About 0.2 g of acid-washed raw material and biochar was
shaken vigorously for 5 min with 25 mL of 0.1 mM methylene blue
and allowed to stand for 24 h at room temperature for the methylene
blue adsorption analysis. Absorbance of the dilute filtrate was
measured with a spectrophotometer at 660 nm. Adsorption
capacity was calculated as: