The pHzpc of water bamboo husk particles was determined by
zeta potential at the range of pH 2–6. As illustrated in Fig. 3, the zeta potentialwas
positive (0.56 mV) at pH 2 but switched to negative at pH 3
(−5.78 mV). Accordingly, the pHzpc value of water bamboo husk particles
should be between 2 and 3, which is slightly lower than those reported
for Pu-erh tea powder (pHzpc = 3.6) [18] and tea waste
(pHzpc = 4.3) [21]. The zeta potential of water bamboo husk continued
to decrease with increasing pH and then remained stable at pH 6.
An optimal negative charge on particle surface was achieved at pH 5
(−26.40 mV). It is also worthy to note that the weakly acidic COOH
groups are usually the main sites on plantwaste sorbents for the uptake
of cationic substances [10,22]. The pKa of COOH group is about 3.5–5.5
so that a majority of carboxyl groups are deprotonated at pH values in
this range, resulting in more negatively-charged sites [22]. Our zeta potential
results are in good agreement with the above explanation.
Moreover, pH 5 is also the optimal value for trapping metal ions by
Ponkan mandarin peel particles [12]. Subsequently, in the following adsorption
processes, the solution pH was fixed at 5 in order to adsorb
plenty of positively-charged metal ions.