# Sustainable Conversion of Pulp Industry Sludge into Activated Biochar for High-Performance Methylene Blue Removal

## TL;DR

This paper shows how waste from the pulp industry can be turned into biochar that effectively removes blue dye from water.

## Contribution

A new method for converting pulp industry sludge into high-performance activated biochar for methylene blue removal is developed.

## Key findings

- Activated biochar produced at 450 °C showed the highest methylene blue removal efficiency and surface area.
- The Langmuir model best fits the adsorption data with a maximum capacity of 390.73 mg g–1.
- Adsorption is chemisorption-controlled, involving electron exchange with oxygenated surface groups.

## Abstract

The pulp and paper industry generates large amounts of
biological
sludge, which can be valorized into activated biochar (A-BC), offering
environmental and economic benefits. In this work, A-BCs were produced
from this residue using H3PO4 as an activating
agent and subjected to different pyrolysis temperatures (400 to 550
°C). H3PO4 was selected for its superior
activation performance over KOH, enhancing porosity and surface functionalization,
while the temperature was chosen to match the main thermal degradation
of the sludge’s lignocellulosic matrix. The A-BCs were characterized
by proximate and elemental analysis (fixed carbon content ∼30%),
FTIR (CO, O–H, O–Si–O, and PO
functional groups), XRD (predominantly amorphous structure), and Raman
spectroscopy (D and G bands). Furthermore, BET surface areas from
7.68 to 1.52 m2 g–1, a higher heating
value (HHV) from 3788 to 4750 kcal kg–1, and a point
of zero charge (pHPZC) from 3.31 to 6.15 were obtained.
Increasing the temperature from 400 to 450 °C increases surface
area via pore formation, while higher temperatures reduce porosity
due to pore collapse and lignin condensation. The A-BC produced at
450 °C (A-BC2) exhibited more than double the surface area and
higher methylene blue (MB) removal efficiency than the other samples,
consistent with the characterization results. The adsorption assays
indicated that the maximum adsorption capacity was 390.73 mg g–1, with the Langmuir model fitting the experimental
data best (R
2 = 0.996, R
2
adj = 0.995, and χ2 = 1.62).
The adsorption kinetics followed the pseudo-second-order model (R
2 = 0.982, R
2
adj = 0.981, and χ2 = 7.78), indicating a
chemisorption-controlled mechanism involving electron sharing or exchange
between cationic dyes and oxygenated biochar surface groups. The study
demonstrates that A-BC from cellulose industry sludge is a viable,
sustainable option for dye-containing effluent treatment, supporting
circular economy principles.