Dear Dr Alemu: The point of zero charge is the pH at which the surface of your adsorbent is globally neutral, i.e., contains as much positively charged as negatively charged surface functions. Below this value, the surface is positively charged; beyond this value, it is negatively charged. For more on this have a look at:

https://www.researchgate.net/post/What_is_use_of_point_of_zero_charge_of_any_adsorbent_How_can_we_use_that_value_to_support_the_results_of_experiment#:~:text=The%20point%20of%20zero%20charge,value%2C%20it%20is%20negatively%20charged.

Regards--

Nice question