The intensification of manganese mining activity in the city of Moanda in recent years is supported by the ever-increasing demand of the world market for manganese. In 2019, Gabon achieved annual production of 6.1 million tonnes against 5.2 million tonnes in 2018, an increase of 17%  . Despite the economic importance of this activity, the fact remains that most of the population of Moanda lives below the poverty line. This poverty forces these people to resort to working the land by practicing rural subsistence agriculture. Agriculture is not without risk in this region due to the proximity of the fields and the various mining activities which can be at the origin of environmental pollution through various scenarios and more precisely for this study pollution agricultural soils. This pollution results in the presence of metallic trace elements at abnormal concentrations on the surface or in the first meters of soil  .
Monitoring soil quality is an environmental and public health issue because the soil in the interface position indeed plays a primordial role in the balance of ecosystems: in climate change, the protection of the quality of the resource by water and as a support for agricultural production .
The absence of public data and soil protection policies so far has little been taken into account in public policies in our country. This shows the need for the implementation of this study to better orient the action of policies in terms of environmental monitoring .
2. Material and Methods
The study was carried out in a region of south-eastern Gabon in the Haut-Ogooué province and more precisely in the mining town of Moanda geo-located at 571 meters above sea level, between (1˚33'25" South latitude and 13˚13'40" East) . The region belongs to a non-metamorphic detrital and volcano-sedimentary series of the Francevillian which outcrops in the south-eastern quarter of Gabon . The climate of the region is relatively similar throughout the country which is characterized by a humid equatorial climate with average temperatures of 24˚C and average annual precipitation of between (800 - 2000 mm) . Mining takes place in an open pit mine on the Bangombé plateau, located above the city. It is one of the largest deposits in the world, with a manganese content of roughly 30% - 46%, one of the highest on the market . Since 2019, the production capacity of the Moanda mine has been increased, making Gabon the world’s second producer of high-grade manganese with 4.8 MT of manganese ore produced in 2019. The landscape of Moanda is recognizable by a mixture savannas and groves, plains, cornices and sandstone plateaus. The relief of Bangombé has become a landscape of small black sterile hills of manganese very poor in minerals and very acidic .
Sampling and sample preparation
Sampling took place during the month of January to June 2016. It was a shallow sampling. In the case of sampling agricultural soils, the depth relevant to the objectives is that between (0 - 15 cm) which constitutes the rhizosphere (the surface explored by the roots). The soil blocks were removed using a 5-point stainless steel helical auger forming a cross on each field. The soils of each site were homogenized in order to have a representative sample from each site  .
The collected soil samples were placed in labeled plastic bags, transported to the laboratory.
Drying in an oven for 24 hours at an average temperature of 105˚C was carried out in order to standardize the residual humidity level and to facilitate better grinding. The weighing after drying made it possible to determine the water contents for each sample. The samples were weighed fresh and after drying to determine the mass water content. This content noted W is calculated using Equation (1)
Mf = fresh leaf mass; Ms = dry leaf mass.
The dried samples were ground in porcelain mortars, the whole sieved to keep only particles of 2 mm of particle size  .
For analysis, the preparation of the pellets and the determination of the total elements were carried out at the CNRST-UATRS Division in Morocco in early July 2016.
Mineralogical analysis of soils
For the mineralogical analysis of soils, wavelength dispersive X-ray fluorescence was the required technique. It allows simultaneous qualitative and quantitative analysis of almost all elements ranging from Boron (Z = 5) to Uranium (Z = 92) in an unknown sample   .
The conversion of the concentration of an element in the pellet (expressed in %) into the concentration of an element i in the dry matter (expressed in mg/kg or ppm) is given by Equation (2):
= Concentration of elementi in the dry matter (in mg/kg or ppm).
= Concentration of an elementi in the pellet in (%).
3. Results and Discussion
The results of the weighing as well as the water contents (W) of the soil samples, before and after passage in the oven are presented in Table 2.
We can, in view of these results, say that the average water content (W) of the soils is of the order of 7% relative to the mass of the fresh sample. This low water content in the soils could be explained by the sampling period which corresponded to the short dry season.
Table 1. GPS Coordinates of the thirteen sites of sampling in the study area.
Figure 1. Geolocation of the sampling area (Moanda).
Table 2. Water content in agricultural soils (W%).
Qualitative analysis by wavelength-dispersive X-ray fluorescence of our agricultural soil samples identified a set of 22 oxides: Fe2O3, Al2O3, MnO2, SiO2, P2O5, TiO2, K2O, MgO, CaO, NiO, ZrO2, Nb2O5, Cr2O3, SrO, ThO2, BaO, SO3, Y2O3, CuO, ZnO, Na2O, CoO, and 4 naturally free elements: I, Ac, Rb, C.
