Production of magnetic sewage sludge biochar: investigation of the activation mechanism and effect of the activating agent and temperature

Irene Sierra,Unai Iriarte-Velasco,Jose L Ayastuy,Andrés T Aguayo

doi:10.1007/s13399-022-02372-w

Irene Sierra, Unai Iriarte-Velasco + Show 2 more

Open Access

https://doi.org/10.1007/s13399-022-02372-w

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Abstract

Sewage sludge-based porous materials were prepared through an integrated and energy-saving procedure that combines chemical activation with an alkali (NaOH or K2CO3) and physical activation with CO2 in a single step. The study was conducted in a wide temperature range (600–1000 °C), using higher temperatures than those commonly used. From the standpoint of textural properties, the impregnation results in a decrease in the optimum activation temperature: 800 °C for untreated samples, and 600–700 °C for impregnated samples. The impregnation increases the amount of –OH and –NH functional groups, and it also leads to the generation of new oxygen- and/or nitrogen-containing functionalities (above 800 °C for the impregnation with K2CO3 and in the whole temperature range for the impregnation with NaOH). The chemical treatment favors the development of magnetic properties on biochars, which is an advantage for its reuse in wastewater treatment, since it favors the removal of anionic surfactants and heavy metals such as lead, copper, zinc, and manganese. The uptake of phenol and methylene blue (MB) is maximized with the combination of the impregnation (with NaOH for phenol and K2CO3 for MB) and the use of a moderate temperature (600–700 °C). The optimum uptake values represent an increase of 87% and 152% for MB and phenol, respectively, compared to the highest value of non-impregnated samples (activated at 800 °C), with the advantage of the lower temperature required.Graphical abstract

Highlights

The amount of sewage sludge is increasing due to population growth, urbanization, and industrial development, and more stringent standards for the discharge of wastewater to aquatic bodies
The objective of the present paper is to investigate the preparation of porous biochars through a procedure that combines chemical activation with an alkali (NaOH or K2CO3) and physical activation with CO2 in a single step
The weight loss of raw sewage sludge during the pyrolysis under nitrogen atmosphere can be divided into three main sections (Fig. 1), in good agreement with the profiles reported in the literature [45]

Summary

Introduction

The amount of sewage sludge is increasing due to population growth, urbanization, and industrial development, and more stringent standards for the discharge of wastewater to aquatic bodies. Sewage sludge is mainly composed of substances responsible for the toxic and pathogenic nature of non-treated wastewater and, its management is an issue of particular concern. Sewage sludge is a heterogeneous mixture of water, organic matter, microorganisms, and inorganic substances. Dry sewage sludge contains nitrogen and phosphorus in different forms. Sludge contains high concentrations of inorganic salts including ions ( CO32−, PO43−, SO42−, and NO3−), heavy metals (e.g., Zn, Pb, Ni, Cd, Cr, Cu, As, and Hg), and other elements (e.g., Si, Al, K, Na, Ca, and Mg) [2]. The composition of sewage sludge is highly variable, since it is affected by different parameters such as the specificity of the sludge source

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