Soil Quality in the Basin of Mine Effluents and the Potential of Alleviation of Metal Dispersion

Abstract

The soils along the basin ofmine effluent were investigated for their role in the mitigation of the pollution of river water in the residential areas around Potchefstroom in South Africa.The results showed that the soils were mainly semi-permeable soil (K from 5.96 ×10-6 to 1.26 ×10-5 m/s) and the cation exchange potential (CEC) values(19.3 to 38.5 cmol/kg)were indicative of the presence of clays in the soils. Investigation of the adsorption potential of soils showed that they had higher affinity for Cu compared to Ni and Zn, and bottom soils were better adsorbents than the top soils. Clayey material in soils along the mine effluent basin enhances interaction with metals, contributing to the mitigation of water pollution. Introduction The mobility of metals in surface water or in the soil is often dependent on the adsorption potential and the permeability of the soil (Shakelford and Daniel, 1991; Jungnickel et al., 2004). The physical and chemical nature of the soil is therefore an important factor controlling the dispersion of metals in the environment. It is reported that clay components of the soil play an important role in the sorption of metals or metal ions from environmental water sources (Antoniadis and McKinley, 2003). The reversible or irreversible fixation of metals ions on the soil components has been described by a migration term which includes advective and diffuse transport as well as a retardation term (Roehl and Czurda, 1998). Clay minerals have different geochemical behaviors and may undergo different physical and chemical responses to environmental conditions. The transport of solute and the cations exchange capacity of the clays will therefore depend of the rock-leachate in a given soil (Cuevas et al. 2009). The mineral component of soil is divided into silt, sand and clay, with clay materials belonging mainly to the phyllosilicates class. The formation of clay materials in soil depends on transformation mechanisms and neo-formation processes; it is therefore evident that the content of the soils will differ with environmental exposure related to the geographical area. The potential of the soil in a river basin to retard metal migration intothe ground water, or the transportation to other surface water sources must be investigated with particular attention to the nature of the specific soil. Metal retention in the soil can be affected by chemical interaction between the charged groups of the soil and the metals or by physical interaction, as metal migration through the soil is also determined by the soil permeability.

Full text

Soil quality in the basin of mine effluents and the potential of
alleviation of metal dispersion
E. Fosso-Kankeu1*, R. Kaitano1, F. Waanders1 and A.F. Mulaba-Bafubiandi2
1School of Chemical and Minerals Engineering, Faculty of Engineering, North West University, Potchefstroom
Campus, Potchefstroom, South Africa, *e-mail: 24838616@nwu.ac.za
2Minerals Processing and Technology Research Center, Department of Metallurgy, School of Mining,
Metallurgy and Chemical Engineering, Faculty of Engineering and the Built Environment, University of
Johannesburg, PO Box 17011, Doornfontein 2028, Johannesburg, South Africa
Abstract
Soil and water samples were systematically collected near the mining sites and along the effluents
flowing in the residential areas. The soils were characterized and used for metal adsorption.
The results showed that the soils were mainly semi-permeable soil (K from 5.96 x 10-6 to 1.26 x 10-5
m/s) and the CEC values (19.3 to 38.5 cmolkg-1) indicative of the presence of clays in the soils.
Investigation of the adsorption potential of soils showed that they had higher affinity for Cu compared
to Ni and Zn, and bottom soils were better adsorbents than the top soils.
This study shows that the nature of soils along the mine effluents enhances their interaction with and
retention of metals and minimizes the surface and ground water pollution.
Keywords Metal retention, mine effluent, soil permeability, clay, cation exchange capacity
Introduction
The mobility of metals in surface water or in the soil is often dependent on the adsorption potential and
the permeability of the soil (Shakelford and Daniel, 1991; Jungnickel et al., 2004). The physical and
chemical nature of the soil is therefore an important factor controlling the dispersion of metals in the
environment. It is reported that clay components of the soil play an important role in the sorption of
metals or metal ions from environmental water sources (Antoniadis and McKinley, 2003). The
reversible or irreversible fixation of metals ions on the soil components has been described by a
migration term which includes advective and diffuse transport as well as a retardation term (Roehl and
Czurda, 1998). Clay minerals have different geochemical behaviors and may undergo different physical
and chemical responses to environmental conditions. The transport of solute and the cations exchange
capacity of the clays will therefore depend of the rock-leachate in a given soil (Cuevas et al., 2009;
Michael et al., 2002). The mineral component of soil is divided into silt, sand and clay,; with clay
materials belonging mainly to the phyllosilicates class. The formation of clay materials in soil depends
on transformation mechanisms and neo-formation processes; it is therefore evident that the content of
the soils will differ with environmental exposure related to the geographical area. The potential of the
soil in a river basin to retard metal migration into the ground water, or the transportation to other surface
water sources must be investigated with particular attention to the nature of the specific soil. Metal
retention in the soil can be affected by chemical interaction between the negatively charged groups of
the soil and the metals or by physical interaction, as metal migration through the soil is also determined
by the soil permeability.
In this study the main focus is on the investigation of the physical and chemical characteristics of
environmental soils likely to contribute to the remediation metal dispersion in surface water.
Methodology
a- Sampling of environmental water and soils
Water samples were collected from effluents around obsolete mines near residential areas of the North
West Province in South Africa. Soils in the same basin were collected at the surface and at a depth of 1
m below the surface. The soils were extracted as block and loose samples for permeability test and
mineralogical analysis, respectively.
Commented [RV1]: Which mining sites and residential areas?
Need to be identified here.
Commented [RV2]: Explain acronym
Commented [RV3]: “Minimizes”? “Reduces” is more likely.
Commented [RV4]: Positively charged groups may play a role as
well, depending on the metal in question.
Commented [RV5]: A map of the general area, mine areas,
residential areas, and sampling points would be very helpful.

b -Characterization of environmental soils
XRD and XRF analyses
Loose soil samples were dried at 100 °C for 24 hours in a oven (Digital Oven 700 L, model code 385).
The dried samples were then crushed using a mortar and pestle to obtain a powder and subsequently
sieve screened to retain a particle size of less than 75 micron.
The recovered powder samples were analyzed by X-ray diffractometer (XRD) and X-ray fluorometer
(XRF) techniques.
The diffractometer used in the present investigation was a Philips model X’Pert pro MPD, equipped
with a 15 place automatic sample changer with utilisation parameters: copper anode tube with a power
of 1.6 kW used at 40 kV, 40 mA; Programmable divergence and anti-scatter slits; primary Soller slits:
0.04 Rad; 2 range: 4°-79.98° with a step size of 0.017; X’celerator detector with a nickel filter and
sample spinner.
The XRF was performed on a MagiX PRO & SuperQ Version 4 (Panalytical, Netherland) apparatus;
the sample was suspended above the x-ray tube in a two position carousel in a holder. A rhodium (Rh)
anode was used in the X-ray tube and operated at 50 kV and current of 125 mA; at a power level of 4
kW. The X-ray spectra were evaluated with the IQ+ program which is part of the SuperQ program.
Permeability test
The undisturbed soil was carefully cut in a mold and soaked for 48 hours till saturation. The saturated
soil in the mold was then introduced into the permeameter. The falling head apparatus had a stand pipe
of 4 mm diameter, which constituted the manometer. The water was allowed to flow in the manometer
and a stop watch was started giving the time for water to flow from level 1 to level 2, which was
recorded. The experiment was repeated three times and the average of readings in close agreement was
considered for the calculation of the permeability according to the following formula:
  
  
 (1)
Where K is the coefficient of permeability, a is the cross sectional area of the stand pipe, L is the length
of the soil sample, A is the cross sectional area of the soil, h1 is the hydraulic head across the sample at
the beginning of the experiment, h2 is the hydraulic across the sample at the end of the experiment
Cation exchange capacity test
The determination of the cation exchange capacity of the soil samples was done through a modified
method, or the BaCl2-compulsive exchange procedure (Gillman, 1979; Gillman and Sumpter, 1986;
Rhoades, 1982) using 1 M of NH4Cl in replacement of BaCl2.
c- Metal adsorption experiments
The dried soil samples, of particle size <75 microns, were exposed to metals in a batch system. A mass
of 0.1 g of soil was mixed in 100 ml solution of Cu and Zn at initial concentration of 5, 10, 15 and 20
mg/L. The mixture was stirred at 160 rpm on a shaker for 5 hours at room temperature (~25 °C); after
exposure the mixture was centrifuged for 5 min at 10000 rpm, and then the residual metal in the
supernatant was measured using an atomic adsorption spectrophotometer (AAS). The adsorption
capacity was determined using the following equation:

