# Reuse of Spent Coffee Grounds: Alternative Applications, Challenges, and Prospects--A Review
## 11 SCGs as a Sorbent  
Currently, natural sorbents are gaining increasing importance in processes for removing contaminants [80]. Sorption processes are relatively simple and inexpensive, which is why they are often used in environmental engineering for water and wastewater treatment. In addition to commercially used sorbents like activated carbon, natural sorbents, particularly waste materials such as orange peels, banana skins, walnut shells, rice husks, and SCGs are playing a larger role [81; 82]. Due to their porous structure, SCGs can be used to remove herbicides and pesticides [83; 84], pharmaceuticals [85], phenols [86], dyes [87; 88], and heavy metal ions [89; 90; 91; 92; 93]. Example substances adsorbed by SCGs are shown in Table 9. It includes the isotherms and process parameters of adsorption. The Langmuir isotherm was used to describe most of the data in the table due to its good fit to adsorption systems. Although other kinetic models, such as the pseudo-second-order model, may better describe the dynamics of the process in certain cases, their application is more demanding. Determining the kinetic parameters for these models requires knowledge of system parameters, such as solution volume, initial concentrations, and process conditions. For each of these elements, the parameters will vary. SCGs are characterized by particle sizes ranging from approximately 100 to 1000 \(\upmu\)m [94]. Due to their porosity and large specific surface area, SCGs are an excellent sorptive material, making them effective in various contamination removal processes. The main components of SCGs include lignin, which provides rigidity and resistance to biological degradation, as well as tannin materials. Tannins, being polyhydroxyphenols, contain functional groups such as -OH, which can form complexes with heavy metals. This allows for SCGs to effectively adsorb heavy metals through complexation processes, making them a valuable material for removing contaminants from water or soil [9]. Typically, adsorption capacity increases with an increase in the specific surface area, as more adsorption sites become available. Additionally, pore size and micropore distribution are closely related to the composition of the adsorbents and the type of biomass used in their synthesis, directly influencing their effectiveness [95]. Enhancing sorptive capacity is beneficial and can be achieved by activating SCGs. Sorbent modification can be performed using physical, thermal, chemical, or biological methods, or their combinations [96]. The disadvantages of using SCGs as sorbents include their relatively low adsorption capacity, which requires modification of the material to improve sorption parameters. Modifying SCGs requires the use of chemical reagents and/or various physical processes, which can be labor-intensive, energy-consuming, and costly. Additionally, the variability in the composition of SCGs (e.g., depending on the species or roasting time) may result in different efficiencies in contaminant removal, making it difficult to comprehensively optimize the sorption methodology.