Abstract
Nine geophagic material samples were analysed in order to characterise their mineralogical and elemental constituents, and infer on their health threats. Most abundant mineral in the samples was quartz, followed by smectite, kaolinite and muscovite in minor; and microcline, plagioclase, and goethite in trace quantities. Dominant major oxides were SiO2 (43 - 74 wt%) and Al2O3(15 - 19 wt%). Chemical Indices of Alteration (79.37-99.34) and Weathering (94.38-99.92) values suggest moderate to extreme silicate weathering, and alkali and alkaline earth metals depletion. Based on molar proportions of Al2O3, CaO + Na2O, and K2O, weathering trend and mineralogical compositions of the soils showed more advanced argillic alteration with rare earth elements being more concentrated compared to Upper Continental Crust. Excessive amounts of quartz and heavy metals in the geophagic soils represent significant health threats to geophagic individuals, though heavy metals had low biaccessibility values. There is need for soils beneficiation before ingestion.
1 Introduction
Geophagia is a universally recognised habit characterised by the deliberate consumption of earthy materials (particularly sediments, soils and clays). Although mostly associated with pregnant and lactating women, the practice is equally reported in children and adolescents across all socio-economic classes, religious and cultural affinities of the world. In Southern Africa, the habit has been documented in Botswana [1], Democratic Republic of Congo (DRC) [2,3], Guinea, Ivory Coast and Senegal [4], Malawi [5], South Africa [6], Swaziland [7], Tanzania [8, 9], and Zimbabwe [10]. Ademuwagun et al. [11] reported geophagia among the Yorubas of Nigeria, Diko & Ekosse [12] mentioned the habit occurring in Cameroon, and Prince et al. [13] reported its practice among the Luo women of Kenya. Geophagia has also been reported in other parts of the globe including Indonesia [14], Turkey [15] and Guatemala [16].
Reasons advanced for sustenance of the habit globally include cultural, medical, nutritional, psychological [17], social [18], spiritual and religious [19], hunger, and physiological fulfillments [20]. The Chaggas of Tanzania consider geophagia to be sacred to women [8]. According to South African urban women, ingesting soils enhances their beauty [6]. Anell & Lagerkrantz [21] have reported that African pregnant women consume soil to facilitate smooth delivery, and enhance dark skin pigment for the baby.
Geophagic materials which are primarily soils are considered as a source of mineral nutrient supplementation. Dreyer et al. [22] and Harvey et al. [23] reported the ingestion of red soils to alleviate symptoms of iron (Fe) deficiency anemia. Geophagic material has been reported to supply 17-55% of recommended pregnancy supplementation of calcium (Ca), magnesium (Mg), zinc (Zn), iron (Fe), copper (Cu), manganese (Mn), selenium, (Se), potassium (K), nickel (Ni) and cobalt (Co) in Guatemala [16]. Geophagic soils have the ability to adsorb plant secondary metabolites and diarrhea causing enterotoxins; detoxify noxious or unpalatable compounds present in diet; alleviate hunger, and gastrointestinal upsets such as diarrhea; and deal with excess acidity in the digestive tract [24]. The Yorubas of Nigeria use clays as one of the active ingredients for the treatment of dysentery and cholera [11].
Brouillard & Rateau [25], Hooda et al. [15] and Severance et al. [26] have shown that ingesting soils with high cation exchange capacity (CEC) could cause Fe deficiency. The consumption of soils, if contaminated, could also lead to health complications including infections from pathogenic soil organisms such as geohelminths [27]. Henry & Cring [28] have indicated that ingesting soils with high concentrations of useful components like iron may also result in health complications like hemosiderosis. Ingesting soils high in coarse particles could affect dental enamel [29] and provoke the rupturing of the Sigmoid Colon [30].
According to Hooda [31], Mahaney et al. [32] and Wilson [24], the positive or negative effects of geophagia depend on the physico-chemical, chemical, biological, and mineralogical properties of the material ingested. The aforementioned properties can be linked to the extent of weathering of the soils and geochemical processes. The ingestion of soil for supplementation of Ca and Fe depends on the concentrations of these elements in the soil which to a large extent depends on the type of soil and the degree of weathering that has taken place. Chemical weathering is one of the principal processes by which weathered material is altered prior to deposition. Whether in allochtonous or autochthonous deposits, weathering products are cumulative results of lithology, degree of physical and chemical weathering experienced by source rock, and subsequent changes during transportation and/or deposition [33, 34]. Chemical weathering is also of fundamental importance for different soil ecosystems, since it contributes to soil macro and micro nutrients. Furthermore, the wide range of element concentrations in soils is the result of interaction between several chemical factors affecting pedogenesis.
Studies carried out by Pebsworth et al. [35] revealed that high content of Fe in geophagic soils does not necessarily translate into high bioavailability, putting the possibility of geophagic soils supplementing for Fe to question. Soil mineralogy plays an important role in the behaviour of soils and contributes to the effect of geophagia among humans through its influence on soil CEC. The alleviation of gastrointestinal upsets and detoxification of noxious or unpalatable compounds present in the diet of individuals by soils depends on the soil sorption capacity which is determined by its CEC, highlighting the significance of the mineralogy of geophagic soils. Given that geophagia is reported to supplement nutrients, in-depth understanding of the geochemical controls of soil nutrients, weathering intensity and trends, and source material characterisation are indispensable. In the Free State Province of South Africa where the habit is persistent, limited efforts have been made to characterise geophagic materials [36]. The aim of this study is to geochemically characterise geophagic soils commonly ingested in the province in order to understand their mineralogical and elemental composition, weathering intensity, trends and provenance. This study is part of a wider project aimed at better understanding of human and enzootic geophagic materials, and practices in Africa.
2 Materials and Methods
2.1 Description of study area
Qwaqwa (28° 24′ 21.8 S; 28° 57′ 10.3 E) and Mangaung (29° 12′ 65 S; 26° 15′ 46 E), both located in the Free State Province of South Africa, are the sites from where samples were collected for the study. Qwaqwa and Mangaung (Fig. 1) were selected as study sites in order to complement previous physicochemical and hematological studies conducted at the two sites [36]. Being a rural community, the main activities in the QwaQwa area are agricultural. Mangaung is a settlement close to the City of Bloemfontein. Both Qwaqwa and Mangaung do not have manufacturing industries. Industrial activities are limited to mainly agricultural and commercial services. The inhabitants of these areas are employed mostly in various low scale service industries.
Map showing location of sampling areas.
The Drakensberg Group, aged approximately 183 million years (Ma) [37], with a thickness of >2000 m features prominently at Qwaqwa having its topographic massif rising to 1800 m above sea level (asl). The main rock types are basaltic lava flows which are fed by Karoo dolerite dykes [38]. This topographic massif is part of the Karoo Supergroup. The soils were formed from the alteration of plagioclase in the basalts, and feldspathic arenites in the Sandstones of the Drakensberg Mountain [39]. The geology of the Mangaung area is dominated by the rocks of the Beaufort Group of the Karoo Supergroup. The rocks, mainly sedimentary, consist of alternating sandstone and mudstone layers with intruding dolerite dykes and sills characterising the topography [40, 41]. Mangaung has an altitude of 1400 m asl. The soils vary from sandy to clayey depending on the variation of the parent material [40].
2.2 Samples collection and analyses
2.2.1 Samples collection
Five of the soil samples were collected from Qwaqwa, a rural settlement, and four of the samples from Mangaung an urban settlement (Fig. 1). The deposits from where the samples were collected were located in various places within the community including the yard, and at the foot of a mountain. Whereas some of the samples analysed were bought, most of them were collected with the assistance of practicing geophagists in the area. The samples were collected mostly from the surface of the deposit after scraping off the top soil. The particle size distribution of the samples was established. The bulk soil samples were mineralogically and geochemically analysed in order to identify, quantify and characterise the minerals, and the major and trace elements contained in them.
2.2.2 Particle size distribution
Particle size distribution (PSD) of the samples was determined using laser diffraction technique as described by van Reeuwijk [42] after dispersal with sodium hexametaphosphate (Na4P2O7), 30% H2O2, and 10% HCl. A Malvern Mastersizer 2000 fitted with Hydro 2000G dispersion unit was then used to determine the particle size distribution as described in Council for Geosciences [43].
2.2.3 Mineralogical analysis
X-ray diffractometry was used for mineral identification. Prior to analysis, the samples were air-dried and gently ground with the aid of mortar and pestle into powdery form. The ground samples were loaded in sample holders and mounted in a Philips PW 3710 X-ray diffractometer system for identification of the mineral phases as described by Ekosse & Anyangwe [1]. The analytical equipment employed a Cu-Kα radiation and a graphite monochromator set at 40 kV and 45 mA. A PW 1877 Automated Powder Diffraction (APD) X'PERT Data Collector software package was used to record the raw data, while a Philips X'PERT Graphics & Identify software package was used for qualitative and semi quantitative identification and analyses. Samples were scanned from 2°2θ to 70° 2θ at a speed of 1°2θ / min. The interpreted results were compared with data and patterns available in the Mineral Powder Diffraction File data book[44]. The phase concentrations of the different minerals identified were determined as semi quantitative estimates, using relative peak heights/areas proportions and reference-intensity-ratio (RIR) method [45].
2.2.4 Geochemical analysis
Major oxides (SiO2, TiO2, Al2O3, Fe2O3(t), MnO, MgO, CaO, Na2O, K2 O, P2 O5, and Cr2 O3) and trace element (As, Ba, Bi, Br, Ce, Co, Cr, Cs, Cu, Ga, Ge, Hf, La, Mo, Nb, Nd, Ni, Pb, Rb, Sc, Se, Sm, Sr, Ta, Th, Tl, U, V, W, Y, Yb, Zn, Zr) concentrations were analysed using the PANalytical Axios WDXRF spectrometer in accordance with the method described by the Council for Geosciences [43], and Fitton [46]. For major and minor element analyses glass disks were used to eliminate matrix effects. Milled samples with grain size of <75 μm were heated at 1000° C for 3 hours to oxidise Fe2+ and S and to determine the loss of ignition (LOI). One gram of heated sample and 9 g of flux consisting of 34% LiBO2 and 66% Li2B4O7 were fused at 1050°C to form stable glass disks. For trace element analysis 12 g of milled sample and 3 g Hoechst wax were mixed and pressed into powder briquette by hydraulic press with an applied pressure of 25 ton. Each sample was analysed thrice and the mean recorded. For quality control, an in-house amphibolite reference material (12/76) was used to ensure accuracy of the data generated by the WD-XRF equipment.
To understand the formation of the geophagic soils, mineral and chemical indices were considered. The Mineralogical Index of Alteration (MIA) for the soils given by the ratio of quartz to the sum of quartz + K-feldspar + plagioclase, provided an assessment of the extent of weathering [47, 48] was used to determine the extent of weathering of the geophagic soils. The Chemical Index of Alteration (CIA) values were calculated from the expression [Al2O3/(Al2O3 + CaO + Na2O + K2O)] x 100, where CaO was considered as that which is incorporated into silicate structure [36]. Chemical Index of Weathering (CIW) of each sample was calculated using the formula [Al2O3/(Al2O3 + CaO + Na2O)] x 100 [49]. Provenance of sediments was inferred from the Index of Compositional Variability (ICV) and ratio of K2O/Al2O3 [50], and the spider plot of the trace elements and rare earth elements (REEs) based on normalisation to UCC values. The ICV is expressed as: ICV = (Fe2O3 + Na2O+CaO+ MgO +TiO2)/ Al2O3.
2.2.5 Analysis for bioaccessibility
The physiologically based extraction test which involves simulating the chemical environment in the human gastrointestinal tract (GIT) to determine the bioaccessibility of ions in the GIT was used to determine the bioaccessibility of heavy metals in the geophagic samples. The procedure involved using gastric juice (prepared by mixing 2.5 g pepsin (Pocrine gastric mucosa), 1 g tri-sodium citrate, 1 g DL Malic acid and 840 μL lactic acid syrup and acidified to pH 1.5 with concentrated hydrochloric acid) and pancreatic juice (500 mg of pancreatin and 175 mg of bile salts per litre of gastric juice solution, and solid sodium hydrogen carbonate (NaHCO3) neutralized to pH 7) to extract heavy metals that are bioaccessible in the stomach and intestines respectively of humans. All reagents were Merck KgaA analysed reagents. Details of the method used for extraction and reagent preparation are found in Intawongse & Dean [51], Sialelli et al. [52], and Li & Zhang [53]. The concentrations of As, Cd, Cr, Co, Cu, Mo, Ni, Pb Se, and Zn in the extracts were determined in the stomach phase (metals extracted with gastric juice), intestinal phase (metals extracted with the prepared pancreatic juice) and residual phase (metals left in samples after extraction with gastric and pancreatic fluids) using a Perkin Elmer Nexion 300 Q ICP-MS. The ratio of the bioaccessible fraction of the metal to its total concentration was taken as the fraction of the metal that an individual is exposed to upon ingestion of these soils. Percent bio accessibility was determined according to equation 1.
In the experiment, samples were analysed in duplicate. In addition, the sum of heavy metals concentration in the stomach, intestinal and residual phases of heavy metals in each of the samples were compared with the single aqua extra extraction to determine element recovery.
3 Results and discussion
3.1 Particle size distribution
The PSD of the geophagic samples reflected clay (≤2 μm), silt (2 - ≤20 μm), and sand (> 20 μm) size particles. The geophagic samples were mostly silty with wt % silt in the different samples ranging from 37% - 80%, whereas wt% sand and clay sized particles ranged from 6% - 60% and 3% - 18% respectively. The shape of the PSD curve for the samples (Fig. 2) indicated the particles were distributed over a wide range as the curves were relatively flat.
Particle size distribution of studied geophagic soil samples.
3.2 Mineralogical characterization
Seven minerals were identified in the geophagic soil samples: quartz, kaolinite, smectite (type identified was montmorillonite), (NaCa)(Al,Mg)6(Si4O10)3(OH)6-nH2O; muscovite, microcline, KAl3Si3O10(OH)2; plagioclase (type identified was albite), (Na,Ca)Al(Si Al)3O8; and goethite. Major diagnostic peaks with reference values [50] identified were as follows: quartz were at 4.24 Å,3.34 Å and I.81 Å and were very close to the values for standard Quartz (46-1045); kaolinite were at 7.12 Å, 4.41 Å and 3.56 Å which corresponded to similar peaks obtained for Kaolinite-1Md (29-1488) from Pugu, Tanzania [50], smectite were 13.6 Å, 4.46 Å and 2.56 Å and were very similar values for standard Montmorillonite-15A (29-1498); muscovite were at 9.95 Å, 3.32 Å and 2.56 Å which were very close to those of Muscovite-2M1(2-263); microcline were at 4.22 Å, 3.26 Å and 3.25 Å which corresponded to Microcline, ordered (19-926); plagioclase were 4.03, 3.20 and 3.18 and were very similar to the values for Albite, Ca-rich, ordered (41-1480); and goethite were at 4.18 Å, 2.69 Å and 2.45 Å and were very close to Goethite (29-713).
Table 1 presents a summary of results of the mineral assemblages of the different soil samples and the relative abundance of the respective minerals identified. Quartz was the most abundant non-clay mineral, whereas smectite was the most abundant clay mineral followed by both kaolinite and muscovite which occurred in minor quantities. Occurring in trace quantities were microcline, plagioclase (in five of the samples), and goethite (in only two of the samples). The mineral constituents of the studied geophagic soils have been identified in other geophagic soils and clays from Guinea, Ivory Coast and Senegal [4], DRC [3], Cameroon and Nigeria [54], and Swaziland [7].
Mineral abundance (wt%) in studied geophagic soil samples
| Sample No | Mineral abundance (wt%) | ||||||
|---|---|---|---|---|---|---|---|
| Quartz | Kaolinite | Smectite[*] | Muscovite | Microcline | Plagioclase[*] | Goethite | |
| S1 | 75 | 9 | 10 | 3 | 1 | 2 | - |
| S2 | 68 | 8 | 17 | 5 | 1 | 1 | - |
| S3 | 69 | 7 | 17 | 6 | 1 | - | - |
| S4 | 71 | 4 | 17 | 6 | 1 | - | 1 |
| S5 | 60 | 5 | 13 | 15 | 1 | 6 | - |
| S6 | 73 | 6 | 15 | 5 | 1 | - | - |
| S7 | 74 | 6 | 16 | 1 | 2 | 1 | - |
| S8 | 79 | 2 | 2 | 7 | 3 | 7 | - |
| S9 | 53 | 29 | 12 | - | - | - | 6 |
Except for sample S9, each sample contained plagioclase and/or K-feldspar with values ranging from 1-7 wt% for plagioclase (albite) and 1-3 wt% for K-feldspar (microcline). Considering values in descending order obtained from MIA calculations, sample S9 = 100%, samples S3, S4 and S6 = 98.6% each, sample S2 = 97.1%, sample S1 = 96.2%, sample S7 = 96.1%, sample S5 = 89.6%, and sample S8 = 88.8%, the geophagic soils could have been developed from intense weathering of primary minerals. Quartz and kaolinite had an inverse relationship. This observation is reflective of incomplete alteration of primary minerals (particularly plagioclase and K-feldspar) in the soil samples. Plagioclase is not usually present in soils because of its nature to chemically weather and alter rapidly [55, 56] to clay minerals.
3.3 Major element geochemistry and weathering characteristics
Concentrations of major oxides in the studied geophagic soil samples are given in Table 2. Certified values of major and trace element concentration in the reference sample (12/76) and values obtained for the reference sample in this research are reported (Table 2). Percentage recovery of major and trace element oxide was between 79% and 125%. The two major oxides which dominated the chemical composition of the soils were SiO2 and Al2O3. Samples with high concentrations of Si oxide had lower concentrations of Al oxide. Geophagic soil samples with high SiO2/Al2O3 ratio had high quartz content.
Concentrations of major oxides (wt%) in studied geophagic soil samples
| Sample | S1 | S2 | S3 | S4 | S5 | S6 | S7 | S8 | S9 | 12/76 | |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Certified | Result | ||||||||||
| SiO2 | 73.87 | 66.39 | 68.55 | 65.99 | 66.81 | 69.11 | 69.85 | 73.62 | 43.46 | 45.78 | 45.07 |
| TiO2 | 0.97 | 0.84 | 0.76 | 0.72 | 0.85 | 0.75 | 0.87 | 0.78 | 1.74 | 1.52 | 1.54 |
| Al2O3 | 15.86 | 18.60 | 17.44 | 17.10 | 17.47 | 17.27 | 17.28 | 14.61 | 25.57 | 16.52 | 16.69 |
| Fe2O3(t) | 1.80 | 4.73 | 4.02 | 6.73 | 4.30 | 3.91 | 3.11 | 3.07 | 17.63 | 9.79 | 9.80 |
| MnO | 0.003 | 0.01 | 0.03 | 0.01 | 0.02 | 0.03 | 0.01 | 0.02 | 0.07 | 0.179 | 0.179 |
| MgO | 0.48 | 0.63 | 1.08 | 0.86 | 0.98 | 1.06 | 0.85 | 0.85 | 0.42 | 8.04 | 8.34 |
| CaO | 0.02 | 0.01 | 0.01 | 0.15 | 0.28 | 0.01 | 0.33 | 0.05 | 0.01 | 10.87 | 10.97 |
| Na2O | 0.22 | 0.10 | 0.04 | 0.08 | 0.76 | 0.05 | 0.07 | 0.79 | 0.01 | 3.58 | 3.69 |
| K2O | 2.33 | 3.20 | 2.56 | 3.19 | 3.50 | 2.51 | 1.56 | 2.79 | 0.15 | 0.68 | 0.68 |
| P2O5 | 0.04 | 0.06 | 0.09 | 0.07 | 0.07 | 0.09 | 0.03 | 0.12 | 0.05 | 0.257 | 0.255 |
| Cr2O3 | 0.01 | 0.01 | 0.01 | 0.01 | 0.01 | 0.01 | 0.01 | 0.01 | 0.03 | 0.071 | 0.075 |
| L.O.I. | 4.57 | 5.65 | 5.58 | 5.22 | 4.54 | 5.46 | 6.66 | 3.51 | 11.11 | 2.61 | 2.63 |
| Total | 100.17 | 100.22 | 100.14 | 100.12 | 99.60 | 100.24 | 100.64 | 100.22 | 100.16 | 99.89 | 99.93 |
Next to SiO2 and Al2O3 in terms of dominating the chemical composition of the geophagic soils was Fe2O3(t) (Table 2). The mean values for the three most concentrated oxides compared to values for UCC [56] were SiO2 being very close (66.00 wt%), and higher concentrations for both Al2O3 (15.2 wt%) and Fe2O3(t) (4.5 wt%). There was also higher concentration of TiO2 in the soil with a mean of 0.92 wt% from the UCC value of 0.5 wt%. The soils had lower concentrations of MgO (mean was 0.80 wt% whereas UCC value is 2.2 wt%), CaO (mean being 4.2 wt% compared to UCC value of 0.14), Na2O (mean was 0.23 wt% whereas UCC value is 3.9 wt%), and K2O (mean being 2.42 wt% compared to UCC value of 3.4 wt%). Apart from SiO2, Al2O3, TiO2 and Fe2O3(t) values obtained for the concentrations of the major oxides were generally very low reflecting very intense weathering of primary minerals.
The Chemical Index of Alteration (CIA) is based on the assumption that the dominant process during chemical weathering is the degradation of feldspar and the formation of clay minerals [57]. Values of CIA (79.37-99.34) and CIW (94.38-99.92) of the geophagic soils suggest intermediate to extreme silicate weathering (Fig. 3). The values are also indicative of low to depleted oxides particularly those of K, Ca, and Na in the soils. The A-CN-K plot [47] presents the relations between Al, Ca, Na and K to quantify the alteration of primary mineral. The samples of the studied geophagic soils plotted near the A area depicting intensive weathering in comparison to the UCC (Fig. 4). Weathering trend from CIA values and mineralogical compositions of the soil showed that the samples were more weathered than the UCC. The observed increase in CIA values was directly proportional to the degree of clay minerals precipitation (Fig. 5).
Plot of Chemical Index of Weathering (CIW) versus Chemical Index of Alteration (CIA) of studied geophagic soil samples.
![Figure 4 Illustrations of weathering trends and approximate mineralogical compositions from: A-CN-K plot; Ka-kaolinite; Gi-gibbsite;IL-illite; M-muscovite, Pl-plagioclase, Ks-K-feldspar, S-smectite. UCC - upper continental crust (Taylor & McLennan [56]). To the left is the CIA (Nesbitt & Young [33]).](/document/doi/10.1515/geo-2017-0009/asset/graphic/j_geo-2017-0009_fig_004.png)
Scatter plot diagram of Chemical Index of Alteration versus clay minerals abundance of studied geophagic soil samples.
3.4 Trace and rare earth elements geochemistry
The trace and REEs concentrations of the geophagic soil samples for As, Ba, Co, Cr, Cs, Hf, La, Nd, Ni, Rb, Sc, Sm, Th, U, V, W, Yb, Zn and Zr are presented in Table 3. The concentrations of Bi, Br, Mo, Se, Ta, and Ti were below the detection limit of the XRF. Except for samples S5 and S8, the samples had lower concentrations of Ba compared to that of UCC. The concentrations of Ce, Cr, La, W, Yb and Zr were higher. Though the concentration of Co was generally higher, samples S1, S7, S8 and S9 had lower concentrations of the element. Hafnium had slightly higher concentration except for samples S3, S6, and S9 in which the concentrations were lower. Rubidium, Th and U had had higher concentrations except for sample S9 in which the concentration of the three elements were slightly low. The concentration of Nd in all the samples was higher except for sample S7 which had lower concentration. Nickel concentration in samples S2, S7 and S8 was low.
Trace elements concentrations (ppm) in studied geophagic soil samples
| S1 | S2 | S3 | S4 | S5 | S6 | S7 | S8 | S9 | UCC | Certified | Results | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| As | 18 | 6.6 | 4.3 | 4.6 | 5.8 | 7.4 | 18 | 20 | 4 | 1.5 | 4 | 4 |
| Ba | 406 | 440 | 453 | 453 | 632 | 449 | 353 | 564 | 98 | 550.0 | 266 | 334 |
| Ce | 83 | 210 | 97 | 152 | 87 | 90 | 37 | 80 | 49 | 64.0 | 44 | 42 |
| Co | 6.6 | 14 | 13 | 9.0 | 12 | 12 | 8.5 | 8.2 | 7.7 | 10.0 | 35 | 40 |
| Cr | 54 | 55 | 49 | 65 | 72 | 49 | 67 | 58 | 181 | 35.0 | 512 | 503 |
| Cs | 5 | 5 | 5 | 5 | 5 | 5 | 5 | 5 | 5 | 3.6 | 5 | 5 |
| Cu | 8.9 | 13 | 23 | 16 | 21 | 23 | 18 | 15 | 106 | 7.8 | 6.2 | |
| Ga | 21 | 22 | 20 | 21 | 22 | 20 | 23 | 19 | 35 | 18 | 19 | |
| Ge | 1.4 | 1.7 | 1.1 | 1.4 | 1.4 | 1.1 | 1.2 | 1.4 | 1.6 | 1 | 1 | |
| Hf | 15 | 10 | 3.2 | 6.3 | 4.5 | 3 | 7.2 | 10.0 | 3.3 | 5.8 | 4 | 3 |
| La | 52 | 91 | 56 | 102 | 46 | 56 | 32 | 48 | 34 | 30.0 | 24 | 37 |
| Nb | 23 | 19 | 16 | 16 | 18 | 16 | 20 | 17 | 12 | 31 | 33 | |
| Nd | 39 | 103 | 50 | 72 | 40 | 51 | 20 | 42 | 29 | 26.0 | 26 | 22 |
| Ni | 7.9 | 18 | 23 | 22 | 28 | 21 | 13 | 18 | 67 | 20.0 | 99 | 103 |
| Pb | 18 | 29 | 25 | 64 | 22 | 26 | 17 | 18 | 13 | 6.2 | 6.9 | |
| Rb | 130 | 205 | 129 | 196 | 172 | 128 | 106 | 135 | 6.9 | 112.0 | 21 | 21 |
| Sc | 12 | 14 | 15 | 17 | 17 | 16 | 17 | 15 | 47 | 11.0 | 39 | 35 |
| Sm | 10 | 22 | 11 | 12 | 10 | 10 | 10 | 10 | 12 | 4.5 | 10 | 10 |
| Sr | 52 | 49 | 68 | 117 | 85 | 67 | 48 | 82 | 2 | 332 | 337 | |
| Th | 19 | 20 | 15 | 16 | 20 | 15 | 18 | 17 | 6.4 | 10.7 | 3.8 | 3 |
| U | 4.4 | 3.6 | 4.3 | 4.4 | 3.6 | 4.3 | 3.7 | 8.2 | 2 | 2.8 | 2.2 | 2 |
| V | 73 | 79 | 83 | 88 | 94 | 82 | 91 | 79 | 249 | 259 | 276 | |
| W | 3 | 3.4 | 3 | 5.3 | 3 | 3 | 3 | 3 | 3 | 2.0 | 3.6 | 3 |
| Y | 41 | 52 | 34 | 126 | 38 | 33 | 29 | 50 | 26 | 27 | 29 | |
| Yb | 5.1 | 4.6 | 4.2 | 9.4 | 4.6 | <3 | 3.3 | 4.4 | <3 | 2.2 | 4.2 | 3.8 |
| Zn | 28 | 74 | 86 | 55 | 88 | 83 | 34 | 64 | 69 | 71.0 | 88 | 88 |
| Zr | 529 | 343 | 217 | 249 | 212 | 215 | 326 | 360 | 192 | 190.0 | 122 | 123 |
The ratio of the mean concentrations of the geophagic soil samples to concentrations of the UCC for the elements were in general slightly above one except for Ta (0.92) and Zn (0.91) which were just below one (Fig. 6). The ratios for Cr (2.06), Sm (2.64) and Yb (2.10) were above two, and As had the highest ratio which was 4.33. Elemental ratios which had values above one implied that the concentration of the element in the soil samples was higher than UCC values, and those below one were indicative of lower concentration values (Fig. 6).
Ratio of mean concentrations of trace and rare earth elements of geophagic soil samples to those of Upper Continental Crust.
3.5 Provenance of the geophagic soils
Index of Compositional Variability (ICV) values for the samples were as follows: S1 = 0.22, S2 = 0.34, S3 = 0.34, S4 = 0.50, S5 = 0.41, S6 = 0.34, S7 = 0.28, S8 = 0.38, S9 = 0.77 and the mean = 0.34. The samples displayed ICV values which were less than one. Argillaceous sediments dominated by clay minerals which display ICV values that are <1.0 could have been derived from a craton environment, and possibly had undergone intensive weathering of their first cycle sediments [50, 58]. The K2O/Al2O3 ratio indicates relative abundance of alkali feldspar versus plagioclase and clay minerals in argillaceous sediments. K2O/Al2O3 ratios of alkali feldspar falls within the range of 0.4 -1, illite (muscovite also by implication of its chemical formula being close to that of illite) = 0.3 and other clay minerals nearly zero [50] including kaolinite and smectite.
A ratio of < 0.4 is indicative of minimal feldspar in the original argillaceous sediments [50]. The ratios of K2O/Al2O3 for the samples were as follows: S1 = 0.15, S2 = 0.17, S3 = 0.15, S4 = 0.19, S5 = 0.20, S6 = 0.15, S7 = 0.10, S8 = 0.19, S9 = 0.01 and the mean = 0.13. These results were consistent with the mineralogical constituents of the geophagic soils, and are indicative of the soils having been formed through intensive weathering processes.
3.6 Bioaccessibility of heavy metals
The percent recovery of heavy metal in the samples ranged from 82-113%. The concentrations of metals in the samples vary as indicated in Table 4. The concentrations of Se, As and Mo in the PBET extracts were below 2 ppb and hence below the detection limit of the ICP-MS used for analyses. Heavy metal concentrations in the samples followed the order Cr > Zn > Cu > Pb > Ni > Co > Cd (Table 4). Samples S9, S5, and S3 had higher concentrations of the heavy metals than the other samples. Among all the metals studied, Zn had the highest percent bioaccesibility followed by Ni, Pb and Cu (Fig. 7). The percent bioaccessibility of the heavy metals was highest in the stomach compared to the intestines for all elements. Cadmium concentration was generally very low hence its low bioaccessibility observed in the GIT.
Concentrations of heavy metals in studied geophagic soil samples
| Sample | Metal concentrations (mg/kg) | ||||||
|---|---|---|---|---|---|---|---|
| Cd | Co | Cr | Cu | Ni | Pb | Zn | |
| S1 | 2.9 | 6.6 | 54.0 | 8.9 | 7.9 | 18.0 | 28.0 |
| S2 | 2.5 | 14.0 | 55.0 | 13.0 | 18.0 | 29.0 | 74.0 |
| S3 | 2.2 | 13.0 | 49.0 | 23.0 | 23.0 | 25.0 | 86.0 |
| S4 | 2.6 | 9.0 | 65.0 | 16.0 | 22.0 | 64.0 | 55.0 |
| S5 | 2.3 | 12.0 | 72.0 | 21.0 | 28.0 | 22.0 | 88.0 |
| S6 | 2.6 | 12.0 | 49.0 | 23.0 | 21.0 | 26.0 | 83.0 |
| S7 | 2.1 | 8.5 | 67.0 | 18.0 | 13.0 | 17.0 | 34.0 |
| S8 | 2.3 | 8.2 | 58.0 | 15.0 | 18.0 | 18.0 | 64.0 |
| S9 | 2.6 | 7.7 | 181.0 | 106.0 | 67.0 | 13.0 | 69.0 |
Bioaccessibility of heavy metals in geophagic soil samples.
3.7 Health implications
The mineralogical and chemical compositions of the studied geophagic soils indicate abundant quartz and smectite content. Quartz in the geophagic materials is as a result of its resistance to chemical alteration. Excess quartz in geophagic materials imparts an undesirable gritty feel during ingestion. In addition, geophagic soils that are gritty contain fine sand particles of quartz and feldspars which may negatively affect dental enamel of geophagic individuals. Quartz having a higher degree of hardness of 7 on Mohs scale compared to dental enamel (5 on Mohs scale), can grind, crack and break dental enamel during mastication [12]. Quartz particles can also erode gastro-intestinal (GI) lining of geophagic individuals with the possibility of perforating the sigmoid colon.
The relative abundance of smectite at the expense of kaolinite imparts higher CEC and an increase in negatively charged sites for adsorption of positively charged ions by electrostatic force. Geophagic soils with large quantities of negative charge readily accommodate cations that may be subsequently available for nutrient supplementation or assist with detoxification by forming complexes with toxins [7].
Results from trace element analysis also revealed significant concentrations of heavy metals such as As, Cr, Cu, Pb and Zn (Table 3). The concentration of As ranged from 4.3 ppm to 20 ppm, with the highest values obtained from samples S1, S7, and S8. Chromium recorded very high concentrations in all samples (49-181 ppm) with the highest value reported in sample S9. Sample S9 equally registered the highest concentration of Cu (106 ppm). Lead concentration in the samples ranged from 13 ppm in sample S9 to 64 ppm in sample S4, whereas Zn concentration ranged from 28 ppm in sample S1 to 88 ppm in sample S5. The heavy metal exposure risks presented by ingestion of these soils may depend on the amount of soils that is ingested per day and the bioavailability of the metal in the soil ingested. The soil matrix, chemical form of the element in question, soil, stomach and intestinal pH, and the soil: solution ratio according to Kutalek et al. [59] determine the bioavailability of elements in soils. Abrahams et al. [2] reported that Pb bioaccessibility in geophagic soils from United Kingdom vary between 3-83%. The bioaccessibility of heavy metals in these samples was therefore within the range of those reported in other studies. The recommended daily dose (RDA) of each metal as well as the reference dose (RfD) of each element also varies with the weight of the individual. The daily intake of heavy metals by individuals ingesting these soils may be low because of the low bioaccessibility but continuous ingestion over a long period may increase the hazard quotient of the soils as a possible route of heavy metal exposure especially for Zn which had the highest bioaccesibility. Though being an essential micronutrient, Zn when ingested in high concentrations or for several months may result in anaemia, damage of the pancreas and a decrease in high level of lipoproteins cholesterol [60]. It should however be noted that not all the bio accessible fraction may be accumulated [61]. Health effects may therefore sometimes be lower than indicated by the bioaccesible fraction of the heavy metal in the GIT.
4 Conclusions
The geochemistry of geophagic soils from Free State Province, South Africa was addressed in this study. Minerals constituents of the soil comprised of quartz, kaolinite, smectite (type identified was montmorillonite), muscovite, microcline, plagioclase (type identified was albite), and goethite. The MIA values were very high and reflective of geophagic soils having developed from intense weathering of primary minerals. The concentrations of the REEs in the soil samples were generally higher than those for UCC though some were lower. Very low mean values of ICV, K2O/Al2O3, and very high values of CIA and CIW of the geophagic soils were all indicative of extreme and intensive silicate weathering environment which characterised the geochemical setting of the soils. The weathered nature of the studied soils indicates poor to moderate geophagic potentials. Furthermore, excessive amounts of quartz and heavy metals such as As, Pb, Cr, Ni and Zn in the soils represent significant health threats for geophagic individuals. Bioaccessibility of heavy metals in the soils was low but health risks especially those associated with Zn exposure could be eminent among those ingesting these soils on a regular basis. Beneficiation of the soils prior to ingestion is recommended.
Acknowledgement
This study is part of the broader UN- ESCO/IUGS/IGCP 545 Project on Clays and Clay Minerals in Africa.
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