## 1 Introduction  
Significant quantities of wastewater are inevitably generated by industrial, agricultural, and domestic activities. Among many inorganic contaminants threatening water quality, such as cadmium, lead, caesium, arsenic, chromium and fluorine, lead (Pb) and arsenic (As) represent typical cationic and oxyanionic pollutants. Arsenic (as AsO\({}_{4}\)\({}^{3-}\)) is a carcinogenic poison occurring naturally or due to human activities. Its inorganic forms are highly toxic and accumulate in soils and sediments through iron binding [1]. Lead (as Pb\({}^{2+}\)) is one such element that has a well known chronic influence on the central nervous system, causing mental retardation, behavioural changes, paralysis and anaemia. Pb is easily accumulated in the body over a lifetime, yet its expulsion is very difficult. Despite the exclusion of tetraethyl lead, other Pb origins, such as battery manufacturers, lead smelters and ammunition industries are significant Pb polluters [2]. Although a number of methods are used in water treatment, such as coagulation, flocculation, precipitation, reverse osmosis, biological processes, gamma radiation and photocatalysis [3; 4; 5; 6; 7], adsorption procedures still take a favoured position due to their undoubted benefits in terms of affordability, easy operation and technical simplicity, and most importantly, the variability of usable adsorbents from a wide range of natural, synthetic and waste materials. Among these approaches, adsorption offers special advantages due to the possible use of a wide range of natural, synthetic, and waste materials to achieve pollutants' removal via chemisorption and/or physisorption [8; 9]. There is currently increasing attention in adsorption employing low-cost adsorbents, such as silicates and aluminosilicates, iron precipitates, biochar, etc. [10; 11; 12]. In this context, fly ash (FA) represents an interesting and promising material mostly utilised in the concrete industry [13; 14]. Furthermore, it finds applications in soil stabilisation, the synthesis of zeolites and now also in wastewater treatment [15; 16; 17]. Clays and other silicate minerals, including kaolin, are generally used in various industrial processes. Some traditional technologies, mostly the manufacturing of high-quality ceramics, paper production, etc., require a high purity and acceptable whiteness of clay material [18; 19]. Therefore, natural clays are also widely used in other industrial and environmental technologies. They have long been considered superior adsorbents due to their wide availability, excellent adsorption properties and environmentally friendly status. The coexistence of various contaminants in industrial and household wastewaters accelerated the development of novel composite adsorbents to overcome some limitations of commercially used one-component materials, such as a narrow selectivity to a specific ion and/or a relatively low adsorption ability towards modern contaminants, namely, antibiotics, aromatics and dyes [20]. Recently, a couple of studies have been focused on the preparation and characterisation of composite adsorbents. Unsurprisingly, aluminiosilicates were often incorporated into composite adsorbents thanks to the above declared benefits. [PERSON] et al. [21] prepared peltiesed adsorbent materials from agro-waste biomass, kaolinite and chitosan, which showed a comparable adsorption capacity to the more expensive food waste (spent coffee grounds). Clays such as bentonite, feldspar, kaolinite and montmorillonite were also mixed with, e.g., waste biomass, particularly with biochar [22; 23], or with FAs [16; 24]. It should be emphasised that the mentioned studies were mainly focused on the characterisation of the prepared composites, while the sorbability of specific contaminants was tested to a limited extent [25]. The aim of this study was to prepare and characterise composite adsorbents from kaolin (SK), as a representative of clays, and two FAs from a heating plant differing in the burned fuel.