Qualitative and quantitative determination of critical coagulation concentration for pristine graphene oxide in various ionic compounds

Graphene oxide (GO) nanoparticles have been widely used in wastewater treatment applications. However, most studies have conducted treatment experiments using synthetic wastewater that does not contain ionic com-pounds. The presence of ionic compounds could suppress the electrical double layer of the GO nanoparticles and thus, promote particle agglomeration. This study aims to evaluate the effect of ion valency (Na+, Ca2+, Al3+, Cl-, and SO42-) and concentration (1 mM up to 200 mM) on the colloidal stability of a pristine GO. The sedimentation profile of GO in those ionic solutions was captured through a novel qualitative photoimaging method coupled with ImageJ analysis. Results showed that a GO with O/C ratio of 0.69 experiences sedimentation even when dispersed in pure distilled water. In line with the classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, the extent of sedimentation increases with the increasing ion concentration in the medium. In terms of cationic valences, trivalent cation (Al3+) was more effective than divalent (Ca2+) and monovalent (Na+) cations in destabilizing GO; here, the corresponding critical coagulation concentration (CCC) was found to be-1 mM,-1.5 mM, and-150 mM, respectively. It was found that NaCl exhibited a higher destabilization effect as compared to Na2SO4 despite the latter is having a higher ionic strength. Besides the expected electrical double layer screening effect, density of the ionic solution also showed substantial effects on the GO's sedimentation profile. The obtained results align with other reported works, thus, confirming the feasibility of the novel photoimaging coupled with ImageJ processing method in assessing colloidal stability of nanoparticles.