**2. Metals in soils: impacts on apiculture**

The origin of heavy metals in soils can be anthropic or natural, and may be associated with different fractions of soil, which determine the mobility and availability of these metals to the surrounding ecosystem. It could affect honeybees or its habitat by polluting plants and water. The availability and mobility of these contaminants could be modified in relation to the physicochemical properties of the soil, for example, pH and organic matter content, among others.

Honey is principally composed of a complex mix of carbohydrates, among which fructose and glucose account for 85–95% of the total sugars. Since glucose is less soluble than fructose, the proportions of these sugars in honey determine overall granulation, with higher fructose quantities lending to honey that remains longer in the liquid state [2, 3]. Other more complex sugars are formed through the bonding of two or more fructose/glucose molecules with trace polysaccharide residues. Honey also contains other substances to lesser degrees, including organic acids, amino acids, proteins, enzymes, minerals, fat-soluble vitamins, flavonoids with antioxidant properties, and hydroxymethylfurfural, a compound that indicates honey freshness [4–8]. Finally, honey can be further classified by melissopalynological analysis as either monofloral or polyfloral in origin. Monofloral honey is of greater commercial value due to 45% of solid residues being single-pollen in origin [9–12]. Altogether, the quality of honey depends on the presence and concentrations for each of the aforementioned compounds, as

The close source-product association between plants-honey means that all honey inherit various characteristics of and share biological properties with their respective botanic sources [13]. Due to this, undesirable compounds or residues can be found in honey if the source plants were exposed to these substances, including those of anthrophic origin. Among the residues that alter the natural composition of honey are metals, which, depending on their concentration in food, can pose as a human health risk [14]. The most common route through which humans ingest and are exposed to metals is through the diet, although the presence of these chemical elements in the air also means intake through

Some heavy metals are essential elements for normal growth of plants such as Co, Fe, Mn, Ni, Zn, and Cu and they have important roles in metabolism, but at higher concentrations, the same metals become toxic. Those increased levels can cause a decrease in percentage of biomass in vegetables and in many other cases, they lead to plant death. On the contrary, some heavy metals such as Pb, `Cd, Cr, and Hg have been marked with high toxicity for plants [15].

Metals have a density, (*d*) > 5 g/mL and atomic number > 20, with the exceptions of alkaline and alkaline earth metals. No more than 0.1% of the earth's crust contains metals. Although the term "heavy metals" primarily refers to elements with elevated cellular toxicity, this definition now extends to include micronutrients that, at high concentrations, represent a risk to human health. Heavy metals without known biological functions are the most dangerous due to high toxicities, including barium (Ba), cadmium (Cd), mercury (Hg), lead (Pb), strontium (Sr), and bismuth (Bi). Trace elements, or micronutrients, toxic at increased concentrations include boron (B), chromium (Cr), cobalt (Co), copper (Cu), iron (Fe), magnesium (Mn), molybdenum (Mo), nickel (Ni), selenium (Se), and zinc (Zn) [16]. Due to the human health risk presented by these heavy metals and micronutrients, regulations exist for the maximum

residual limits permitted in various foods destined for human consumption [17, 18].

The origin of heavy metals in soils can be anthropic or natural, and may be associated with different fractions of soil, which determine the mobility and availability of these metals to

**2. Metals in soils: impacts on apiculture**

well as on classification as either mono- or polyfloral.

inhalation.

312 Honey Analysis

Undoubtedly, this will have an important effect in the metal content in honey, since honeybees are able to take water from these polluted sources. Moreover, bees may transport these pollutants to beehives by fixing them to their bodies after their contact with the polluted plant species.

Soils systems are complex and vary in traits based on mineral and organic residue compositions. In particular, heavy metals of both anthrophic and geochemical origins can affect soil characteristics. For example, Chile is the leading producer of copper worldwide, with this metal constituting a primary source of both net national income and employment. Nevertheless, copper mines generally overlap with the Transverse Valleys of the "Norte Chico" region of Chile, which is also an important region for agriculture and apiculture. Due to this spatial crossover, controversies exist between the mining and apiculture industries regarding mining-produced wastes. Specifically, these toxic residues are discharged into the air, soil, and water of valleys with human populations and with ranch, farm, and apiculture productions.

The presence of heavy metals in soils is not only due to external contamination, but can also be of geochemical in origin. Indeed, high copper contents can be found internationally in a number of soils [19]. This can occur due to mixed causes, such as abnormal native geochemical contents being complemented by mining contaminants. Generally, copper contamination is accompanied by high contents of other metals, such as arsenic, lead, cadmium, and zinc. Soils are open systems that exchange energy and organic matter with proximal environments. These exchanges are typified by a heterogeneous mix of three principal components—solid, gaseous, and liquid fractions of organic, inorganic, and microorganic components [20]. Several analytical approaches exist for determining total heavy metal contents or the fraction of total soil content represented by these elements. This fraction can be used to determine metal availability and mobility. Element availability in soils is the most representative way to estimate total element content as this fraction facilitates establishing assumptions of mobility, plant absorption, and possible contamination [21]. The availability of distinct contaminating elements depends on properties inherent to each element, including the tendency to form complexes with organic material; mineral chemisorption; precipitation as insoluble sulfides, carbonates, phosphates, and oxides; and co-precipitation in other minerals [22].

One of the most important chemical processes in soils is adsorption. This process determines the quantities of nutrients, metals, pesticides, and other organic chemical components retained on the soil surface. Due to these functions, adsorption clearly participates in regulating nutrient and contaminant transport in soil. Chemical and physical forces act during adsorption in direct relation to soil-surface functional groups and the ion or molecule of the solution. The interplay between both these relations gives rise to surface complexes that can be classified as either internal or external sphere complexes. Internal sphere complexes are established by chemical forces that are generally irreversible and slightly affected by changes in ionic strengths. In turn, external sphere complexes primarily involve Coulombic interactions that, through a reversible process, are affected by ionic strengths in the aqueous phase [23].

Most soils are heterogeneous and constituted by different minerals, solids, and organic compounds. Various interaction mechanisms of soil with heavy metals have been described, including diffusion through micropores and adsorption at sites with variable reactivity. It is not possible to discriminate between these mechanisms, being more appropriate to use the term "sorption" in order to describe the retention of heavy metals by these three pathways [24]. The type of sorption and metal-binding mechanisms depend on various factors, such as ionic radius, electronegativity, surface type, valence electrons, and ionic strength of the solution. Currently, strict regulations exist for metals due to residual accumulations and persistence in the environment, as supported by findings after specific contamination events [25, 26]. Furthermore, a number of studies have established the threat posed by the possible contamination of water and soil resources destined for agricultural ends. Any subsequently produced plants would represent healthy risk to consumers [27–29].
