Metal oxides (solid or porous) contain large no. of hydroxyl groups on their surface. Hence bare particles tend to aggregate faster in real applications in which their colloidal suspension is prepared in water or other disperdents. With DLS technique you should determine the average particle size(hydrodynamic diameter) of your cubes. Then construct the aggregation profile to know its colloidal stability. Check the zeta potential values at various pH to know the iso-electric point of your material. In the application/s you are mentioning normally the material is tested at various pH so this Zp curve will give you information about charges on your cubes at various pH. For testing you may need to functionalize your cubes with organic compounds or polymers to increase their interactions with the target metal ions. You will get lot of papers in the literature about use of porous materials for metal ion removal or trapping.

Hi  Jinlin 

Currently widely used sorbents are activated carbon or zeolites. In your case you are going to use cobalt oxide. This oxide may be dissolved in certain condition (e.g. if solution will contain HNO3 or even worst mixture of HNO3 and H2O2) so before use of this material you need to take into account all of these restrictions. Also very important is the type of pores. In general based on pore types the porous materials can be divided as open pore, close, interconnected, etc. for sorbents it is very important pore type which should be open or interconnected. If you have body with high porosity but pores are close type then it can’t be used as sorbent. Therefore in very beginning you need to establish pores’ type.    

In the thermodynamics viewpoint the metal ion sorption are divided in two common types: endothermal and exothermal sorption processes. Depending on which kind of ions will be absorbed it can be estimated whether heat treatment will be needed or not in your case.

For more detail please refer to these two reviews which may be helpful for your research.

http://engj.org/index.php/ej/article/viewFile/341/276

http://test.benthamopen.com/contents/pdf/TOCOLLSJ/TOCOLLSJ-4-19.pdf

 @Sushilkumar A. Jadhav

First of all, thanks a lot for your answers to my question. You know that Ni ions are positive, so the surface charge of Co3O4 should be negative and the larger electrical potential difference, the more adsorption.  So if porous cobalt oxides were dispersed in nickel acetate, which is alkalescent, will the successful adsorption occur?  Or else, if I put the Co3O4 with an average size of 200 nm into alkaline solution as pre-treatment, will the aggregation be reduced ( It is known to us all that particles with the same charges (negatively charged or positively charged) will not aggregate.) and make for the adsorption of metal ions? Last but no least, although my major is materials science,  I have no chance to study relevant subjects on  hydrodynamics and I am eager to teach myself about it, you know that it is always hard  for a novice to choose a proper book, so could you recommend some valuable books about hydrodynamics. Thanks very much.

@Hayk L. Khachatryan 

Thanks a lot for your advice and paper, which benefit me a lot. Now, I am sure that my porous materials are interconnected. Then how can I determinate whether the metal ion sorption is endothermal or exothermal? I prefer to adsorb the divalent Ni ions.

Dear Jinlin,

I would like to add an information that the adsorption on metal oxides pores or  surface is a physical process not chemical process. therefore, we should not say chemically adsorb metal ions.

Best regards and wishes from Jordan,

Ayoup Ghrair