Heavy metals discharge to the environment has become a point of attention for
many researchers due to their toxic and carcinogenic nature. Sources of heavy
metals can be natural (such as volcanic activity and weathering of rocks) or
anthropogenic (such as electroplating and metal finishing industries, metallurgical
industry, tannery, chemical manufacturing, mine drainage, battery manufacturing
and landfills leachates). The effluents from these sources contain Zn(II) along
with other toxic metals such as chromium, lead, cadmium, nickel, mercury and
copper. Zn(II) is an essential trace element for humans and takes part in a
number of enzymatic reactions. When concentration of Zn (II) increases above
a limit (e.g., the threshold limit value (TLV) for Zn(II) in drinking water
is 3 mg L-1), it becomes toxic to man, animals and plants life; causing
carcinogenesis, mutagenesis and teratogenesis as a result of its bioaccumulation
(Salim et al., 2003; ATCDR,
1993). Based on Malaysian Drinking Water Quality Standards, the maximum
acceptable limit of Zn(II) is 3 mg L-1, while WHO recommended as
5 mg L-1.
In order to safeguard public health, water and wastewater containing heavy
metals must be treated. Several techniques have been suggested for treatment
of wastewater containing heavy metals. These include precipitation, coagulation,
complexation, ozonation, ion exchange, electroplating and electrodialysis. The
problems associated with these techniques are high initial and operational costs,
high reagent and energy requirement and the generation of toxic sludge containing
heavy metal ions or waste products, which require proper disposal (Volesky,
1990; Pitsari et al., 2013).
Adsorption is one of the most effective techniques for the removal of various
pollutants including heavy metals from wastewater. Activated carbon is the most
widely used adsorbent but the high price of activated carbon limits its use
on a large scale (Isa et al., 2008). Hence, there
is a need to develop low-cost adsorbents that are effective for adsorptive removal
of heavy metals. Rice husk is an agricultural waste. It consists of cellulose
(32.23%), hemicelluloses (21.34%), lignin (21.44%) and mineral ash (15.05%)
(Rahman et al., 1997); with high percentage
of silica (96.34%) in the mineral ash (Rahman and Ismail,
1993). It is therefore expected that rice husk-based adsorbents would be
effective in adsorbing heavy metals from aqueous solution. However, the rice
husk needs to be modified or treated before being applied for adsorption of
heavy metals (Chakraborty et al., 2011). Chemical
or thermal treatment reduces cellulose, hemicelluloses and lignin crystallinity,
leading to an increase of specific area for adsorption (Daffalla
et al., 2010).
In this study, rice husk was treated with sulfuric acid to develop an Acidically
Prepared Rice Husk Carbon (APRHC). The efficiency of the produced carbon to
remove Zn(II) from aqueous solution was tested and adsorption mechanism was
investigated. The aim of the study was to develop a low-cost carbon for inexpensive
removal of heavy metals.
MATERIALS AND METHODS
Preparation of APRHC: Rice husk was collected from a rice mill and was
washed several times with tap water followed by distilled water in order to
remove dust and dried in an oven at 105°C for 24 h. The APRHC was prepared
as follows (El-Shafey, 2005). The cleaned and dried
rice husk (20 g) was weighed in a clean dry beaker of capacity 500 mL. One hundred
milliliters of 13 M sulphuric acid were added to the rice husk and the mixture
was heated at 175-180°C for 20 min with occasional stirring. The resulting
black mixture was allowed to cool in a desiccator to room temperature and then
filtered using a Buchner funnel under vacuum. The filtrate was then dried in
an oven at 120°C to constant weight and cooled in a desiccator. The resulting
APRHC was ground to a finer size of 212-500 μm and used in various adsorption
Adsorption experiments: Unless specified, batch adsorption experiments were carried out by shaking 100 mL of Zn(II) solution of desired concentration with 0.2 g of APRHC in a conical flask at room temperature (22°C), using an orbital shaker at 150 rpm. After a predetermined contact time, the flask was removed from the orbital shaker and the supernatant was filtered through 0.45 μm membrane filter and analysed for remaining Zn(II) concentration using a spectrophotometer. The effects of pH (1-6), contact time (5-120 min), Zn(II) concentration (20-80 mg L-1) and adsorbent dose (1-6 g L-1) on Zn(II) adsorption were also determined. The pH of the solution was adjusted by 0.1 N NaOH or 0.1 N HCl. Adsorption isotherms and kinetics were studied based on batch equilibrium test using optimum contact time and pH.
RESULTS AND DISCUSSION
Characterisation of APRHC: The values of BET surface area, micropore area, micropore volume and average pore diameter are 58.54 m2 g-1, 14.53 m2 g-1, 0.007209 mL g-1 and 45.46 Å, respectively. Scanning electron micrograph of APRHC (Fig. 1) shows presence of macro- and micro-pores.
|| Scanning electron micrograph of acidically prepared rice
||Effect of pH on adsorption of Zn(II) by acidically prepared
rice husk carbon
||Effect of contact time on adsorption of Zn(II) by acidically
prepared rice husk carbon
Effect of pH: pH of an aqueous solution is one of the important controlling
parameters in the adsorption process. In order to investigate the effect of
pH on the adsorption of Zn(II) by APRHC, pH of the solution was adjusted. The
pH range was chosen as 1-6 to avoid precipitation. It is found that pH affected
adsorption of Zn(II). Maximum adsorption of 92% for 20 mg L-1 Zn(II)
concentration occurred at pH 3 and the results are shown in Fig.
