Studies on Zn (II) Ion Adsorption by Rice Husks Digested with Nitric Acid

The use of rice husks as activated carbon as a potential adsorbent for the removal of zinc
from aqueous solution has been investigated. The objectives of this experiment are to
create a highly available, effective and inexpensive adsorbent for adsorption process in
removal of metal ions that present in waste water streams. Besides that, characteristics of
rice husk as adsorbent are carefully studied.
Due to health, safety and environment concerns, highly toxic metals that present
in waste stream are dangerous and required to be eliminated through a chemical process
called adsorption. Generally, adsorption is done in the plant using adsorption column,
conventional methods such as ion exchange process, and other biological processes.
Since, the conventional method consumes a lot of money hence, a newer and cost
effective method is applied by using agriculture wastes which face highly disposal
problem or other activated carbon from a variety of materials such as peach stones,
almond shells, peanut skins and others.
This laboratory scale experiment was conducted by adsorbing 25 ml of 25 ppm
of Zn2+ solution with digested and carbonized rice husk into 500 ml conical flask.
Agitation of sample was operated at 150 rpm, 2 hours at room temperature. Adsorption
sample was filtered and the filtrate solution was tested on Atomic Absorption
Spectrophotometer (AAS) to determine concentration left after adsorption. The AAS
results were analyzed basedon various factors tested. Brunauer - Emmet - Teller(BET)
method was studied and Scanning Electron Micrscopy (SEM) was employed to
characterize the rice husk fibrous structures.
Scopes of this experiment which are on the effects of various factors, e.g., nitric
acid concentration, carbonization temperature and time, particle size of rice husk, rice
husk loading, adsorption contact time and shaker speed on adsorption capacity, were
quantitatively determined.
Nitric acid concentration effect at 1.0Mfor acid treatment, 500 °C and 1 hour
carbonization prove the highest adsorption capacity. Particle size at 0.500 mm has the
greatest effect to adsorb Zn2+ in terms of surface area and engineering economic
analysis. Higher mass loading was found to adsorb more Zn2+ quantity and at 30th
minute, adsorption equilibrium had been achieved. During adsorption experiment,
shaker speed of 200 rpm recorded the highest Zn2+ uptake. The two-parameter
equilibrium models (Langmuir and Freundlich equations) were discussed. It was found
that Freundlich best fitted on the experiment.
Overall, rice husk has a potential for removing Zn2+ in waste stream but is
required to improve further on the rice husk preparation. Future work discussed is
required to be alarmed for the betterment of this process technology.