Adsorption Mechanisms and Structures of Arsenic Removed

Abstract

The toxicity of arsenic ions and salts in the environment (air, water, and land) has
made it among the most common elements that have to be removed or reduced in
the environment. This paper proposed possible mechanisms of arsenic removal or
reduction by powdered raw eggshells through adsorption techniques. Raw eggshells
from chickens were collected from an institutional farm in Nigeria. These collected
raw eggshells were washed with distilled water to remove impurities, air dried to
reduce the moisture content and powdered using a Silver Crest blender. Energy
Dispersive Spectroscopy and scanning electron microscope images of the powdered
eggshell were evaluated at selected magnifications. The study revealed that the
major elemental components of the powdered eggshells are aluminium (between
1.25 and 7.58 % of the adsorbent as aluminium), Calcium (between 3.7 % and 10.12
% of the adsorbent as calcium), Silicon (between 11.15 % and 25.62 % of the
adsorbent as silicon), Carbon (between 0.00 % and 27.75% of the adsorbent as
carbon) and oxygen (between 48.75and 58. 13% of the adsorbent as oxygen) had
the highest weight percentages. The other elements present in the adsorbent
structure were Na, Mg, K, and Fe. It was revealed based on the components of
eggshell and previous studies that the possible mechanisms of arsenic removal by
powdered eggshell include: ( ) 2 4 3s
Fe OH H AsO Fe OAsO H OH− −
 − + → − +2 2 4 2 4XMnOOH H AsO XMnOOH H AsO+ −
+ → −( )2 2 4 2 2 42
XMgOH H AsO XMgOH H AsO+ −
+ → −3 4 4 2SOH H AsO SHAsO H O H− +
+ + +( ) ( )2 4 2 2 4( )2 ss
M OH H AsO M OH H AsO− −
+  −
The study concluded that the removal or reduction of arsenic by powdered eggshells
is a function of the following factors pH of the aqueous solution, the composition of
the aqueous solution, the presence of hydroxyl groups ions and the nature of the
aqueous solution. The mechanism of arsenic removal includes redox, H-bond and
surface complexation formations, electrostatic attraction, oxic and anoxic
reactions.