# Use of Mineral Waste for the Production of Artificial Aggregates
## 4 Non-Ferrous Metallurgical Waste
### Threats to the Aquatic and Soil Environment  
The impact of slags on the environment is determined by the results of leachability tests (dynamic and static), in which natural slag weathering processes or landfill conditions are simulated. These tests help determine the mobility of the elements contained in the slags and predict their long-term behavior (up to several hundred years) in the environment [61; 62].  
The results of leaching tests are often supplemented by geochemical modelling. For over 20 years, many researchers have used elemental speciation modelling to study the reactivity of landfilled waste (including slags) in hypergenic environments. Geochemical modelling is also the basis for identifying secondary minerals that crystallize in the slags due to environmental factors [54].  
The rate of leaching of elements from the waste into the environment is determined by a number of factors, such as the fineness and morphology of the waste lumps, their chemical and phase composition (including the proportion of amorphous matter), ambient temperature, liquid/solid ratio, redox potential, and contact time of the material with precipitation water. One of the key parameters governing the leaching process is the pH of the surrounding environment in which the waste is situated [60; 61; 62; 63]. A factor that significantly reduces the release of elements into the environment is, for example, the use of carbonates (limestone, dolomite) as a buffer layer in landfills [63; 64; 65].  
Studies conducted by the authors have shown a significant degree of elemental leaching from Zn-Pb metallurgical slags, with some of the elements subsequently bound in secondary minerals formed under hypergenic environmental conditions, which significantly reduces their mobility [62; 63; 65; 66; 67; 68; 69].  
As noted, the solubility of the components present in slag and their mobility in the environment is driven by their mineralogical and chemical composition. The silicate and spinel phases have been shown to be the most resistant to weathering processes, whereas the sulphide, glass, and intermetallic phases displayed considerably higher solubility in soil and water environments [70; 71].  
The elements with the greatest solubility (>2.0 mg/L) that are released in the highest quantities from slags include Zn, Pb, and Fe. Zn leaching occurs over an extended period; for example, [PERSON] et al. observed that after 12 years, the Zn concentration in eluate exceeded the 200 mg/kg limit for hazardous waste by a factor of 40 [72]. Under the pH conditions of a hypergenic environment (4.5-6.0), Zn occurs mainly in the form of Zn\({}^{2+}\) and ZnCO\({}_{3}\), and rarely as ZnSO\({}_{4}\).7H\({}_{2}\)O. Zn mobility in near-neutral environments is limited, as the element is readily adsorbed by oxides, hydroxides, and aluminosilicates [58; 59].  
Despite its considerable solubility in water (>2 mg/L), the leaching degree of lead from slags after 12 years is significantly lower than that of zinc [73]. This is because the release of Pb is partly limited by the formation of new phases, such as lead carbonates, sulphates (e.g., cerusite (PbCO\({}_{3}\)), and anglesite (PbSO\({}_{4}\))), which are insoluble in water [74]. Increased lead mobility is significantly influenced by the salinity of water in the subsurface zone, which promotes the formation of soluble PbCl\({}_{2}\), PbCl\({}_{4}\)\({}^{2+}\), and PbCl\({}^{+}\) complexes [75].  
The main source of iron in the hypergenic environment may be the release of Fe\({}^{2+}\) and SO\({}_{4}\)\({}^{2-}\) ions due to the oxidation of pyrrhotite in the hypergenic environment. As indicated by the geochemical modelling results, the dominant iron phases in the near-surface conditionsare as follows: melanterite FeSO4, FeSO4, \(\cdot\) 7H2O, goethite FeO(OH), ferrihydryte Fe5 HO8, \(\cdot\) 4H2O, jarosite KFe3+3(OH)6(SO4)2, and soluble iron sulphates [60,76].