The mining industry is an important asset for the economy of many regions across Morocco, particularly with respect to exports and employment, but also through numerous technological developments.

 

Active mining operations must respect governmental environmental criteria. Thus, the  introduction of contaminants in the environment is limited to a level considered acceptable by the overseeing governments. Nevertheless, some abandoned / functional mine sites are not well controlled and have significant impacts on the environment (El Adnani et al., 2007).

 

Mining    operations    generate    different     types of wastes which are potentially environmentally harmful (El Gharmali et al., 2004; Aikpokpodion et al., 2013). Mining and milling/beneficiation processes (crushing, grinding, washing,) generate four major categories of waste, that is, (1) Mine waste (low-grade ore, overburden and barren rocks), (2) Tailings, (3) Dump heap leach and (4) Acid mine water (Esshaimi et al., 2012). These wastes loaded enriched in heavy metals are disposed in surrounding land and water in a more or less environmentally acceptable manner.

 

 

 

 

 

 

The sampling of the tailings was carried out in 2 sampling spots (S1, S2) and (S3, S4) respectively from each tailings reservoir TR1 (155 m/100 m/13 m) and TR2 (150 m/125 m/19 m) (Figure 2) by using a specially designed cylindrical stainless steel corer. After collection, the tailing samples were carefully transferred to clean and dry self-sealing polyethylene bags and transported to laboratory. After being air-dried in paper lined propylene trays at room temperature and disaggregated with a wooden roller, all samples were sieved through <2 mm or <100 μm.

 

The tailings samples from each zone were thoroughly mixed  and homogenized by coning and quartering. Finally the tailings samples were stored at 4°C in tightly sealed polyethylene bags until its analysis. Due to the strong association and affinity of heavy metals with fine grained soil components, the <100 μm tailing fraction were selected for the sequential extraction and total chemical determinations (Pickering, 1986; Ramos et al., 1999; Cuong and Obbard, 2006). The <2 mm tailing fraction was used to determine the pH, electrical conductivity (EC), organic matter (OM), organic carbon (OC) and carbonate contents. The physical-chemical characterization consisted in the determination of the tailings sample pH, electrical conductivity (EC), organic matter content (OM) and the carbonate content. The soil pH was measured with the aid of pH meter while EC was done with the use of electrode meter (Badmus et al., 2014). Organic carbon content of the soil sample was estimated by following Walkey and Black method (1934). 10 g of sieved soil sample was taken into a 500 ml wide mouthed Erlenmeyer flask. To this, 10 ml of 1 N K₂ Cr₂ O₇ was added and swirled the flask gently to disperse the soil in the solution. 20 ml of Conc. H₂SO₄ was then added slowly followed by vigorous shaking for 1 min. Distilled water (200 ml) was added to the flask and the suspension was filtered. Few drops of o-phenanthroline indicator was then added to the filtered solution and titrated against 0.5 N FeSO₄ H₂O. Sample without soil was kept as a blank. The amount of organic carbon in the soil sample was calculated by using the following formula:

 

 

The results obtained for the tailing grain-size, pH, EC, OM,     OC     and     CaCO₃     content      measurements 

 

 

corresponding to the old Draa Lasfar tailing mine are presented in Table 1. These results show that OC of different tailing samples depends on the sand composition of tailings. The low OC values of different soils is related with the low rate of degradation by microbial-mediated processes due to the heavy metal contamination. These are also probably related to the poor absorbability of organics on negatively charged quartz (Aysha et al., 2015) which predominate in solid tailings of Draa Lasfar mine. In addition, the constant flushing activity by rain can support the low percentage of OC in these soils.

 

Results show also that Draa Lasfar mine tailing samples have neutral to alkaline pH for the majority of the samples. Neutral and alkaline pH in mining residues could be attributed to the presence of carbonates with a high concentration ranging from 162.1 to 196.4 mg g^-1 for tailings, due primarily to the mixture of these mining residues with soil very rich in carbonates. The EC measurements revealed that all tailings samples presented high values ranging from 1257,9 to 1865.4 µS cm^-1. Results showed that the most alkaline tailings samples have the lower EC values. This correlation between high pH and low EC value can be explained by the presence of low amounts of sulfur ions that causes a decrease of the EC. Regarding the carbonate content, it can be stated that tailings with a pH of 7.1 and higher generally have high calcium carbonate content. Total element concentrations in the investigated soils varied  in a narrow range of values (Table 2). In this sense, alkaline tailing samples together with high amounts of OM and the presence of carbonates increase the retention of heavy metals in these tailing samples.

 

However, these values do not provide sufficient information of its potential hazardous effects on environment because the mobility and eco-toxicity of heavy metals depend strongly on their specific chemical forms or binding. Heavy metals may be distributed among many components of the tailings and may be associated with them in different ways (Aikpokpodion et al., 2013) and the determination of the heavy metals fractionation is useful in order to assess toxic effects. The chemical form of heavy metals in tailing components is of great significance in determining the potential bioavailability and translocation of the metals to other environmental compartments like water, soil, plant and microorganisms when physicochemical conditions are favorable (Cuong and Obbard, 2006).

 

Overall, Cd is well distributed among the different fractions in all tailing samples (Figure 3) and seems to be very available with more than 34.1 to 42.7% of this metal bound to the short-term mobile fraction F1 and F2 respectively. The long-term mobile fraction (F3, F4 and F5) homes more than 34.5 to 38.5% of Cd. The immobilized fraction (F6) traps about 18.3 to 24.5%.

 

Copper in tailings (Figure 4) is essentially bound to soluble (F1) and exchangeable fractions (F2) with more than 34.7 to 37% of the total metal content. Copper is also widely present (26.5 to 31.5%) in the residual fraction (F6). The oxidizable (F3), acid-soluble (F4) and reducible factions (F5) are represented by small percentages in these tailings, representing respectively about 7.9 to 8.5, 10.9 to 11.7 and 14.8 to 16%.

 

 

The high percentage of Cu in the residue is likely due to the fact that Cu is easily chemisorbed on or incorporated in clay minerals (Pickering, 1989). The significant Cu concentrations in organic phase can be justify by Cu  affinity with OM, thus it can be hypothesized that Cu is bound to labile organic matter such as lipids, proteins, and carbohydrates. Lead distribution (Figure 5) in tailings is characterized by a strong dominance of this metal bound to long-term mobile fraction (F3, F4 and F5), with more than 50.4 to 60% of the total metal.  The short-term mobile fraction (F1 and F2) represents about 18 to 21.5%. The remaining fraction (F6) traps about 19 to 28% of the total metal.

 

The relatively high percentage of Pb in reducible phase is in agreement with the known ability of amorphous Fe–Mn oxides to scavenge Pb from solution (Yusuf, 2007). Thus, a high Pb percentage in the reducible fraction is a hazard for the aquatic environment because Fe and Mn species can be reduced into the porewaters during early diagenesis by microbially mediated redox reactions (Oluwatosin et al., 2008). Dissolution will also release Pb associated with oxide phases to the porewater possibly to the overlying water column and to benthic biota (Kabala and Singh, 2001). The major sources of Pb are from intensive human activities,  including  agriculture  in   the drainage basin (Rodriguez et al., 2009). In addition, a substantial contribution from the factories located in the upstream of the Tensift river dealing with Pb play a vital role in soils contamination by heavy metals as referred by Sarkar et al. (2007).

 

 

Zinc in tailings (Figure 6) is essentially bound to Fe–Mn oxide phase (F4) with more than 32 to 39% of the Zn content. This result can be justified by the high stability of Zn on oxides. Iron oxides adsorb considerable amounts of Zn and these oxides may also occlude Zn in the lattice structures (Banerjee, 2003). Zinc is also widely present (21 to 26%) in the residual fraction (F6). The water soluble (F1) and exchangeable fractions (F2) are represented by relatively small percentages in these tailings (about 13 to 22% and 9 to 14% respectively).  

 

Environmental pollution by heavy metals originated from functional mines can become a very important source of contamination both in soil and water. Therefore, the characterization of chemical and physical properties tailings is important to assess the risk of potential environmental mobility of toxic trace metals that are contained in this kind of waste. Taking into consideration the high mobility and potential bioavailability of heavy metals in this fraction and their total concentration, it can be concluded that Draa Lasfar mine tailings could have potentially hazardous effects on the environment. Cadmium and copper were predominantly associated with short-term mobile fraction (F1 and F2) while lead was largely associated with long-term mobile fraction (F3, F4 and F5) and Zn was essentially bound to Fe–Mn oxide phase (F4). 

The authors have not declared any conflict of interest.

Author would like to thank Ait Said Khalid for his invaluable technical assistance. We gratefully acknowledge the assistance provided by Mme Nadira TALBI in proof-reading the manuscript and Mme Maaroufi Ghizlan for the collect of samples.