RHC was ground to powder (particle size 6 0.28 nm) and a 6 g
sample was sulfonated using 60 mL concentrated sulfuric acid
(95–98%) for 0.5 h in a 250 mL flask controlled at 90 C in an oil
bath. After cooling to ambient temperature, the mixture was added
to distilled water, stirred, and filtrated. Then the precipitate was
washed repeatedly with hot distilled water until the filtrate was
free from sulfate ions. Following filtration, the sample was dried
at 80 C for 24 h in an air-drying oven.