**6. Biological applications of lanthanides**

where uptake or removal of elements is metabolically controlled [86, 87]. Some metals belong to the group of essential micronutrients, being important for growth and development of plant cells, and are involved in active metabolism [88]. Bioaccumulation of chemical compounds depends on rates of uptake and metabolism, and on the ability of the organism to degrade or store compounds. In essence, the process of accumulation of elements in algal cells is very complicated and depends on the properties of the species (type, size, form, and state of development), the element (charge, chemical form, and concentration), and the medium (pH, type, and concentration of metal salts or presence of complexing agents) [89]. As can be seen in **Table 4**, accumulation, biosorption, and desorption of lanthanides occurs in micro- and macroalgae, including brown, green, and red algae, algal flagellates, and also cyanobacteria. The potential for biosorption of cerium ions by cyanobacteria *Arthrospira* (*Spirulina*) was also tested [100]. Live and dead algae were shown to efficiently accumulate these metals because

94 Lanthanides

**Figure 2.** Intracellular localization of different lanthanides in *Desmodesmus quadricauda.* The absorbed lanthanides (horizontal rows) were visualized in cells stained with the fluorescent dye Fluo-4 (left column). Chloroplasts are visualized by autofluorescence of chlorophyll (middle column). In merged photos (right column), the localization of lanthanides seen either inside chloroplasts (Nd, Ce) or in the cytoplasm (La, Gd) (according to Řezanka et al. [109]).

In biological systems, lanthanides are applied for different purposes such as growth promoters, fertilizers, water bloom killers, or as detection tools (bioindicators, tracers, and markers). Lanthanides have been proposed as growth stimulators for various animals such as pigs and other livestock [110]. Algae were also used as a feed additive to improve the condition of domestic animals [111]. Lanthanide-rich algae are a potential alternative to food supplements or functional foods. However, only one study on young abalones was performed to demonstrate that lanthanide-enriched algal biomass was an effective growth promoter [82]. Therefore, it would be important to increase the number of studies, to obtain relevant data on the effects of lanthanide transmission and to assess the risk of human exposure through food derived from animals [35].

Many microorganisms, including blue-green algae (e.g., *Microcystis* or *Alexandrium* spp.), cause water blooms with negative impacts on health, ecology, and economics. Water blooms produce harmful toxins (e.g., microcystins and saxitoxins) with detrimental effects on humans and animals [84]. Lanthanides affect algal physiology and their impact on the level of microcystins was demonstrated in *Microcystis aeruginosa* [112, 113]. There was a close relationship between lanthanides, phosphorus content and the growth characteristics of cyanobacteria [113]. New techniques of dephosphatisation of the environment include the use of Ln-modified clays [83, 84]. The advantage of these methods is the low level of side effects on living organisms.

One reason is the risk of reduced availability of resources (China owns more than 95% of natural sources) or their relatively rapid depletion from other sources. Replacement of lanthanides with alternate substances in industrial applications is currently not possible [132, 133]. Due to their unique chemical and physical properties and their extensive applications in industrial products, the importance and demand for these elements is constantly increasing [131, 134]. The economic impact of an emerging lanthanide shortage increases the urgency for efficiently using renewable energy sources from the ever increasing number of different types of waste products worldwide. At present, research is focused on the progressive and costefficient recycling of lanthanides for industrial processes [4, 95, 102, 135, 136], which would reduce risks associated with inaccessibility or depletion of natural resources while minimiz-

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ing environmental problems associated with their extraction and processing [137].

source of various lanthanides, either in the form of dry powder or wet mud [138].

and at the same time environmentally friendly technologies.

Lanthanides from waste sources can be recycled by chemical separation from solutions (e.g., chemical precipitation, electrochemically, membrane division, reverse osmosis, etc.). These methods are comparatively costly and, moreover, are often a source of other nonorganic wastes [139]. Methods such as pyrometry and hydrometallurgy for the extraction of lanthanides from ores have significant negative impacts on the environment and involve high costs [126]. The other serious disadvantage is the dependence on a single and limited source and possibly the depletion of other natural resources [126, 140, 141]. These traditional physicochemical processes are expensive or even inefficient for the treatment of sewage containing low concentrations of metal ions [142]. A by-product of conventional methods is the associated large volume of contaminated water, high temperatures and a high consumption of chemical compounds [143, 144]. Researchers are therefore looking for low-cost approaches

As a biotechnological approach, biosorption is considered to be a more efficient and cheaper alternative to conventional chemical methods of recycling lanthanides [133, 145, 146]. Various different organic residues of animal or plant origin, including resin, activated charcoal, or

**7.1. Chemical recycling**

**7.2. Biosorption**

One of the most widespread lanthanide-containing wastes is electrical and electronic equipment, including lighting equipment, computers, or photovoltaic panels. This waste is a growing threat to the world's environment, and lanthanide recovery is therefore becoming economically attractive. The main sources for recycling are luminophores, powder mixtures obtained from electronic waste and containing high concentrations of lanthanides. Luminophores are obtained from television screens or monitors, as well as energy-saving bulbs and lamps, where they are used to convert cathodic tube radiation or ultraviolet electric discharge into mercury vapor and visible light. These luminophores occur as a powder attached to the inner surfaces of mesh or tubes. The glass parts of these waste networks, monitors, screens, and light bulbs can be easily recycled, but luminophore layers must be removed because the luminescent compounds would reduce the quality of recycled glass. The luminophores as waste represent a toxicity problem but, on the other hand, are a concentrated

The unique chemical features of lanthanides make them ideal tracers for geochemical processes in nature [9]. They represent alternative, nonradioactive, highly detectable labels. They were used, for example, to confirm the impact of cyanobacterial mats on deep waters outside French Polynesia, providing evidence for an end-ascending flow [114]. They enable scientists to follow oceanic cycles, petrogenesis, the chemical evolution of the Earth [16, 29], or palaeo-environmental conditions [115–118]. Lanthanides can also serve as anthropogenic activity indicators [27]. Because of their particular affinity to algae, the lanthanide profile may be a useful indicator for exploring the ecology of marine environments [10] and can also be used to monitor sources of pollution from natural events such as volcanic activity [25]. In combination with macroalgal sampling, the lanthanide profile may help to characterize coastal water quality and pollution [22, 23, 27].

Lanthanides have been used for their inert nature as detection agents in various experiments, for example, in studies of the rate of passage and digestibility of nutrients in humans and animals [119–121]. Lanthanide oxides have been used as markers in sea cucumber (*Apostichopus japonicus*) grown on a variety of macroalgal diets [121].

In the development of new, sensitive detection methods, active chelates of lanthanides have been obtained and tested. They are used in sensitive immunoassays to suppress the background [122] or as very sensitive fluorescence probes [123]. An example of their use is the labeling of the cyanotoxin microcystin [124, 125].
