3.2. Optimization of adsorption parameters

The percentage removal of As(III) in solution was calculated using Eq. (1):

trations of the analyte, respectively.

28 Arsenic - Analytical and Toxicological Studies

where qe is the metal uptake (mgg�<sup>1</sup>

3. Results and discussion

Figure 1. FTIR spectrum of avocado fruit waste seed (AFWS).

tration (mgL�<sup>1</sup>

R% <sup>¼</sup> <sup>C</sup><sup>0</sup> � Ce Co

quantity added to the biomass and metal content of the supernatant (Eq. (2) [14, 15]:

3.1. Characterization of the raw avocado seed and activated carbon material

qe <sup>¼</sup> ð Þ <sup>C</sup><sup>0</sup> � Ce M

where R is the percentage (%) removal and Co and Ce are the initial and equilibrium concen-

The amount of metal adsorbed by adsorbent was calculated from the difference of metal

The FTIR spectrum is an important technique which provides the surface functional groups that significantly contribute in the enhanced adsorption efficiency of the adsorbent. FTIR was used to determine the surface functional groups of raw avocado fruit waste seed. In Figure 1, the spectrum of the powdered avocado seed is represented, where the band located at 3259 cm�<sup>1</sup> corresponds to v (OdH) vibrations in the hydroxyl group, while the strong peaks

), V is the volume of the solution (mL), and M is the mass of the adsorbent (g).

� 100 (1)

� V (2)

), C<sup>0</sup> and Ce are the initial and equilibrium metal concen-

pH is one of the most important parameters that influence the adsorption of the analyte. In this study, the amount of AsIII adsorbed on avocado seed was the highest at pH 6 and gradually decreased as the pH increased up to 9 (Figure 3a) [7]. However, the highest removal was observed with avocado seed due to the presence of carboxylic group on the surface which increased the affinity toward arsenic to the adsorbent (Figure 1). Oxygen of the carbonyl group easily formed the complex with the arsenic [23]. Arsenic(III) adsorption decreased as the pH goes below 6 due to the increasing ionic strength [24].

The effect of the concentration was carried out by increasing the initial concentration from 5 to 30 mg L<sup>1</sup> , and the solutions were adjusted to pH 6 at 25C. It was observed that the percentage removal increased with the increasing concentration of the analyte; this is due to

Figure 2. Avocado seed images of (a) SEM and (b) EDS.

The effect of contact time is an important factor in adsorption because it affects the adsorption kinetics of an adsorbent at the given initial concentration of the adsorbate [26]. The batch adsorption experiments were carried out to investigate the effect of agitation time on the adsorption of As(III). Adsorption rate initially increased rapidly, and the highest removal was reached at 120 min (Figure 3d). Further increase in contact time did not show a significant

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31

Temperature is one of the parameters that affect the equilibrium and solubility and can also initiate the chemical reaction. This is because temperature can either increase or decrease the activation energy of the analyte. The effect of temperature on the adsorption of arsenic was investigated from room temperature of 40�C. From the results obtained in Figure 3e, the temperature did not have any effects since there is no significant increase or decrease in the

Under optimized conditions, 2 mg L�<sup>1</sup> AsIII standard solution was adsorbed by the avocado peels, and 75% AsIII was removed (Figure 4). The adsorption capacity was 93.75 mg/g when

Adsorption is described by the functions which connect the amount of adsorbate on the

The distribution of metal ions between the liquid phase and the solid phase is described by

Ce þ

where Ce is the equilibrium concentration (mg/L), qe is the amount of arsenic adsorbed onto the solid phase (mg/g), b is the equilibrium adsorption constant related to the affinity of binding

1 KLqmax

(3)

several isotherm models such as Langmuir and Freundlich [27].

Ce qe

<sup>¼</sup> <sup>1</sup> qmax

Figure 4. Determination of percentage (%) removal from 2 mg L�<sup>1</sup> AsIII standard solution by ICPOES.

The Langmuir equation can be written in the form of Eq. (3):

change in the percentage (%) removal of arsenic.

percentage (%) removal of arsenic.

Eq. (2) was applied.

adsorbent.

