(Shimadzu, Japan) with detecting wavelength at λ 400
nm (Zhang and Bai, 2003; Doulia et al., 2009; Zhang et
al., 2012; Zulfikar et al., 2013). As the absorbance was
pH-dependent, calibration lines were made for each
required pH in the study. The percent of humic acid
removal from peat water was calculated using the
following equation: