Sewage Sludge Hydrochar: An Option for Removal of Methylene Blue from Wastewater

TLDR

Adsorption of dyes involves multiple phenomena such as physio‑ and chemisorption, acid–base, and redox equilibria. The study develops hydrochar from municipal sewage sludge via hydrothermal carbonization to achieve high adsorption capacity for dye removal. Three hydrochars were produced at 190–250 °C for 3 h, some KOH‑treated, and their physicochemical properties and methylene‑blue adsorption were characterized in batch studies. KOH‑modified hydrochars removed over 95 % of methylene blue within 15 min, outperforming raw chars, especially at high concentrations, due to surface homogenization, notably for the 250 °C sample.

Abstract

Municipal sewage sludge was subjected to a hydrothermal carbonization (HTC) process for developing a hydrochar with high adsorption capacity for water remediation in terms of dye removal. Three hydrochars were produced from municipal sewage sludge by performing HTC at 190, 220 and 250 °C, with a 3 h reaction time. Moreover, a portion of each hydrochar was subjected to a post-treatment with KOH in order to increase the adsorption capacity. Physicochemical properties of sludge samples, raw hydrochars and KOH-modified hydrochars were measured and batch adsorption studies were performed using methylene blue (MB) as a reference dye. Data revealed that both raw and modified hydrochars reached good MB removal efficiency for solutions with low MB concentrations; on the contrary, MB in high concentration solutions was efficiently removed only by modified hydrochars. Interestingly, the KOH treatment greatly improved the MB adsorption rate; the modified hydrochars were capable of capturing above 95% of the initial MB amount in less than 15 min. The physicochemical characterization indicates that alkali modification caused a change in the hydrochar surface making it more chemically homogeneous, which is particularly evident for the 250 °C hydrochar. Thus, the adsorption process can be regarded as a complex result of various phenomena, including physi- and chemi-sorption, acid–base and redox equilibria.