higher than non-charged organics of the same size
(molecular weight). Kimura et al. [82] investigated
the rejection of organic micropollutants
categorized as DBPs and pharmaceutically active
compounds using polyamide NF/RO membranes
based on bench-scale filtration experiments. Their
results clearly showed that charged compounds
could be rejected to a great extent (i.e., >90%)
regardless of physicochemical properties of the
tested compounds. In contrast, rejection of noncharged
compounds was found to be influenced
mainly by the size of the compounds. This rejection
mechanism occurs as the negatively charged
solutes and the negatively charged membrane
surface prevent solutes from approaching and the
solutes are not allowed to permeate through the
membrane. The negative properties of membrane
surface usually are contributed by the sulfonic
and/or carboxylic acid groups which are deprotonated
at neutral pH [78,79]. Different pH conditions
will substantially change the membrane
surface charge. Studies [83,84] revealed that by
increasing pH, the negative surface charge of
membranes would be increased and thus higher
rejections would be expected especially for
negatively charged compounds.