From Fig. 4a, the adsorption capacities of the GO series would
increase as oxidation degree increase due to the fact that there are
more active adsorption sites in GO at higher OD, which is fully consistent
with the results of zeta potentials. More interestingly, MB
uptake of each graphite-based sample is almost pH independent in
measured range, but its negative charge is enhanced with increasing
pH as mentioned above. It may be ascribed to the fact that MB
has stronger affinity to GO, which could compete with proton and
seize the active adsorption site on GO even at low pH.