Here, we propose
a low-cost, sustainable, and viable adsorbent
(pine tree-derived biochar) to remove acid dyes such as acid violet
17 (AV), which is used in the silk dyeing industry. As a case study,
the AV removal process was demonstrated using synthetic effluent and
further as a proof of concept using real dye effluent produced from
the Sirumugai textile unit in India. The pine tree-derived biochar
was selected for removal of aqueous AV dye in batch and fixed-bed
column studies. The adsorbent material was characterized for crystallinity
(XRD), surface area (BET), surface morphology and elemental compositions
(SEM–EDX), thermal stability (TGA), weight loss (DGA), and
functional groups (FTIR). Batch sorption studies were performed to
evaluate (i) adsorption at various pH values (at pH 2 to 7), (ii)
isotherms (at 10, 25, and 35 °C) to assess the temperature effect
on the sorption efficiency, and (iii) kinetics to reveal the effect
of time, adsorbent dose, and initial concentration on the reaction
rate. After systematic evaluation, 2 g/L biochar, 25 mg/L AV, pH 3,
40 °C, and 40 and 360 min in a completely mixed batch study resulted
in 50 and 90% dye removal, respectively. The isoelectric point at
pH 3.7 ± 0.2 results in maximum dye removal, therefore suggesting
that monitoring the ratio of different effluent (acid/wash/dye) can
improve the colorant removal efficiency. The Langmuir isotherm best
fits with the sorption of AV to biochar, provided a maximal dye uptake
of 29 mg/g at 40 °C, showing that adsorption was endothermic.
Fixed-bed studies were conducted at room temperature with an initial
dye concentration of 25 and 50 mg/L. The glass columns were packed
with biochar (bed depth 20 cm, pore volume = 14 mL) at an initial
pH of 5.0 and a 10 mL/min flow rate for 120 min. Finally, the regeneration
of the adsorbent was achieved using desorption studies conducted under
the proposed experimental conditions resulted in 90–93% removal
of AV even after five cycles of regeneration.
