Waste garlic stems make an adsorbent for arsenic removal

Professor Monoj Kumar Mondal and Anuj Kumar Prajapati from the Dept. of Chemical Engineering and Technology, Indian Institute of Technology have used nanoporous activated garlic stem carbon (AGSC) — prepared from garlic stem waste — to remove arsenide ions [As(III)], from synthetic water samples and from groundwater.

They performed batch adsorption experiments to study the adsorption of As(III) onto AGSC. They obtained maximum removal of 93.3% of As(III) at optimum condition pH = 6, adsorbent dose of 5 g/L, equilibrium time of 150 min, initial As(III) concentration 400µg/L, and temperature 298K. The maximum adsorption capacity of AGSC for As(III) removal was found to be 192.30µg/g. The adsorption process of As(III) onto AGSC was shown to be both exothermic and spontaneous.

Arsenic is one of the most toxic metalloids present in groundwater and other water bodies. It is found in various oxidation states, but is mostly found in tri- and pentavalent form. The U.S. Environmental Protection Agency and the World Health Organisation have set the maximum allowable level for total arsenic in water for human use at 10µg/L. Several technologies have been used to keep the concentration of arsenic in water below 10µg/L, however these technologies often involve high process cost, low removal efficiency and toxic reagents. Adsorption can be economical if the adsorbent is prepared from abundantly available waste materials, is of inexpensive preparation, and exhibits good regeneration ability.

India is the world’s second largest producer of garlic. Garlic stem is a waste burned by farmers. This decreases air quality, and is one of the reasons why the authors of this study chose the waste garlic stem for producing active carbon.