Contribution of waste and biochar amendment to the sorption of metals in a copper mine tailing
Introduction
Mine soils are highly degraded at both physical and chemical levels, which prevents them from carrying out their functions and plants growing on their surface. Mine soils are a source of pollution that affects the area where the mine operates and the adjacent areas. This is due to the contamination of underground waters as a result of acid runoff from the mine, or atmospheric pollution caused by fine particles that are carried off into the air. One of the main impacts that mining has on soils is their high concentration of metals, acidity and low organic matter content (Akala and Lal, 2001, Vega et al., 2005, Santibañez et al., 2007, Zanuzzi et al., 2009, Asensio et al., 2013). It is important to understand how these metallic elements behave to their concentration and the ease with which they migrate through these types of soils.
One of the most recently used techniques to recover mine soils involves amendments made in situ with different types of waste material (such as municipal solid wastes ‘(MSW)’, sewage sludge or waste from the food industry). Using these types of amendments has a double benefit. Firstly, it returns the components of these wastes to the biogeochemical cycle, and secondly, it adds organic matter to the soil, increasing the pH and immobilizing metals (Theodoratos et al., 2000, Paradelo et al., 2011). The contribution of organic matter is important, as previous studies (Karami et al., 2011) have shown that increasing it in the soil leads to a descent in the assimilable metal content. Organic matter also adds essential nutrients that improve the fertility level of the soils, and helps to improve certain physical–chemical properties of the soil (Illera et al., 2000). Sometimes, the waste material used as an amendment contains contaminants such as metals or organic compounds, which means that many of them cannot be used on any type of soil. It is also necessary to control the proportion of amendment/soil used, in order to ensure a balance between their potentially beneficial qualities and possible problems of contamination (Paradelo et al., 2011). Adding organic matter to these types of soils leads to a descent in the assimilable metal content, as this is capable of forming strong bonds with metals and therefore retain them firmly in the soil (Kabata-Pendias, 2001). Different studies have shown that adding amendments made of waste materials reduces the assimilable metal content in the soil (Brown et al., 2003, Natal-da-Luz et al., 2012). The increased content of organic matter provided by the residue causes an increase in the sorption capacity of heavy metals in soil (Kocasoy and Güvener, 2009). These studies were based on understanding how the available amount of metals varied in the soil, but not on how these metals behaved in the soil once the amendments had been applied.
Another option, despite still being costly, is to use biochar in combination with these amendments to recover degraded soils. Biochar is obtained by biomass pyrolysis (using both plant and animal waste) using substoichiometric combustion or without oxygen in pyrolytic ovens. The biochar, in the same way as the amendments made using waste material, provides organic matter, especially carbon and nitrogen. However, unlike the amendments, the waste used to make biochar usually contains very few or no trace elements, such as forestry biomass. They also have the added benefit that the C and N they provide are much more recalcitrant. Different studies (Beesley et al., 2010, Beesley and Marmiroli, 2011, Park et al., 2011) have shown that biochar can reduce available contents of metals such as Cu, Pb and Zn, and increase the soil pH. The biochar used in this study was made using Acacia dealbata Link. This is a shrub from the legume family (Fabaceae) that is considered an invasive species in the Iberian Peninsula. Using biochar made of A. dealbata as an amendment would therefore serve two functions — recovering the soil and using the waste matter from a type of invasive plant. The biochar increases the sorption capacity of heavy metals in soil; this increase is due to the high pH, organic matter, and large surface area (Beesley et al., 2010, Beesley and Marmiroli, 2011, Uchimiya et al., 2011).
An understanding of metal sorption processes can help in providing very valuable information about the effect of amendments once they have been applied to the soil. Therefore, the aim of this study was to compare the sorption capacity of Cu, Pb and Zn in a copper mine soil after applying a combined amendment of waste and biochar, and to understand the behaviour of the metals included in the amendments. Copper, lead and zinc are usually found in high concentrations in the soils from the mine in the study, and in the waste materials used as amendments.
Section snippets
Study area and amendments
The sample zone is in the mine in Touro (Northwest Spain; Lat/Lon (Datum ETRS89): 8° 20′ 12.06″ W 42° 52′ 46.18″ N). The climate in this zone is Atlantic (oceanic) with precipitation reaching 1886 mm per year (with an average of 157 mm per month) and a mean daily temperature of 12.6 °C. The average of relative humidity is 77% (AEMET 2014). For this study, a mine tailing was chosen from the mine's settling pond (S). The amendment made of wastes was provided by the company Tratamiento Ecológicos del
Characteristics of the mine tailing (S), amendment (T) and biochar (B)
The mine tailing (S) had an acidic pH, while the amendment and the biochar used (T and B, respectively) had a significantly higher pH than the untreated soil (Table 1). The biochar and the amendment also had a significantly higher concentration of carbon (C) than the mine soil. The highest concentration of both quasitotal and CaCl2-extractable Cu was observed in the soil, and the amendment and biochar had concentrations significantly lower. The highest concentration of quasitotal CaCl2
Characteristics of the mine tailing (S), mine spoil material amended with amendment mixtures (STB20, STB40%, STB60%) and biochar amendment (TB100%)
The pH value of the mine tailing (S) amended with different mixtures of waste material and biochar (STB20%, STB40%, STB60%) constantly increased as the proportion of amendment applied increased. This increase in the pH value is due to the waste matter used to make the amendment, such as solid urban waste, ash from a paper factory, agri-food waste, purification plant sludge and biochar (Canet et al., 2007, Hackett et al., 1999, Nicholson et al., 1996, Tejada et al., 2010). This increase in the
Conclusions
The application of an amendment made of a mixture of waste matter and biochar to a mine tailing increased the capacity of the soil to sorb Cu, Pb and Zn. Also, practically all of these metals contained in the amendment were retained. It is interesting to note that Zn is the element that was sorbed the least from the three elements studied. For all of these reasons, the contribution of metals by the amendment used is offset by the increase in the sorption capacity of these amendments for these
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