2. Similar observation has been found by Ghorbani et
al. (2012) for the removal of Zn ion from aqueous solution using polyaniline
nanocomposite coated on rice husk. All subsequent adsorption tests were conducted
at pH 3.
Effect of contact time and initial Zn(II) concentration: Effect of contact
time and initial Zn(II) concentration on adsorption by APRHC is shown in Fig.
3. The contact time was varied in the range 5-120 min at Zn(II) concentrations
20, 40, 60 and 80 mg L-1, APRHC dose of 2 g L-1 and pH
||Effect of APRHC dose on adsorption of Zn(II) by acidically
prepared rice husk carbon
The extent of Zn(II) adsorption increased as the initial Zn(II) concentration
decreased and as the contact time increased. Equilibrium adsorption was attained
in 60 min. A contact time of 60 min has also been reported for the adsorption
of Zn(II) by bed sediments (Jain, 2001). A contact
time of 60 min was used in further adsorption tests.
Effect of APRHC dose In order to assess the APRHC dose on Zn(II) adsorption, an experiment was conducted under optimum conditions (contact time 60 min and pH 3). The effect of APRHC dose (1-6 g L-1) on adsorption of Zn(II) is shown in Fig. 4. The adsorption was found to increase with APRHC dose and maximum adsorption of 97.2% for 80 mg L-1 Zn(II) concentration occurred at 4 g L-1.
Adsorption isotherm: In a solid-liquid system during adsorption, the
distribution ratio of the solute between the solid and the liquid phase is a
measure of the position of equilibrium. The preferred form of representing this
distribution is to express the solute adsorbed per unit weight of the solid
(qe) as a function of equilibrium concentration (Ce) at
a fixed temperature. This type of expression is known as an adsorption isotherm
(Weber, 1972). The Langmuir and Freundlich adsorption
isotherms are represented in Eq. 1 and 2,
where, Q° is the amount of solute adsorbed per unit weight of adsorbent
in forming a monolayer on the surface (monolayer adsorption capacity), b is
a constant related to the energy of adsorption, Kf is the Freundlich
constant (adsorption capacity) and 1/n represents the adsorption intensity of
Zn(II) by adsorbent or surface heterogeneity.
||(a) Langmuir isotherm and (b) Freundlich isotherm for Zn(II)
adsorption acidically prepared rice husk carbon
Linear forms of the Langmuir isotherm (Ce/qe = 1/(bQ°)+Ce/Q°)
and the Freundlich isotherm (log qe = log Kf+(1/n) log
Ce) were fitted to the adsorption data; shown in Fig.
5a and b, respectively. Langmuir constants Q° and
b were 44 and 0.06 and Freundlich constants Kf and 1/n were 7.31
and 0.38, respectively. The Freundlich isotherm model gave better fit of the
experimental data than Langmuir isotherm model.
Adsorption kinetics: For adsorption kinetics, the experimental data of Zn (II) adsorption by APRHC were applied to the two commonly used models, i.e., the pseudo-first-order and pseudo-second-order kinetic models. The linear form of the pseudo-first-order model is written as in Eq. 3:
||(a) Pseudo-first-order kinetic plot and (b) Pseudo-second-order
kinetic plot of Zn(II) adsorption by acidically prepared rice husk carbon
and the linear form of the pseudo-second-order model is in Eq. 4:
where, qe and qt are the amounts of metal ion adsorbed
(mg g-1) at equilibrium and any time t, respectively, k1
is the equilibrium rate constant for pseudo-first-order kinetics (min-1)
and k2 is the equilibrium rate constant for pseudo-second-order kinetics
[g (mg-1 min-1)]. Plots of log (qe-qt)
versus t and of t/qt versus t are presented in Fig.
6a and b, respectively. The experimental data fitted better
to pseudo-second-order kinetic model than pseudo-first-order kinetic model,
based on high R2 for pseudo-second-order kinetic model. Hence, it
is concluded that the adsorption of Zn (II) by APRHC is chemisorption. Similar
observations have been reported for removal of Zn(II) using carbonaceous sorbent
chemically prepared from rice husk (El-Shafey, 2010),
Van apple pulp (Depci et al., 2012) and marine
green algae (Kumar et al., 2007).
Acidically prepared rice husk carbon was found effective in the adsorption of Zn(II) from aqueous solution and maximum adsorption occurred in 60 min at pH 3. Freundlich isotherm model fitted the experimental data better than Langmuir isotherm model. Langmuir constants Q° and b were 44 and 0.06 and Freundlich constants Kf and 1/n were 7.31 and 0.38, respectively. The pseudo-second-order kinetic model fitted the experimental data well, indicating chemical adsorption of Zn(II). Being a low-cost agricultural by-product, rice husk is a good source for the preparation of rice husk carbon that can be employed as an effective adsorbent for the removal of Zn(II) and potentially for other heavy metals from aqueous solution and wastewater in developing countries.
The authors are thankful to Universiti Teknologi PETRONAS (UTP) for providing facilities for this research and Graduate Assistantship to the first author.