Major environmental problems from coal mining are mostly associated with high concentrations of sulfur, salinity, metals, and acidity [1
] produced by surface deposits of waste rocks (mine waste) [2
]. These components can affect the quality of surface waters and shallow groundwater near the mines. The presence of high concentrations of salts in coal mining soils resulting from the use of coatings rich in salts or as a consequence of the pyrite oxidation in the mine spoils can decrease the productivity of the soil [4
]. In particular, dissolution of salt from saline mine spoil during rainfall events may result in local or regional dispersion of salts through leaching, or in the accumulation of dissolved salts in soil pore water and inhibition of plant growth [5
]. Therefore, despite the application of amendments to carbonaceous wastes, coal mines can still be a source of high concentrations of metals and dissolved salts such as Mn, Zn, Al, Fe, K, Cu Se, Na, Ca, Mg, Cl, NO3−
, and SO42−
]. Salinity and osmotic tension have been found to be responsible for the inhibition of plant growth and the delay of germination [8
]. Salinity adversely affects crop seed germination either through the creation of osmotic potential outside the seed, which hinders water absorption, or through the toxic effect of Na+
]. Salinity is normally evaluated through electrical conductivity (EC) readings of soil solutions or saturated soil pastes, which are highly correlated with the concentration of total salts. Efflorescent salt in coal mine spoil piles is dominated by sulfate salts of sodium, magnesium, and calcium [1
], a product of the evaporation of acid mine drainage (AMD) in surficial environments. These minerals can be considered as the highest polluting residue in the mining environment. Because of the high solubility of efflorescent minerals, climate plays an important role in their mineral chemistry [11
]. The minerals commonly associated with saline soils are gypsum, thenardite, mirabilite, hexahydrate, epsomite, bloedite, konyaite, loweite, eugstgerite, wattevilleite, halite, nitratine, and natrojarosite [1
The use of stabilization materials or reagents for the remediation in situ of soils contaminated with metals and organic constituents has been extensively studied, and they are increasingly being accepted as a remediation technology [12
]. The main goal of in situ remediation technologies is to stabilize and contain pollutant materials in the soil to reduce their ability to pollute water or biota and thereby reduce their potential for toxicity and transport. These technologies also aim to improve the soil’s water retention capacity [13
], cation exchange capacity (CEC), and the availability of nutrients such as P, Ca, and K [16
], and to improve mine soil aggregation and root length [19
]. Recently, soil recalcitrant organic carbon (also known as biochar), which is a black carbon material, has been studied as a soil amendment [18
]. The application of biochar improves some physical and chemical soil properties [23
]. For example, in one study, biochar application increased soil aggregation, water holding capacity, and cation exchange capacity [24
]. The type of biochar and its application rate influence wheat seed germination and seedling growth [25
]. However, some studies have reported that maize seed germination and early growth are not significantly affected by biochar [26
]. The biochar remediation of Cu, Zn, and Cd in soil samples has also been evaluated [27
]. The capacity of activated charcoal to absorb a variety of salts has long been noted [28
], and charcoal has also been utilized in industrial desalination processes [29
]. However, the potential use of biochar as a soil amendment to mitigate salt-induced plant stress appears to have received essentially no prior research attention. In this way, the use of biochar as an amendment and remediation medium has been widely accepted for soil quality improvement [18
]. Additionally, the presence of graphene in the biochar used for environmental remediation purposes opens up the possibility of new potential applications [32
], provides a slow release fertilizer and serves as a microbial inoculum [33
]. The stimulation or inhibition of seed germination through biochar application has been investigated primarily [25
] as a phytotoxic bioassay and for its potential role in the rehabilitation, revegetation, and repair of contaminated soils [35
The main goal of this work is to assess the action and effectiveness of biochar as an amendment in the remediation of soil contaminated with efflorescent salts in a coal mining area in the north of Colombia.
The research here was aimed to study the effect of biochar from oil palm crop residues on the germination and elongation of the brachiaria grass root in mining soils affected by the presence of efflorescent salts by coal mining activity in Cesar.
3. Results and Discussion
The physical and chemical characteristics and elemental composition of the efflorescent salts and biochar are summarized in Table 1
. The high EC in the efflorescent salts in comparison to the biochar is due to the higher content of charged species. As can be observed, these residues are rich in iron with high content of sulfates, calcium, and aluminum. Additionally, a significant presence of Na, followed by minor amounts of Mg, K, Cu, and Mn, was detected. The EC of the efflorescent salts was higher than 8 dS/m but lower than 16 dS/m, hence the material can be classified as highly saline sodic [42
The waste heaps in the coal mines studied show features similar to those studied in North-Central Colorado, USA, which appear to be a potential source of trace elements and salinity [1
The XRD pattern of the efflorescent salts is displayed in Figure 2
. Quartz = Q (SiO2
) and gypsum = G (CaSO4
O) are the major minerals although considerable amounts of alunite = A (KAl3
) and natrojarosite = N (NaFe3
) and minor amounts of calcite = C (CaCO3
) and halite = H (NaCl) were also observed. The diffraction pattern of the salts’ residues confirms the diversity of crystalline solids waste in the sample. Although pyrite and schwertmannite have often been reported in drainage acids of coal mines [43
], it was not possible to identify these phases in the XRD analysis. Although elemental analysis in efflorescent salts showed the presence of sulfur, it may be that pyrite cannot be observed in the XRD pattern due to the insensitivity of the technique.