These chemical elements have been grouped as follows: the alkali metals namely: Na, K and Rb; alkaline earths: Mg, Ca, Sr and Ba; the transition elements: Ti, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Cr and Mn; the presence of Al is a metal not metalloid; The presence of non-metals namely: O, C, P, S and Si; detect halogen: I; two rare earths: Ac, Th. Among these elements, we note the main nutrients of the soil which are: metallic cations: calcium (Ca), magnesium (Mg), potassium (K), sodium (Na) and the main anions: phosphorus (P). These nutrients contribute to biological functioning and are referred to as major building blocks in animals and plants . We have the presence of chemical elements whose low content is recognized as responsible for the deficiency of plants or their phytotoxicity: These are the so-called essential elements: Fe, Mn, Zn, Cu and Co and other elements considered beneficial for the plant: Cr, I, Ni, Rb, Se, Si, Sr and Ti. Its last two categories are also called trace elements. The absence of pollutants in all the soil samples analyzed, i.e. elements whose essential character for living beings has not yet been demonstrated . This absence may be linked to the practice of essentially artisanal agriculture without the use of fertilizers and pesticides.
The mineralogical concentrations (in % dry mass) of the major elements and trace elements in the agricultural soils of Moanda are presented in Table 3 and Table 4. The average contents of major elements in the sampled soils Fe2O3 (23.90), Al2O3 (16.26), MnO2 (4.17), SiO2 (38.98), P2O5 (0.18), TiO2 (2.46) are higher than the contents found at the level of the earth's crust, ie 5.00; 8.10; 0.10; 27.70; 0.11; 0.44 for Fe2O3, Al2O3, MnO2, SiO2, P2O5, TiO2; except for the concentrations of MgO (0.27), K2O (1.04) and CaO (0.44) in the agricultural soils of Moanda against 2.00; 2.50; 3.63 in the earth’s crust . The average contents of Al2O3, MnO2 and Fe2O3 are respectively 2, 4 and 5 times higher than those found in the earth’s crust. The non-detection of Mn on the sites (1, 2, 6, 7 and 8) corresponds to soil samples coming from plantations far from the town of Moanda, that is to say. Sites (1 and 2) corresponding to the soil samples taken on the Franceville-Moanda road; Sites (6, 7 and 8) for soil samples on the Moanda-Mounana road and site 12 on the Moanda-Bakoumba road. Detection of Cu (Sites 13 and 16), Co (Sites 3 and 11), Cr (Site 1), Zn (3, 7, 14 and 17) and Ni (1, 6, 7, 8, 10, 12 and 16) would correspond to point pollution, the exact origin of which is not known, but most of which are due to natural contamination in the vicinity of mining deposits and to atmospheric pollution.
Table 3. Concentration (in %) of major elements in agricultural soils of Moanda.
P.a.f: loss on fire; n.d: not detected; ! : set values below detection limits; CEC: crude earth content.
The soil nutrient concentrations (in mg/kg) vary from (31,400 - 3080) for K, (5620 - 1630) for Mg, (3200 - 800) for P, (33,000 - 862) for Ca, (1592 - 874) for S
Table 4. Concentration (in %) of trace elements in agricultural soils of Moanda.
P.a.f: loss on fire; n.d: not detected; !: Fixed/<<: lower to the limits detection.
Table 5. Concentration of nonmetallic elements (in mg/kg) in Moanda dry agricultural soils.
n.d: not detected; <<: lower to the limits detection.
Table 6. Concentration of metallic elements (in mg/kg) in Moanda dry agricultural soils.
n.d: not detected; <<: lower to the limits detection.
and (2040 - 1014) for Na at the sites detected. The average total concentrations of the elements K (17,188.24), Mg (3252.35), Ca (6348.47059), are higher than the average concentrations of its elements observed in the soils cultivated around Moanda by the extraction method and of mineralization in the work (Ondo et al.)  —For Ca (178.6 ± 24.8b), Mg (1.037 ± 430b), k (8.796 ± 785b). Only the Na (464.24) in our samples is lower than the Na (891.1 ± 47.2a) of the soils cultivated in the previous works.
The average concentrations of metallic trace elements (in mg/kg dry mass) in the Moanda agricultural soil layers between (0 - 15 cm) were decreasing values for Fe (334,282.35), Al (172,129.41), Mn (5583.53), Ni (2622.35), Nb (1466.12), Ac (922.82), Y (730), Zn (250.71), Th (143.53), Co (117.76), Cu (106.35), Co (94.47). A comparative study was carried out with the critical threshold toxicity standards recommended for its components to ensure the safety of agricultural products (Table 7).
The average concentrations of Cu, Fe, Mn, Zn, Ni, Co are well above the critical toxicity limits recommended for these elements for a safety of agricultural products which are (8 - 40 mg/kg) for Cu, (10 - 50 mg/kg) for Fe, (200 - 1000 mg/kg) for Mn, (10 - 100 mg/kg) for Zn, (10 - 100 mg/kg) for Ni, (15 - 50 mg/kg) for Co . These elements can cause phytotoxicity if their bioaccumulation is significant in plants grown in this region. However, we must qualify because the toxicity of an element depends among other things on its speciation, influenced by factors related to the soil (pH, redox potential, cation exchange capacity), to the plant (species, variety, stage of development, root excretion) and in the soil microorganisms (biological activity), climate (temperature, humidity), cultivation techniques, element speciation and interactions between elements. It would be difficult to assess the health impact from toxicological threshold values from often disparate publications .
Mineralogical distribution in the soils of Moanda
The distribution of the main metallic trace elements presenting pollution risks in the agricultural soils of Moanda (Figure 2) shows a preponderant accumulation of Fe, i.e. on average 59%, Al 31%, and Mn 9%. Oxides of (iron, aluminum, manganese, silica) and clay minerals (kaolinite) are the main minerals abundant in the ferralitic soils characteristic of Moanda soil . The Ni content is 1% while Cu, Cr, Co are less than 1%.
Moanda's soil is relatively acidic, with a relatively pH < 5 Ondo et al. , these soils will enhance the assimilation of certain soluble elements compared to other. The Mn, Fe, Cu and Zn will be the main trace elements which will be assimilated more easily according to their bioavailability .
The detrimental effects of soil acidity on plant physiology and yield reductions are generally attributed to the presence of soluble aluminum and/or in some soils, to that of soluble manganese . The toxicity of aluminum is most often indirect
Table 7. Critical micronutrient levels in soils.
Figure 2. Distribution of the main trace elements found in Moanda dry agricultural soils.
because aluminum disrupts the water supply of plants, inhibits the absorption of phosphorus and calcium and, on the contrary, enhances that of manganese .
Mn can quickly become the most available element for cultivated plants in these soils dedicated to agriculture (Anguile et al., 2013) .
These data show the major environmental stake of this study of the evolution of these trace elements in the agricultural soils of Moanda.
In order to judge the homogeneity on the spatial distribution of the chemical elements present in the soil samples from the point of view of the concentrations for each element, a simple comparison of the dispersion parameters (maximum content, minimum content, mean and standard deviation) was initially allowed to judge the homogeneity of our samples (Table 8).
The very high values of the standard deviations reflect the significant
Table 8. Relative statistical results to the variability of soils.
Figure 3. Distribution of Fe in the 17 agricultural soils sites of Moanda.
dispersiveness of the soils with the concentrations of the elements varying considerably from one site to another. Thus, for O, Al, Fe, Si and Mn, the concentrations vary respectively by ±41,252, i.e. 2.%; ±31,597 or (2%); ±106,741 or (5%); ±112,581 or (6%); ±74,053 or (4%) around their respective averages. For the rest of the elements, there is little variability, i.e. values less than 1% from one site to another compared to the average.
These data reflect the dispersive nature of the distribution of chemical elements in the composition of soils from one site to another in the same region. Figures 3-7 illustrate the distribution of the elements Mn, Ni, Fe, Al and Zn in the soil samples.
A high availability of these ETMs in general and of Mn in particular in the soil solution would present a real risk on the food security of agricultural products from this region.
Figure 4. Distribution of Al in the 17 agricultural soils sites of Moanda.
Figure 5. Distribution of Mn in the 17 agricultural soils sites of Moanda.
Figure 6. Distribution of Ni in the 17 agricultural soils sites of Moanda.
The analysis of the chemical composition of soils by X-ray fluorescence with wavelength dispersion shows the effectiveness of this analytical technique from a qualitative and quantitative point of view because it has revealed the presence of 27 elements in the soils. sampled in Moanda: O, Si, Fe, Al, C, Mn, Ti, K, Ca, Zr, Mg, Ni, P, Nb, I, Sr, S, Ba, Rb, Ac, Y, Na, Zn, Th, Co, Cu and Cr.
Among the elements, the main ions essential for the efficiency of agricultural production are: Ca, Mg, K, Na, P and S. Trace elements are: Fe, Al, Mn, Cu, Ni, Cr and Co. The set of dispersive parameters around the mean showed heterogeneity in the chemical composition of soils in a given region.
The concentration levels of the trace metal elements Cu, Fe, Al, Mn, Zn, Ni, Cr and Co found in agricultural soils of Moanda are above the critical toxicity limits recommended for the safety of agricultural products.
Moanda soils, being relatively acidic, will promote the assimilability of soluble Mn in soils rich in manganese ore as is the case in the Moanda region and increase the risk of toxicity to crops. The risk of manganese poisoning is very real in the mining town of Moanda. The practice of agriculture remains a concern for food security.
The lack of previous data did not allow us to estimate increases in soil levels as a result of human action. A periodic assessment as part of a monitoring program will show the evolution of metals, accidental pollution and chronic pollution corresponding to the impact of mining activity.
The authors thank the National Center for Scientific and Technological Research Division UATRS of Morocco for having accepted to analyze these samples, the research team of the Inorganic Materials Chemistry Laboratory (L.A.C.M.I.) for the efforts made to carry out this work as well as all the farmers who accepted the sampling of the fields all the people who contributed to the realization of this work.
ETM: Metallic Trace Element.
ICP/MS: Plasma couplé à un système d’induction hautes fréquences/spectromètre de masse (Plasma coupled to a high frequency induction system/mass spectrometer).
UATRS: Unités d’Appui Technique à la Recherche Scientifique (Scientific Research Technical Support Units).
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