Color Removal from Anaerobically Digested Sugar Cane Stillage by Biomass from Invasive Macrophytes

The ability of untreated and acid-treated biomass from Pistia stratiotes (PL and APL, respectively) and Eichhornia crassipes (ELS and AELS, respectively) to remove color from anaerobically digested sugar cane stillage (ADS) was investigated. The effects of pH (3–8), particle size (< 0.75, 0.75–1, 1–4 mm), and biomass concentration (5–15 g/L) on decolorization of ADS were assessed using untreated biomass. After acid modification of biomass (acid-treated), the effects of pH (3–8), biomass concentration (6–10 g/L), time (20–480 min), and ADS dilution (non-diluted, 1:2, 1:10, 1:20) on color removal from ADS were evaluated. Scanning electron microscopy and Fourier transform infrared spectroscopy (FTIR) analyses were also performed. A clear effect of particle size on ADS decolorization was found (21.04 ± 0.75 and 27.87 ± 0.30 % for 0.75–1 and <0.75 mm, respectively, for ELS; 31.65 ± 0.23 and 37.82 ± 0.53 for 1–4 and 0.75–1 mm, respectively, for PL). Decolorization also increased when the untreated biomass concentration was higher (15.41 ± 0.3 and 27.89 ± 0.2 % for 5 and 10 g/L, respectively, for ELS; 15.61 ± 0.11 and 33.06 ± 1.09 % for 5 and 10 g/L, respectively, for PL). The use of acid-treated biomass enhanced the effect of pH on color removal (48.30 ± 1.27 and 12.96 ± 0.27 % for pH of 3 and 7, respectively, for AELS; 47.11 ± 1.72 and 6.62 ± 0.21 % for pH of 3 and 7, respectively, for APL). The highest rate of color removal obtained using acid-treated biomass was 55.58 ± 1.82 and 56 ± 0.77 % for AELS and APL, respectively. The FTIR spectra analysis suggested the electrostatic attraction between protonated carboxylic groups on biomass and anionic colored compounds as being one of the adsorption mechanisms for ADS decolorization. The use of dry biomass from invasive macrophytes is an effective alternative for color removal from ADS.