3.3. Adsorption kinetics

Figure 3. Optimization of (a) pH of the solution, (b) concentration of the analyte, (c) bio-adsorbent dosage, (d) contact time between the bio-adsorbent and the analyte, and (e) temperature of the solution.

the fact that as the concentration increased more ions were available in the solution for adsorption [25]. It was observed from the results in Figure 3b that the highest removal (65%) was reached using 20 mg L<sup>1</sup> and after there was no significant increase in the percentage (%) removal of AsIII.

The amount of adsorbent is one on the important factors that affects the adsorption capacity. The adsorbent amount of raw avocado seed on the efficiency of adsorption was investigated, and adsorbent amount was varied from 0.025 to 0.8 g. The results observed indicated that the adsorption increased with increasing adsorbent dosage till 0.8 g (Figure 3c). The increase in the percentage removal is due to the availability of active sites for adsorption [26]. It was found that after the dosage of 0.4 g there is no significant change in the percentage removal of arsenic. Then, 0.8 g was used throughout the experiments.

The effect of contact time is an important factor in adsorption because it affects the adsorption kinetics of an adsorbent at the given initial concentration of the adsorbate [26]. The batch adsorption experiments were carried out to investigate the effect of agitation time on the adsorption of As(III). Adsorption rate initially increased rapidly, and the highest removal was reached at 120 min (Figure 3d). Further increase in contact time did not show a significant change in the percentage (%) removal of arsenic.

Temperature is one of the parameters that affect the equilibrium and solubility and can also initiate the chemical reaction. This is because temperature can either increase or decrease the activation energy of the analyte. The effect of temperature on the adsorption of arsenic was investigated from room temperature of 40�C. From the results obtained in Figure 3e, the temperature did not have any effects since there is no significant increase or decrease in the percentage (%) removal of arsenic.

Under optimized conditions, 2 mg L�<sup>1</sup> AsIII standard solution was adsorbed by the avocado peels, and 75% AsIII was removed (Figure 4). The adsorption capacity was 93.75 mg/g when Eq. (2) was applied.

#### 3.3. Adsorption kinetics

the fact that as the concentration increased more ions were available in the solution for adsorption [25]. It was observed from the results in Figure 3b that the highest removal (65%) was reached using 20 mg L<sup>1</sup> and after there was no significant increase in the percentage (%)

Figure 3. Optimization of (a) pH of the solution, (b) concentration of the analyte, (c) bio-adsorbent dosage, (d) contact

time between the bio-adsorbent and the analyte, and (e) temperature of the solution.

The amount of adsorbent is one on the important factors that affects the adsorption capacity. The adsorbent amount of raw avocado seed on the efficiency of adsorption was investigated, and adsorbent amount was varied from 0.025 to 0.8 g. The results observed indicated that the adsorption increased with increasing adsorbent dosage till 0.8 g (Figure 3c). The increase in the percentage removal is due to the availability of active sites for adsorption [26]. It was found that after the dosage of 0.4 g there is no significant change in the percentage removal of arsenic.

removal of AsIII.

30 Arsenic - Analytical and Toxicological Studies

Then, 0.8 g was used throughout the experiments.

Adsorption is described by the functions which connect the amount of adsorbate on the adsorbent.

The distribution of metal ions between the liquid phase and the solid phase is described by several isotherm models such as Langmuir and Freundlich [27].

The Langmuir equation can be written in the form of Eq. (3):

$$\frac{C\_{\varepsilon}}{q\_{\varepsilon}} = \frac{1}{q\_{\text{max}}} C\_{\varepsilon} + \frac{1}{K\_{L} q\_{\text{max}}} \tag{3}$$

where Ce is the equilibrium concentration (mg/L), qe is the amount of arsenic adsorbed onto the solid phase (mg/g), b is the equilibrium adsorption constant related to the affinity of binding

Figure 4. Determination of percentage (%) removal from 2 mg L�<sup>1</sup> AsIII standard solution by ICPOES.

sites (L/mg), and qmax is the maximum amount of arsenic per unit weight of adsorbent for complete monolayer coverage.

