## 1 Introduction  
Fresh water is vital for human health, for nature, and for the functioning of economies and societies. Because freshwater resources are threatened by various pressures, such as higher water consumption, urban development, and climate change, it becomes important to consider rainwater as an alternative resource. Furthermore, rainwater harvesting can be potentiated in adaptation to climate change [1], since the occurrence of episodes of high precipitation are becoming frequent all over the world [1]. Several potentially toxic elements (PTEs) have been found in rainwater, such as chromium, copper, lead, mercury, nickel, cadmium, and zinc [2, 3, 4]. Some of those have been found in high concentrations, such as copper, lead, and zinc. These PTEs can pass through the human food chain, increasing the probability of health problems when in high concentrations: copper can cause headache, nausea, and gastrointestinal illness [5]; lead influences the reproductive, nervous, cardiovascular, and renal systems [6]; and zinc can cause nausea, headaches, loss of appetite, and low copper levels in the body [7]. Table 1 shows the values of Zn(II), Cu(II), and Pb(II) concentrations found in rainwater events in different locales of the world, reported since 2015, and presents the maximum admissible concentrations for the same elements in accordance with European [8] legislation for drinking water. Such legislation can be considered a reference point for rainwater since there is no legislation for this type of water, and in some parts of the world, it is used as drinking water. In accordance with the European standard, Cu(II) and Pb(II) are subjected to maximum allowable concentrations of 2000 ug/L and 5.0 ug/L, respectively [8], while Zn(II) does not possess a specific limit value under this directive. However, the maximum admissible value of 5.0 ug/L for Zn(II), indicated previously in Portuguese legislation for drinking water, may be used as a reference [9]. Given that the concentrations of Cu(II), Pb(II), and Zn(II) in rainwater exceed the maximum admissible values for drinking water many times (e.g., [10; 11]), to ensure water quality and to mitigate potential health risks, it becomes necessary to remove them from the water before using it, namely when it is intended to use rainwater for domestic purposes. Several processes have been used to remove PTEs from water sources, including chemical precipitation, ion exchange, electrochemical remediation, sedimentation and flocculation, membrane separation, and sorption [27]. However, except for sorption, they present some limitations in their effectiveness when dealing with low concentrations of elements, besides being costly and complex [27]. Sorption has gained significant attention in recent years for water treatment, and biosorption is a subfield of sorption that has attracted considerable attention as an eco-friendly and sustainable approach to water treatment [28]. Biosorption is based on the inherent affinity between the biomaterial (either deceased or living biomass--biosorbent) and the target species [29] through different mechanisms, such as ion exchange, precipitation, reduction, electrostatic attraction, and complexation [29; 30]. Considering the advantages of biosorption, becomes important to explore it in rainwater