# Adsorption of Heavy Metals from Low Concentration Solutions onto Dried _Chlamydomonas reinhardtii_
## 1 Introduction  
Increasing anthropogenic and industrial activity leads the water streams to be easily contaminated by a wide variety of organic and inorganic pollutants. Among other sources of environmental pollution, heavy metals (HMs, hereafter), which are toxic even at a low level of exposure, can be found in the majority of wastewaters due to their usage in many industrial branches [1; 2; 3]. In particular, the removal of Pb, Cr, and Cd is of paramount importance, since they can cause different serious pathologies such as kidney necrosis, anaemia, and hypertension [4; 5; 6; 7; 8; 9]. Unlike organic substances, HMs are non-biodegradable, thus persistent [10]. HMs have a high mobility in water and they can be adsorbed to and accumulated in the environment and in living organisms such as plants and animals, causing them various disorders and health problems [11]. The spread of HMs can lead to the deterioration of agricultural land, phytotoxicity, and disequilibrium in soil microbial processes. Hence, wastewaters must be treated before their reintroduction into the environment, mainly due to the presence of HMs in low concentrations [12; 13; 14; 15]. Furthermore, there is an increasing awareness that the prevention of emissions, rather than remediation of damage, is a more cost-effective and efficient approach for the mitigation of impacts on the environment and on human health [11]. Various methods for heavy metal removal have been proposed [16; 17; 18], for instance chemical precipitation, ion-exchange, membrane filtration, coagulation-flocculation, flotation, and electrochemical methods. However, these methods are expensive and requirehigh energy and the usage of harmful chemicals. Moreover, conventional methods are only feasible for high metal pollution concentrations. Adsorption presents a successful technology thanks to a wide variety of available sorbents, high recovery efficiency, fast extraction time, low cost, and low consumption of organic solvents. In this respect, common examples of sorbents are activated carbon, clay minerals, inorganic oxides, porous organic polymers, or organic-inorganic hybrid materials. However, considering the new circular and greener economy paradigm, the development of more environmental sustainable sorbents is a key point [19; 20]. An innovative strategy to treat wastewater is presented by the use of microorganisms such as microalgae, being cost effective, very efficient in the presence of low metal concentrations, and, in principle, recyclable and reusable [9; 10; 21; 22; 23; 24; 25; 26]. Microalgae are unicellular and eukaryotic microorganisms which can be solitary or colonial, some of them are able to move thanks to flagella [27]. Microalgae can be autotrophic or heterotrophic depending on the mechanism of energy production, the first one being related to photosynthesis with CO\({}_{2}\) consumption, while the second one to redox reactions [28]. According to [29], many factors may influence the biosorption of HMs by means of microalgae, such as the initial metal concentration, temperature, pH, time of contact, micronutrients, CO\({}_{2}\), light and time of lightness, stirring, and microalga characteristics, e.g., the microalga species. The capture of heavy metals by microalgae can occur both actively or passively [30]. Passive capture occurs as biosorption of the metals on the cell surface. Active capture implies two mechanisms: first biosorption at the cell wall, via passive chemical and physical processes, and subsequently bioaccumulation inside the cell, via active transport. The HMs are then deposited in different organelles inside the cells. This last mechanism is metabolic- and energy-dependent; hence, living microalgae are more susceptible to toxicity, and environmental parameters. Bioaccumulation occurs more slowly than biosorption and, in principle, it is not reversible and more stable since the metals are accumulated in cell organelles [29].