Freundlich equation is represented as shown in Eq. (4):

$$\text{Logq}\_{\epsilon} = \log K\_f + \frac{1}{n} \log \mathbb{C}\_{\epsilon} \tag{4}$$

due to the carboxylic groups that are the surface of the avocado seed [28], meaning that the chemisorption took place. To prove that the data belonged to Langmuir isotherm, the separation value, RL value from Eq. (5) was calculated. RL proves whether the Langmuir adsorption nature

1 þ ð Þ 1 þ KL � C0

where C<sup>0</sup> is the initial concentration and KL is a Langmuir constant obtained from plotting 1/qe versus 1/Ce. The results in Table 1 indicated that the equilibrium sorption was favorable for

Analytical figures of merit for the quantitative analysis of arsenic(III) such as limit of detection

deviation (RSD) were calculated. In order to determine the LOD, the blank solution was subjected to the optimum experimental conditions, and the signals for ten blank samples were measured (n = 10). The limit of detection (LOD), calculated based on 3S/m (where S is the standard deviation of the blank and m is the slope of the calibration curve) was 0.10 mg L�<sup>1</sup>

The limit of quantification (LOQ = 10S/m) was 0.20 mg L�<sup>1</sup> for arsenic(III). The linear calibra-

The precision (repeatability) of the batch adsorption method was studied by measurements of eight replicates of 2.0 mg L�<sup>1</sup> standard solution of AsIII as shown in Table 2. The precision,

A water sample from East London municipality was adsorbed by the raw avocado seed under the optimized conditions. It is shown in Figure 6(a and b) that the bio-adsorbent is removed (54 and 55%) from sampling area A and B, respectively. During the adsorption of AsIII from environmental water samples, an interference can be experienced from metal ions such as FeIII,

This indicated that avocado seed has the great potential in removing heavy metals like AsIII in

RL 0.17 R<sup>2</sup> 0.97

Adsorption isotherm Parameter Value Langmuir KL 0.0022 L/mg

Freundlich R<sup>2</sup> 0.72

(5)

33

.

), and the relative standard

Bio-adsorbents for the Removal of Heavy Metals from Water

http://dx.doi.org/10.5772/intechopen.73570

RL <sup>¼</sup> <sup>1</sup>

is favorable if RL > 0, unfavorable if RL > 1, and irreversible if RL = 0:

(LOD), limit of quantification (LOQ), correlation coefficient (R<sup>2</sup>

tion curve was plotted with a correlation coefficient of 0.98.

expressed in terms of standard deviation (%RSD), was 2.1.

FeII, ZnII, CdII, NiII, MnII, AlIII, PbII, and CuII [10].

environmental water samples without being modified.

Table 1. Adsorption isotherms for AsIII adsorption by a bio-adsorbent.

3.5. Application of the avocado seed adsorbent in real water samples

Langmuir isotherm.

3.4. Analytical figures of merit

where Ce is the equilibrium concentration (mg/L), qe is the amount of arsenic adsorbed onto the solid phase (mg/g), Kf is an indicator of the adsorption capacity, and n is the heterogeneity factor.

The results in Figure 5(a and b) and Table 1 showed that the correlation coefficient for linear Langmuir model (R2 , 0.97) was higher than the Freundlich model (R2 , 0.72). The data was best fitted in Langmuir model, and this signified that the adsorbent had high affinity for arsenic(III)

Figure 5. (a) Langmuir isotherm displaying the adsorption of AsIII onto the surface of the avocado seed by plotting Ce/qe against equilibrium concentration (Ce). (b) Freundlich isotherm showing the adsorption of AsIII onto the surface of the avocado seed by plotting ln Ce against equilibrium concentration ln qe.

due to the carboxylic groups that are the surface of the avocado seed [28], meaning that the chemisorption took place. To prove that the data belonged to Langmuir isotherm, the separation value, RL value from Eq. (5) was calculated. RL proves whether the Langmuir adsorption nature is favorable if RL > 0, unfavorable if RL > 1, and irreversible if RL = 0:

$$\mathbf{R}\_{\rm L} = \frac{1}{1 + (1 + \mathbf{K}\_{\rm L} \times \mathbf{C}\_{0})} \tag{5}$$

where C<sup>0</sup> is the initial concentration and KL is a Langmuir constant obtained from plotting 1/qe versus 1/Ce. The results in Table 1 indicated that the equilibrium sorption was favorable for Langmuir isotherm.
