Rice husk and activated carbon for waste water treatment of El-Mex Bay, Alexandria Coast, Egypt

2.1 Preparation of activated rice husk (1)

Rice husk was obtained from local rice mills and was washed several times with bi-distilled water followed by filtration. In the first step, (carbonization), a 25 g of rice husk was heated gradually at the temperature rate (50 °C/15 min), in a stainless steel pipe (furnace tube) up to 500 °C. In the second step (activation), the temperature of the furnace tube was raised suddenly to 900 °C for 1 h. At this temperature the carbon dioxide was supplied from a gas cylinder at a constant flow rate. The product was cooled giving the desired pure adsorbent.

2.2 Preparation of activated rice husk (2)

Natural rice husk was stirred with 4% KOH and heated to boiling for 30 min, then the mixture was left overnight, and filtered. To the filtrate, 10% HCl was added until the pH reacted (5–7), where a white precipitate was formed, then filtered, dried at 105 °C. This precipitate was used as sorbent (2).

3.1 Treatment of water using rice husk

It seems that, the (%R) of the heavy metals in wastewater of El-Umum drain after treatment using activated rice husk (1) is better than that of the absorbent (2), due to the higher percent of ash content (Abdo, 2002). The (%R) of rice husk (1) for Fe (III) and Mn (II) is 99% and 98% respectively. Also, it is deduced that the activated rice husk (2) is successful for the removal of Mn (II) but it is useless in Fe (III) treatment. It appears that, the type (1) has a basic surface and type (2) has an acidic one. This imports the predominant effect on the removal of the heavy metal ions under investigations. The ash content of the adsorbents imports some surface chemical characteristics where as the ash% increases, the uptake increases too (Abdo, 2002), Table 1.

Since the main constituent of ash content is silica, the ion-exchange reaction on the silica surface is accomplished through the substitution of protons of the surface silanol groups by the metal ions from solution as: $$M^{n+}+\chi (\equiv \text{SiOH})\leftrightarrow M{(\equiv \text{SiOH})}_x^{(n-x)}+\chi H^{+}$$ Where:

• Mⁿ⁺ = metal ion with n⁺ charge.

• SiOH = group Silanol on the SiO₂ surface.

• χH⁺ = number of protons released.

Since electrostatic attraction was not possible between positively charged adsorbent and positively charged metal ion species, it seems that some non-electrostatic force called “specific adsorption” was involved in the process of adsorption (Abdo, 2002). However the possible adsorption mechanism includes the following: $${\text{SO}}^{-}+{\text{M}}^{2+}\to {\text{SOM}}^{+}$$ $${\text{SO}}^{-}+{\text{MOH}}^{+}\to \text{SOMOH}$$ $$\text{SOH}+{\text{M}}^{2+}\to {\text{SOM}}^{+}+{\text{H}}^{+}$$ $$\text{SOH}+{\text{MOH}}^{+}\to \text{SOMOH}+{\text{H}}^{+}$$ Where SO⁻ denotes negatively charged surface, SOH denotes neutral surface and M denotes metal ions.

Besides, two major types of chemical bindings can be responsible for the adsorption of various metal ions onto carbon surface: covalent and hydrogen bonding (Khalil and Girgis, 1994). The covalent bonding results from the sharing of free electron pairs between the surface oxygen atom and the metal atom or the formation of an O---M bonding. The hydrogen bonding results from the surface oxygen atom and the hydrogen atom of the hydrated metal ions (Abdo, 2002).

3.2 Adsorption isotherms

Adsorption data were analysed according to Langmuir (Langmuir, 1915) and Freundlich (Freundlich, 1926) models. Fe(III) and Mn(II) were selected for the studies, as representative examples, where both gave interesting results.

The Langmuir equation has been successfully applied to many adsorption systems given by the equation:

• X = x/m, the amount of solute adsorbed x per unit weight of adsorbent m

• Ce = equilibrium concentration of the solute

• Xm = retention capacity of the adsorbent

• b = constant related to the energy of adsorption

For linearization plotting of the data, equation (1) can be written in the form:

A linear plot of 1/X versus 1/Ce was obtained using activated rice husk 1, Figs. 1 and 2.

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Figure 1

Langmiur adsorption isotherm for iron using activated rice husk.

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Figure 2

Langmiur adsorption isotherm for manganese using activated rice husk.

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Regression data and adsorption parameters obtained are indicated in Table 3. An advantage of the Langmuir equation in its linear form points to the high successful application of rice husk for the removal of metals. The data for the sorption of metals onto activated rice husk were fitted to the Langmuir model. r² = 0.94 and 0.91 for Fe and Mn, respectively, indicate the monolayer coverage of metals on the outer surface of sorbent, in which the mechanism of adsorption occurs uniformly on the reactive part of the surface. The fits are quite well for the sorbent, to suggest the applicability of the Langmuir model for the investigated system. Xm and b, Langmuir constants, are the measures of maximum adsorption capacity. The values of Langmuir parameters with correlation coefficients were computed from the intercept and the slope of the fitted Langmuir equation.

It seems that XmFe > XmMn and r² Fe > r² Mn, to assign the successful application to adsorbent Fe more than Mn. The essential characteristics of a Langmuir isotherm can be expressed in terms of a dimensionless constant separation factor RL as follows.(El-Nemr et al., 2005) $$\text{RL}=1/[1+({\text{bC}}_{\text{o}})]$$ Where C_o is the initial concentration and RL indicates the trend of the isotherm. The types of equilibrium isotherms are related with the RL values, for RL > 1 unfavourable process is the dominant and for values 0 < RL < 1 favourable mechanism could be reached. In the present work, Table 4, the values of RL were found to be less than (1) and slightly higher than (0) indicating the favourable adsorption of Fe(III) and Mn(II) metals on rice husk.

