# Biomass Fly Ash Self-Hardened Adsorbent Monoliths for Methylene Blue Removal from Aqueous Solutions
## 1 Introduction  
Global water demand for irrigation, domestic use, manufacturing, livestock, and electricity is expected to increase by 55% between 2000 and 2050 (3500 km\({}^{3}\) in 2000 for approximately 5500 km\({}^{3}\) by 2050) [1]. To avoid water shortage, the proper treatment of wastewater is of utmost importance [1] since it may provide a source of clean water and soften the water scarcity [2]. The use of dyes in industries such as textile, leather, paper and plastics generates considerable amounts of tinted wastewater [3]. In the textile industry, methylene blue (MB) is the most-used dye to colour cotton, wood, and silk [3]. Yet, MB may cause damaging consequences to human health, such as blindness, respiratory distress and abdominal disorders. Therefore, its removal from wastewaters is extremely important [3, 4]. adsorption is considered the most effective and simple method of water decontamination [2; 3]. It has a low cost, simple layout, easy operation and insensitivity to toxic pollutants [3]. Due to this, the number of published research works on MB removal from wastewater is very significant (5545 for \"adsorbents and methylene blue\", data obtained from the Scopus database in December 2021). However, the development of environmentally friendly low-cost adsorbent materials remains a challenge. Nowadays, the combustion of biomass is a relevant alternative technology to generate useful energy (heat and power) since it is considered carbon-neutral [5; 6; 7]. Nonetheless, this process produces huge quantities of biomass ash (BA). In 2018, it was estimated that nearly 10 million tonnes of BA were generated worldwide only for power production [8]. The amount is certainly much higher nowadays since new conversion plants were built due to the implementation of decarbonisation policies, such as the European Union's low-carbon economy roadmap [9]. The proper management of the produced ashes is essential for the sustainability of the biomass to energy practice [10]. One of the most used biomass combustion technologies for heat and power production is bubbling fluidised bed combustion (BFBC). BFBC generates two ash fluxes: biomass fly ash (BFA) that is collected in the super-heaters, economisers and air pollution control devices (cyclones, bag-filters, electrostatic precipitators); and, bottom bed ash (BBA), usually coarser in size, resulting from periodic discharges of the sand bed mixed with BA [10]. Both BFA and BBA are composed of particles from the original sand bed, inorganic content of the biomass, exogenous inorganic materials (soil and small stones) attached to biomass and unburnt fuel [10]. Commonly, these two streams of ash are landfilled [10], so their recycling is important from environmental and economic viewpoints. One innovative and environmentally friendly application for BFA is its use as a raw material for the production of bulk adsorbents, exploiting its self-hardening ability. BFA, when mixed with water, can harden [5; 6; 7] and form monoliths with enough mechanical strength for some applications. The adsorption capacity of the samples can be enhanced by increasing their porosity using a foaming agent, such as aluminium powder (AP). A dense structure limits the permeability to liquids, so the existence of interconnected pores will favour the adsorption performance. The use of bulk adsorbents represents an improvement on the use of BFA by itself because it will allow their direct utilisation in packed beds (powdered adsorbents may need to be used inside a container [2]) and, after use, they can be easily collected [4]. In this work, and for the first time, low-cost BFA self-hardened bulk absorbents with controlled porosity were produced. Their performance as MB adsorbents from aqueous solutions was evaluated in batch tests. A commercial Portland limestone cement (PLC) was added to prevent porous coalescence by accelerating the hardening process. PLC also enhances material strength, especially at an early age.