(73c) Overcoming Kinetic Limitations of Cr(VI) Adsorption Onto Biosorbents: Biomas-Magnetite Bionanocomposite

INTRODUCTION

Biosorbents
are a promising class of adsorbents discovered in the last decade for their
potential application in water and waste water treatment (Abdolali et al., 2014). One class of biosorbents
are the lignocellulosic materials obtained from plant wastes or by-products (Ngah and Hanafiah, 2008). Lignocellulosic
materials are composed of macromolecules such as lignin, cellulose and hemicellulose
compounds  which are known to contain groups that exhibits exchange and
complexating properties (Ofomaja and Ho, 2007). The lignin
portion of lignocellulosics acts a binding agent holding the celluloses
fraction to the hemicellulose fraction and keeps the plant materials erect and
firm. As a result particles of biosorbents materials applied as adsorbents
possess very low porosity and internal surface causing restriction of fluid
flow.

This
feature of biomasses, as they are called generally leads to several challenges
when during application of biomasses as adsorbents and these includes (i) low
diffusion rates of pollutants within the adsorbent particles during adsorption (Thinh
et al., 2013), (ii) high operating pressures when applied in packed columns
(Yavuz et al., 2006), low adsorption
rates and capacities (Thinh et al., 2013). To overcome these
shortcomings researchers have resorted to reducing the sizes of the biomass
materials by crushing or grinding them to smaller particles sizes. Small
particle sized materials for adsorptions are also known to come with its
challenge, which is the separation of the adsorbent particles from the treated
solution at the end of contact (Safarik et al.,
2007).

Currently
researchers have turned to the use of bio-nanocomposites consisting
biomaterials coated with magnetic nanoparticles to overcome these limitations.
Although much success have been achieved in applying these novel biomaterials
composites yet little is known about how and to what extent these limitations
are dealt with. This paper therefore seeks to study how and to what extent to
which kinetic limitations are avoided when pine biomass coated with magnetite
is applied as adsorbent for Cr(VI) from aqueous solution.

METHODS:

Pine cone was treated
with 0.15 mol/dm³ NaOH and the 1.0 g of the product added to a
solution containing a mixture of Fe²⁺/Fe³⁺ of molar ratio
2:1. Magnetic was then precipitated onto the pine surface using NH₄OH.
The biocomposite and the raw pine biomass were characterized. Batch kinetic
adsorption studies of Cr(VI) from aqueous solution was performed using both
materials from a 150 mg/dm³ solution at 26, 31, 36, 41 and 46 ^oC.
Three nonlinear kinetic and three diffusion models were applied to test the
kinetic data. 

RESULTS:

FTIR
spectra for the NaOH treated and MNP-Pine composite are shown in Figs. 1 and 2.
The presence of Fe-O and Fe-OH bonds in the composite confirms that presence of
magnetite in the bio-nanocomposite.

Fig.
1: FTIR of NaOH treated pine

Fig. 2: FTIR of MNP-Pine
Composite.

Surface characteristics
of the NaOH treated and the bio-composite are shown in Table 1. The results
shows that an increase of BET surface from 2.67 m²/g to 68.91 m²/g
in the bio-composite. The external surface and pore width were also increased
drastically with the composite. The small size of the magnetite coating as well
as the possibility of magnetite formation within the pores of the pine causing
expansion and breakup of the pine particles may be responsible for this
happening. 

The kinetic data for
the adsorption of Cr(VI) onto NaOH and MNP-Pine composite were fitted to the
intraparticle diffusion model in Fig. 3. The results indicate the while the
metal uptake per square-root of time profile for the NaOH treated pine can be
broken into two sections, the MNP-Pine composite profile can be broken into
three sections.

 Fig.
3: Intraparticle Diffusion plots for NaOH treated pine and MNP-Pine.

The implication of this
is the decrease in kinetic restriction by the creation of increased internal
surface in the composite. The intraparticle diffusion constants were found to
be higher for the bio-composite than for the NaOH treated pine signifying free
flow of aqueous solution through the internal surface of the material. The
relationship between intraparticle diffusion constant and temperature is shown
in Table 2. Also it was observed that the constants increased with increasing
solution temperature. Increasing solution temperature increases the rate of
migration which is responsible for the increased diffusion of the pollutant
through the adsorbent material.

CONCLUSION:

Kinetic limitations
associated with the use of biomaterials as adsorbents can be eliminated by coating
of biomaterials with nanoparticles. The resulting bio-composite processes better
surface properties and improved diffusion properties for pollution removal from
aqueous solution.

References

Safarik, I., Lunackova,
P., Mosiniewicz-Szablewska, E., Weyda, F., Safarikova, M. Adsorption of
water-soluble organic dyes on ferrofluid-modified sawdust. Holzforschung 61
(2007) 247-253.

Yavuz, H., Denizli, A.,
Güngünes, H., Safarikova, M., Safarik, I. Biosorption of mercury on
magnetically modified yeast cells. Separation and Purification technology 52
(2006) 253-260

Abdolali, a., Guo,
W.S., Ngo, H.H., Chen, S.S., Nguyen, N.C., Tung, K.L. Typical linocellulosic
wastes and by-products for biosorption process in water and wastewater:
Bioresource Technology 160 (2014) 57-66.

Ngah.,W.S.W., Hanafiah,
M.A.K.M. Removal of heavy metal ions from wastewater by chemically modified
plant waste as adsorbents: A review. Bioresource Technology 99 (2008) 3935-3948.

Table
1: Surface Properties

Properties                                NaOH
treated Pine                 MNP-Pine Composite

Surface area (m²/g)                  2.67                                         68.91   

Pore volume (cm³/g)                0.002                                       0.153                                      

External surface (m²/g)           2.25                                         48.79

Ave.
pore width (nm)                         8.89                                         33.10

Table 2: Diffusion parameters for Cr(VI) adsorption onto NaOH treated and MNP-Pine at
different temperatures

Temp.
(^oC)                               Intraparticle diffusion rate
constant (mg/g min^0.5)

                                    NaOH
Treated Pine                            MNP-Pine Composite

                                    k_i1                                                        k_i1                                k_i2

26	0.1465	0.1404	0.6828
31	0.1994	0.2192	0.8464
36	0.2450	0.2662	1.0963
41	0.2690	0.3142	1.3046
46	0.3110	0.3789	1.7028