Abstract:
Groundwater contamination with elevated levels of fluoride has been an age-long
environmental problem in many countries including Nigeria. Adsorption is an effective
defluoridation technique; however, most of the effective adsorbents are not readily available. It
is therefore necessary to explore the adsorptive potentials of local materials such as Raffia
Palm Shells (RPS) for fluoride containment in groundwater. This study was designed to
produce and characterise activated carbon from RPS for groundwater defluoridation.
Fluoride contents of groundwater in Makurdi, Nigeria, were determined using 63 samples
collected from boreholes in 21 locations following standard procedures. The USEPA Hazard
Quotient (HQ) was used to evaluate the human health risk potentials in relation to fluoride
contamination for infants, children, teenagers and adults. The RPS were sourced from Ugbema
market in Benue state and processed into activated carbon using phosphoric acid as activating
agent. Response Surface Methodology (RSM) was used to optimise the quality (Specific
Surface Area (SSA) and Carbon Yield (CY)) of Raffia Palm Shell Activated Carbon (RPSAC).
The surface of RPSAC was coated with aluminium hydroxide to produce Aluminium-oxideCoated-RPSAC (ACRPSAC) using functionalization principle. Physical characteristics
(Brunauer-Emmett-Teller Surface Area (BETSA), Bulk Density (BD), Moisture Content
(MC), Total Pore Volume (TPV), Average Pore Diameter (APD) and pHpzc) of the adsorbents
were determined using standard methods. Adsorbents’ surface compositions were determined
by SEM/EDX, FTIR and XRD analyses. Groundwater defluoridation potentials of the
adsorbents were evaluated using batch adsorption method in comparison with a Commercial
Activated Carbon (CAC). Linear regression and ANOVA at α0.05 were used to analyse the data
sets.
The fluoride contents in the water exceeded the WHO limit of 1.5 mg/L in 33.3 % of the
samples and ranged from 0.32 – 2.06 mg/L (mean=1.26±0.41). The HQ exceeded the threshold
value of 1 in 66.7, 71.4, 52.4 and 9.5 % of the water samples for infants, children, teenagers
and adults, respectively. Optimum conditions for the synthesis of RPSAC were 524 oC, 77.0
%, 4.00 g/mL and 104 minutes for temperature, concentration, impregnation ratio and time,
respectively. The optimized values of SSA and CY were 1762.93 m2/g and 78.0 %,
respectively. The physical characteristics of RPSAC and ACRPSAC were 456.1 and 715.8
m2/g, 0.45 and 0.37 g/cm3, 18.5 and 4.2 %, 0.25 and 0.47 cm3/g, 2.13 and 1.85 nm, 2.10 and
4.05 for BETSA, BD, MC, TPV, APD and pHpzc respectively. The SEM/EDX showed that the
adsorbents had both micro and meso-porosities. The abundance of hydroxyl functional groups
on the adsorbents’ surface was evident. The RPSAC was found to be amorphous, while the
ACRPSAC was microcrystalline due to the formation of graphite-like structures. Batch
fluoride adsorption performances of the adsorbents were in the order of ACRPSAC > RPSAC
> CAC (removal efficiency= 80.0–99.0%) and were significantly different. Fluoride removal
obeyed the Langmuir (R2=0.8802–0.9751) and Pseudo second order (R2=0.9974–0.9999)
models which signified that its adsorption by the adsorbents was chemisorption-controlled.
Aluminium-oxide-coated raffia palm shell activated carbon is a suitable adsorbent for
groundwater defluoridation in batch systems.
