# Sewage Sludge Derived Materials for CO2 Adsorption
## 1 Introduction  
Nowadays, climate change has attracted serious attention. Despite the economic shutdown caused by the coronavirus pandemic response, carbon dioxide emissions, which are main factor in the greenhouse effect (along with methane and nitrous oxide but in a higher concentration), reached a concentration in the atmosphere in 2020 of 414 ppm, constituting a continuous increase according to National Oceanic and Atmospheric Administration Global Monitoring Laboratory [1]. Therefore, different technologies for carbon capture and storage, which involve chemical and physical adsorption, cryogenic distillation, and membrane separation, were proposed [2, 3, 4, 5, 6, 7]. These processes have both advantages and disadvantages, such as high energy consumption for cryogenic distillation or the trade-off relation between the membrane permeability and selectivity. The adsorption technique has attracted serious attention due to low energy consumption and cost-efficiency ratio [8, 9, 10, 7]. In order to be considered potential candidates for CO2 adsorption, the materials must also possess some properties, such as high surface area, high pore volume, low affinity to moisture, regeneration ability, thermal and mechanical stability, and high selectivity [11]. As a consequence, various materials were obtained and proposed for CO2 adsorption, including activated carbon, zeolites, silica or metal oxides [12, 13, 14, 15]. The high demand of activated carbon for pollutants adsorption resulted from different processes and for food or pharmaceutical obtaining can bejustified due their textural properties, availability and low cost [16]. Generally, this type of materials has significant advantages, such as lower regeneration energies compared to others adsorbents [17], ability for functionalisation with different groups [18] and increased capacity to adsorb contaminants from gases and liquids matrices [19]. Commercially available activated carbons were obtained from coal, peat and petroleum coke [11; 18]. However, the researchers turned their attention to huge availability of wastes rich in carbon (e.g., plastics, agricultural) as raw materials for activated carbon synthesis, trying to solve two issues at the same time: waste disposal and CO\({}_{2}\) capture [20; 21; 22]. In this context, the sewage sludge resulting from wastewater treatment plants could be a promising candidate for CO\({}_{2}\) adsorbents obtaining and energy or fuel production through different processes, such as pyrolysis [8], gasification [23] or combustion [24]. The pyrolysis products--gas, oil and char--were studied in order to get more information about their potential application and impact on the environment. Usually, the process is performed at 180-550 \({}^{\circ}\)C, where the moisture and volatiles losses are reached up to 390 \({}^{\circ}\)C and the formation of CO and CO\({}_{2}\) occurs due to bond disruption, continuing with the conversion of less reactive compounds up to 550 \({}^{\circ}\)C [8; 25; 26; 27]. During the gasification process, the sludge is converted in synthesis gas, due to high content of moisture [23; 28]. The gasification yield and cost-effectiveness were studied by optimization of process conditions [29; 30]. Additionally, it was discovered that the use of sewage sludge during coal gasification, a process called co-gasification, contributed to the increase of the calorific value of the resulting gas [31]. Another advantage of this process is the minimization of the pollutant gas emissions [32; 33]. Combustion is large-scale process using the sewage sludge as raw material [34]. It is achieved at a temperature above 850 \({}^{\circ}\)C in a fluidized bed of inert material. The combustion requires a prior drying, in order to reach a sewage sludge moisture less than 65% [35]. A serious drawback is the high pollutant gas emission, requiring purification systems. The solid residual char resulting from these processes is rich in metals, and its disposal is an issue that must be solved in terms of the environment protection.