# Adsorption of Heavy Metals from Low Concentration Solutions onto Dried _Chlamydomonas reinhardtii_
## 1 Introduction
In passive capture, the interaction occurs between the algal cell wall (negatively charged) and the metal ions (positively charged) with the functional groups present at the surface; the process is metabolic-independent, fast, and reversible [29]. Metal binding can occur with different mechanisms, such as electrostatic interaction, surface complexation, ion exchange, and precipitation, that can occur individually or in combination. During the adsorption process, the cell wall characteristics are of upmost importance. Indeed, different algal biomasses are characterised by different types of cell wall and thus, by various functional groups (-COOH, -OH, -NH\({}_{2}\), -SH) able to bind metal ions [31; 32]. For biosorption, dried microalgae are advantageous since they are not affected by the toxicity of HMs and do not require a continuous supply of nutrients. The reversibility of biosorption on one side is an advantage, allowing for the sorbents' reusability, but, on the other hand, it could lead to instability of the capture. The application of microalgal biomass in wastewater treatment systems should also include a systematic economic evaluation. The removal of heavy metals through an adsorption process includes the costs of production as well as, concerning the dried biomass, those of the drying process. According to [33], the operational and energy consumption costs depend on the technology chosen for harvesting and dewatering the microalgae. Of upmost importance is also the kind of cultivation used; generally, close reactors require lower operational and energetic costs than open ponds. Drying process costs can be lowered by using by-products coming from different waste streams as possible biosorbents; moreover, dried biomasses can be, in principle, easily regenerated and reused, reducing the costs of the cultivation step [34]. Different algal strains, such as _Chorella vulgaris, Spirulina maxima, Spirulina platensis, Scenedesmus_ sp., _Scenedesmus almeriensis_, _Chbrella sorokiniana_, _Parachorella_ sp., and _Chlamydomonas reinhardtii_, _Chlorophyceae_ sp., are potentially suitable for heavy metal removal [10; 23; 35; 36; 37; 38; 39; 40; 41; 42; 43] (Table S1). Among the others, _Chlamydomonas reinhardtii_ (C. reinhardtii, hereafter) has received lot of attention in recent years thanks to simple culture conditions and to its high tolerance to heavy metals [37; 42; 44]. In the literature, to determine the effect of solid/liquid ratio among different heavy metal ions, high metal concentrations have been used [35; 45]. The Best Available Techniques (BATs) reference documents of different productive sectors emissions in wastewaters, e.g., ceramic manufacturing, ferrous metals processing, and surface treatment of metals and plastics [46; 47; 48], report the industrial wastewaters pollution data (Table S2). Here, low concentrations of heavy metals are released in wastewaters. However, Italian and EU regulations impose strong limitations of HMs concentrations in water streams before their reintroduction into the environment and for human consumption (i.e., Pb < 0.005 mg/L, Cr < 0.025 mg/L, and Cd < 0.005 mg/L, [49]). In this paper, the green microalga _C. reinhardtii_ is exploited in its dried form to remove HMs from low-polluted environments. The aim of this work is to study the capture capability of dried microalgae towards low concentrations of HMs, such as Pb(II), Cr(III), and Cd(II), present in different productive sectors. Initial metal concentrations were fixed at 0.5 mg/L, 0.2 mg/L, and 0.2 mg/L, respectively, for Pb(II), Cr(III), Cd(II), and are related to real data reported by the BATs reference documents of industries such as ceramic manufacturing, ferrous metals processing, and surface treatment of metals and plastics [46; 47; 48]. Low concentration adsorption onto dried _C. reinhardtii_, with a preliminary comparison of Cd adsorption with living _C. reinhardtii_, was performed.