The XRD pattern of biochar is presented in Figure 3
. Two broad bands centered at 2θ = 22.5° and 2θ = 43° indicate the presence of graphene in this sample. The formation of graphene sheets could occur during the pyrolysis process of waste materials derived from oil palm cultivation. Similar diffraction patterns were obtained by Zhang et al. [32
] and Saikia et al. [44
] in studies of biochar and bituminous coal graphenes, respectively.
shows SEM images of efflorescent salts (top panel) and biochar (bottom panel) at different magnifications. In the efflorescent salts, the presence of irregular particles with a wide range of sizes is observed, along with a great and well-defined regular structure, with either a hexagonal and/or monoclinic habit, possibly related to the presence of gypsum and/or alunite-natrojarosite. On the other hand, according to the bottom panel, the biochar seems to have developed high irregular longitudinal porosity, keeping the structural components of the biomass, indicating that thermal degradation had not been completed.
shows the FTIR spectrum in the wavenumber range of 800–4000 cm−1
for the biochar sample. The broad band centered at around 3422 cm−1
is assigned to the stretching vibrational modes of hydroxyl groups. The spectrum exhibits two very weak peaks located at ~2850 and 2925 cm−1
due respectively to C–H bond bending and stretching from aliphatic groups. The absorptions at 1560 cm−1
and 1380 cm−1
can be attributed to the antisymmetric and symmetric C=O stretching modes, commonly associated with the carboxylate functional group. Finally, the bands between 1050 cm−1
and 1300 cm−1
may be assigned to C–O bonds. The functional groups identified in this spectrum are similar to these found by Cantrell et al. [45
] in biochar poultry litter studies. The waste heaps in the coal mines studied show features similar to those studied in North-Central Colorado, USA, which appear as a potential source of trace elements and salinity [1
As shown in Figure 6
a, a significant increase in EC was observed following the increase from 5% to 10% biochar. Similar behavior was reported by Abdel-Fattah [46
], in which the salts can be readily leached during the first leaching period. Figure 6
b shows the pH variation as a function of the biochar content. It can be seen that the biochar addition increased the pH of the saline substratum. Additionally, when the efflorescent salt was mixed with 5% biochar, the pH changed from 8.32 to 8.38, which is equivalent to 31.58% of the total variation achieved. Many amendments and soil improvement products require long periods of time to adjust and balance the pH and EC.
The germination percentage and root length of brachiaria grass in the presence of efflorescent salts and biochar are shown in Figure 7
a,b. The inhibition of germination and root elongation compared with that of the control depended on the salt content. In general, the percentage of seed germination and root length increased with the addition of biochar. Because of the results obtained, Petri dish bioassays are considered a valid method for assessing toxicity problems associated with various materials, including mining waste and biosolids [25
The ANOVA results showed that the germination percentage and root lengths of the brachiaria grass were significantly affected by the concentration of the salts (p
< 0.05) (Table 2
), and that biochar positively improved these properties. The germination percentage was higher for treatments with the highest percentage of biochar (13.33%) and for the control, which lacked both salt and biochar (12%). The root lengths were also recorded for the highest dose of biochar (22.37 cm) and targets (20.03 cm). The germination percentage of brachiaria significantly (p
< 0.05) decreased with increasing salt content; high concentrations of soluble salts were also detrimental to the growth of this plant.
According to the multiple percentage test (Table 3
), the germination and root length of the control target showed significant differences for the groups treated with efflorescent salt and biochar. The presence of efflorescent salt negatively affected the elongation of the brachiaria grass roots (p
< 0.05); for the treatments carried out with the presence of salt. Differences were only observed between the groups with the highest and lowest salt concentrations. The effect of the addition of biochar on germination and root growth in the presence of salt showed that the group treated with the highest dose of biochar was significantly different from the others. The results are conclusive: the biochar enhanced the germination and root length properties. Similar findings have been reported with other tested precursors of biochar [35
]. The reduction in phytotoxicity after the application of biochar can probably be explained by the decrease in metals and osmotic toxicity, as identified by Abari et al. [49
]. According to Wang et al. [50
], the results obtained from the efflorescent salt waste and basic information on the phytotoxicity of such waste with respect to germination and root elongation is required for the evaluation of ecological risk type. As a rapid phytotoxicity test method, the approach used here presents advantages such as sensitivity, simplicity, low cost, and convenience with respect to the use of unstable chemicals or samples.
In the near future, more tests will be required to reduce any errors arising from the experiments. Another constraint is the scaling of biochar remediation from Petri dishes to pots and experimental plots.