Adsorption of Trace Levels of Arsenic from Aqueous Solutions
by Conditioned Layered Double Hydroxides: Batch and Flow
Megha Dadwhal, Muhammad Sahimi, and Theodore T. Tsotsis
Arsenic is found in water in the form of oxyanions. Relatively high
concentrations of As have been reported both in power plant discharges,
as well as, in fresh water supplies. Inorganic As may be present in the
3+ (As(III), arsenite) and 5+ (As(V), arsenate) oxidation states1.
Protracted contact with As-containing water is thought to cause
arsenicosis, a form of poisoning in humans2. The International Agency
for Research on Cancer (IARC) currently classifies As as a group 1
chemical, which is considered to be carcinogenic to humans.
The focus of the present work is to do a systematic study of the
adsorption of As by conditioned calcined layered double hydroxide
(LDH) adsorbents. Conditioning the adsorbent significantly reduced the
dissolution observed with uncalcined and calcined LDH3. The adsorption
rates and isotherms have been investigated in batch experiments using
particles of four different particle size ranges. As(V) adsorption is shown
to follow a Sips-type adsorption isotherm. The As(V) adsorption rate on
conditioned LDH increases with decreasing adsorbent particle size; the
adsorption capacity, on the other hand, is independent of particle size.
A homogeneous surface diffusion model and a bidisperse pore model
were used to fit the experimental kinetic data. The homogenous surface
diffusion model estimated diffusivity values dependent on the particle
size, whereas the bidisperse pore model predicted an intracrystalline
diffusivity, which is fairly invariant with particle size4.
The removal of As(V) on conditioned, calcined LDH adsorbents was also
investigated in continuous operations. In packed-bed column
experiments, the impact of important solution and operational
parameters such as influent As concentration, pH, sorbent particle size
and flow rate were studied. An early breakthrough and saturation was
observed at higher flow rates and at higher influent concentrations. On
the other hand, a decrease in the sorbent particle size and decrease in
influent pH resulted in an increase in the bed volumes at breakthrough.
A film surface diffusion model, which accounts for both the external
film diffusion in the liquid phase and intraparticle diffusion resistance
within the sorbent, has been developed to simulate the fixed bed
sorption of As (V) onto LDH. Mass transfer coefficients were calculated
using correlations. Predicted simulation results were found to be in
close proximity to the experimental results.
(1) Ferguson, J.F.; Gavis, J.; A Review of the Arsenic Cycle in Natural
Waters. Water Res. 1972, 6, 1259.
(2) Lepkowski, W.; Arsenic Crisis in Bangladesh. Chem. Eng. News
1998, 76 (46), 27.
(3) Yang, L.; Shahrivari, Z.; Liu P.K.T.; Sahimi, M.; Tsotsis, T.T.
Removal of Trace Levels of Arsenic and Selenium from Aqueous
Solutions by Calcined and Uncalcined Layered Double Hydroxides (LDH).
Industrial & Engineering Chemistry Research, 2005, 44(17), 6804-6815.
(4) Yang, L; Dadwhal, M; Shahrivari, Z; Ostwal, M; Liu, P.K.T; Sahimi,
M; Tsotsis, T.T. Adsorption of Arsenic on Layered Double Hydroxides:
Effect of the Particle Size. Industrial & Engineering Chemistry Research,
2006, 45(13), 4742-4751.
Keywords: Layered double hydroxides, Hydrotalcites, Arsenic,
Adsorption, Packed bed Columns