3.2.1

3.2.1 Freundlich adsorption isotherm

Freundlich adsorption isotherm was used, based on the following equation (Freundlich, 1926):

(1)

$$\text{X}/\text{m}={\text{K}}_{\text{F}}{\text{Ce}}^{1/\text{n}}$$ Where:

• X/m is amount of metal adsorbed per unit weight of adsorbent (mg/g).

• Ce is the metal concentration at equilibrium (mg/1).

• K_F, 1/n are Freundlich constants, which are the empirical constants and indicative of sorption capacity and sorption intensity, respectively.

The calculated K_F and 1/n values were fitted by logarithmic transfer of Eq. (1) to:

(2)

$$\mathit{log}\text{X}/\text{m}=\mathit{log}{\text{K}}_{\text{F}}+1/\mathit{nlog}\text{Ce}$$ The Freundlich adsorption isotherms with the correlation coefficient are presented in Figs. 3–6 and Table 5. The observed linear relationships as evidenced by the r² and 1/n values indicating the applicability of adsorption isotherms and the monolayer coverage on adsorbent surface. The monolayer adsorption capacities of the adsorbents possess high adsorption capacity and hence it could be employed as low-cost adsorbent for the removal of metals. The KF values are in harmony with the Xm values i.e. as Xm increases, KF increases in turn.

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Figure 3

Relation between X and Ce of iron using activated rice husk.

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Figure 4

Relation between X and Ce of manganese using activated rice husk.

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Figure 5

Freundlich plot for iron adsorption onto activated rice husk.

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Figure 6

Freundlich plot for manganese adsorption onto activated rice husk.

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Table 5 Freundlich isotherms for the adsorption of Fe(III) and Mn(II) using rice husk 1.

Ion	KF	1/n	r2
Fe(III)	4.7 × 1013	6.25	0.84
Mn(II)	3.3 × 105	2.38	0.94

3.2.2

3.2.2 Kinetics of adsorption

Effect of time on adsorption of both Fe(III) and Mn(II) using activated rice husk (1) was studied. Figs. 7 and 8, and Tables 6 and 7 showed that the concentrations of the metals ions are decreased with increasing time (min).

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Figure 7

Effect of time on the adsorption of iron using activated rice husk.

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Figure 8

Effect of time on the adsorption of manganese using activated rice husk.

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Table 6 Effect of time on the adsorption of Fe(III) using activated rice husk 1.

Fe(III) (mg/l)	2.0	1.6	1.2	0.7	0.09	0.04	0.02
Time (min)	0	20	40	60	80	100	120

Table 7 Effect of time on the adsorption of Mn(II) using activated rice husk 1.

Fe(III) (mg/l)	1.0	0.6	0.2	0.07	0.05	0.01
Time(min)	0	20	40	60	80	100

The rates of adsorption are measured by determining the change of concentration of the metal as a function of time. Linearization of the data is obtained by plotting the log C_t, (C_t is the concentration of metal at different times) versus time (min), Figs. 9 and 10. The adsorption rates were calculated from the slopes (slope = −K/2.303), Table 8. The process followed first order reaction.

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Figure 9

Relation between time (min) versus concentration of iron using activated rice.

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Figure 10

Relation between time (min) versus concentration of manganese using activated rice husk.

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Table 8 Adsorption rates of Fe(III) and Mn(II) ions on activated rice husk.

Ion	Rate of reaction (K)
Fe(III)	0.058
Mn(II)	0.048

4.1 The data give the following

• -

  Decreasing of Fe(III) and Mn(II) concentrations after passing the samples through active carbon filters occurred, due to adsorption.

• -

  The adsorption of the metals is increased with the increase of the weight of activated carbon.

4.1.1

4.1.1 Adsorption isotherms

Adsorption data were analysed according to Langmuir (Langmuir, 1915) and Freundlich (Freundlich, 1926) models. The effect of different weights of activated carbon (3000–3800 mg) on the adsorption of Fe(III) and Mn(II) ions was monitored. The data are calculated and given in Table 9(a,b,c,d,e,f). The metals adsorption data were plotted according to linear Langmuir isotherms. Linearized plots of 1/X versus 1/Ce were obtained, Figs. 11 and 12. Simply, we can say that: x = Co–Ce and X = x/m.

Regression data and adsorption parameters are given in Table 10. Langmuir equation in its linear form points to the high successful application of activated carbon for the removal of metals. (Figs. 13–15)

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Figure 13

Relation between X and Ce of iron using activated carbon.

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Figure 14

Relation between X and Ce of manganese using activated carbon.

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Figure 15

Freundlich plot for iron adsorption onto activated carbon.

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4.1.2

4.1.2 Freundlich adsorption isotherms

A straight line with a slope 1/n and an intercept log KF is obtained. This reflects the satisfaction of the Freundlich isotherm model for the adsorption of Fe(III) Mn(II) ions. The log KF, is an indication of adsorption capacity and the slope, 1/n, is a criterion for adsorption intensity. The Freundlich parameters for the adsorption of the metal ions are given in Table 11 and Figs. 10–16.

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Figure 16

Freundlich plot for manganese adsorption onto activated carbon.

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It is apparent that Fe(III) and Mn(II) could be treated by activated carbon in sequence depending on the K_F, Xm, b, r² values. All these parameters are high for Fe with respect to Mn.

In spite of that both rice husk and activated carbon could be successfully used for the removal of Fe and Mn. But, it seems that rice husk is the best (as controlled from Xm and K_F values). In Egypt due to the problems that arise from the storage of rice and the firing clouds obtained from the husk, its use for the treatment is excellent for the costs and environmental aspects.