Starch And Cyclodextrin Based Adsorbents For Removal And Preconcentration Of Polycyclic Aromatic Hydrocarbons And Phthalates In Water

ABSTRACT

The release of Polycyclic Aromatic Hydrocarbons (PAHs) and phthalates from

anthropogenic sources into the aquatic environment is of public health concern. The C18

bonded silica and styrene-divinyl-benzene polymer adsorbents which are used for

preconcentration and remediation of PAHs and phthalates polluted water are expensive

and non-biodegradable. Hence, there is need to source for alternative adsorbents. Starch

and cyclodextrin based polymers have been reported to be cheap and biodegradable, but

have not been applied as adsorbents for PAHs and phthalates. In this study, starch and

cyclodextrin based adsorbents were synthesised and employed for removal and

preconcentration of PAHs and phthalates in water.

Starch (15.0 g), β-cyclodextrin (12.5 g), and γ-cyclodextrin (8.0 g) were cross-linked with

epichlorohydrin (EPI) (34.0-340.1 mM), 1,6-hexamethylene diisocyanate, (HDI) (7.04-

70.4 mM), and 4,4-methylene diphenyl diisocyanate, (MDI) (7.04-70.4 mM) to produce

EPI, HDI, and MDI cross-linked adsorbents [EPI-starch, EPI-β-cyclodextrin, and EPI-γ-

cyclodextrin; HDI-starch, HDI-β-cyclodextrin, and HDI-γ-cyclodextrin; and MDI-starch,

MDI-β-cyclodextrin, and MDI-γ-cyclodextrin]. The adsorbents were characterised using

infrared spectrophotometry, Brauner-Emmet-Teller surface analysis, Scanning Electron

Microscopy (SEM) and elemental analysis. Effects of time, temperature, and initial

adsorbate concentration on the adsorption of PAHs (acenaphthylene, phenanthrene,

fluorene, benzo(a)anthracene) and phthalates (dimethyl and diethyl phthalates) were

studied using standard methods. Data generated were used to study the adsorption

kinetics and thermodynamics of the adsorption process, and also fitted to four isotherm

models. The adsorbents’ efficiencies were evaluated by their respective adsorption

coefficients,

d K . The preconcentration studies applied off-line column Solid Phase

Extraction (SPE) standard procedure using the adsorbents (250 mg) as the solid phase and

the adsorbents were validated by recovery studies and detection limit.

The observed infra-red peaks of aromatic (3036, 1598, and 819 cm–1), amine (1721 and

751 cm–1), and carbonyl (1671 cm–1) functional groups in the adsorbents indicated

successful cross-linking process. Surface areas of adsorbents (3.3-40.4 m2/g) were higher

than those of native starch (1.0 m2/g) and cyclodextrins (0.7 m2/g). Increment in porosity

UNIVERSITY OF IBADAN LIBRARY

iii

was observed from SEM images, which confirmed the enhancement of surface area of

adsorbents. The adsorbents had higher carbon content (42.7-59.9%) and lower hydrogen

content (5.7-7.6%), an indication of increased hydrophobicity. The adsorption data for

PAHs and phthalates were best described by pseudo-second order kinetics (r2>0.996),

which confirmed that surface adsorption was the rate-limiting step. The adsorption free

energy values were negative, and thus confirmed the spontaneity of the adsorption process.

Freundlich isotherm (r2>0.746) best described PAHs adsorption, an indication of

multilayer adsorption; while phthalate adsorption was best fitted by Langmuir isotherm

(r2>0.882), which was suggestive of monolayer adsorption. The log d K

values of the

adsorbents (4.0-5.0 for PAHs and 2.7-3.0 for phthalates) indicated good sorption

efficiency. The values of recoveries (71.7-126.0% for PAHs and 81.5-104.6% for

phthalates) and detection limit (0.9–153.4 ng/L for PAHs and 78.1-117.3 ng/L for

phthalates) indicated high analytical performance for SPE preconcentration method using

the adsorbents.

Starch and cyclodextrin based adsorbents were effective in the removal and

preconcentration of polycyclic aromatic hydrocarbons and phthalates in water; and hence

are potential alternatives for the control of these chemical pollutants.