(V) adsorption by phosphate is due to the similar chemical properties of arsenate with phosphate in aqueous solution at pH 6.0 (Hughes, 2002). Similar results were reported by Rahman et al. (2008) and Mkandawire et al. (2004) about the removal of arsenic by Spirodela polyrhiza L. and Lemna gibba L., respectively. These results probably meant that these dried plants had also high adsorption ability to phosphate or sulfate under some conditions that we had in our experiment. The results also show that the arsenate uptakes by both dried plants were not affected by the presence of simultaneous bivalent cations, nitrate, and chloride.

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Figure 12

Effect on the removal of As(v) on C. rhizoma and W. frutescens with coexisting ions at initial concentration for each ion was 100 mg/l; temperature was 22 °C; contact time was 12 h; and initial pH_i was 6.

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3.8 Comparison of adsorption capacity with different adsorbents reported in literature

In order to justify the validity of dried plants as adsorbents for adsorption, its adsorption potential must be compared with other various adsorbents used for this purpose. The values of maximum adsorption capacity of the adsorbents for the removal of As(V) reported in the literature are given in Table 5. The direct comparison of the adsorbent capacity of studied adsorbents with other adsorbents capacity reported in the literature is difficult due to the varying experimental conditions employed in those studies; however, both dried plants in this study possess reasonable adsorption capacity in comparison with other adsorbents. The adsorption capacity differences of arsenate ions uptake are ascribed to the properties of each adsorbent such as adsorbent structure, functional groups and surface areas (Ozsoy and Kumbur, 2006).