  (2)
Where: qe is adsorption capacity (mg/g), C0 is the initial concentration of metal (mg/L), Ce is the
concentration of metal at equilibrium (mg/L), V is the volume of the solution (L) and m is the mass of
adsorbent (g)
Results and discussion
Mineralogical composition of soils in effluent basin

The XRD analysis showed that the soil samples mainly constituted consisted of aluminosilicate
minerals with a dominance of phyllosilicates which included muscovite, bentonite, montmorillonite and
illite. This result clearly indicated that there was a considerable amount of clayey materials in the soil
essentially downstream of the mine areas (points 7, 8 and 9). Major trace metals identified by XRF
analysis included zinc, copper, nickel and cobalt, relatively abundant in soils around mine areas (points
1 and 2).
Water quality in the effluents
The analysis of water in the effluents was done to determine what major metal pollutants in the mine
effluents were transported in the surface water in the residential areas. As shown in Table 1, nickel and
zinc were present in relatively high concentrations in the mine effluents, but there was a significant
decrease of these concentrations in water downstream, below recommended values for drinking water
use (SABS, 2005).
Table 1 Concentrations of selected elements in mine effluents
Sampling
points
Elements concentration mg/L
Na
Mg
Fe
Co
Ca
Al
Si
Ni
Zn
Cu
1
132.1
186
0
0
206.04
86.4
175.7
2.42
4.46
0
2
131.04
485.4
0
0
1028
35.58
156.7
2.56
5.607
0
3
132.2
250.4
0
0
181.8
21.6
161.3
2.43
4.965
0
4
130.5
112
0
0
168.17
0
170.7
0.35
0.27
0
5
131.6
87.4
0
0
169.6
0
173.03
0
0.402
0.2
6
131.14
92
0
0
171.35
0
167.9
0.5
0.36
0
7
206.7
45.09
1.442
0.013
111.6
1.439
na
0.008
0.171
0
8
197.1
53.46
0.420
0
138.9
0.082
na
0.019
0.096
0
9
162.6
64.38
11.71
0
140
15.06
na
0.028
0.178
0
Physico-chemical characteristic of soils
The physico-chemical properties of the soil samples were determined by the permeability and CEC
tests. It was observed (Table 2) that the soils with a relatively high content (points 7, 8 and 9) of clay
minerals (points 7, 8 and 9) had a very low permeability (K < 10-4 m/s), while the soils around the mine
areas (points 1, 2 and 6) had a relatively higher permeability (K ~ 1 x 10-1 m/s). The CEC values were
generally between 19 and 48 cmol/kg, which is to a large extentd also indicative of claying soils. There
was were no major differences among the physico-chemical properties of the top and bottom soils,
although the CEC values tend to increase in the bottom soils at sampling points 6, 7, 8 and 9; different
trend was observed at sampling points 1 and 2 where the soils have been disturbed by mining activities.
Table 2 Permeability and CEC values of top and bottom soils
Sampling
points
Sample types
Top soil
Bottom soil
Permeability (m/s)
CEC (cmolkg-1)
Permeability (m/s)
CEC (cmolkg-1)
1
5.784x10-1
37.81
1.542x10-1
13.81
2
1.2
21.71
na
22.65
6
5.702x10-1
30.86
1.02
48.23
7
1.294x10-5
19.3
6.108x10-6
34.32
8
6.469x10-6
21.13
7.188x10-6
38.5
9
8.06x10-6
25.723
7.188x10-6
35
Metal adsorption potential of soils
Commented [RV6]: This does not mean much without a map.
Commented [RV7]: It would be good to include these in Table 1.
Commented [RV8]: This is not possible. Please include
detection limits instead.
Commented [RV9]: What does this mean?
Commented [RV10]: Clayey?

The metal adsorption capacity of soil indicates its ability to reduce the mobility of metal ions in the
effluent. The main metal ions observed in the mine effluents were exposed to powdered dried soils in a
batch system, to estimate the possible interaction. Results in Table 3 show that the soil samples from
sampling points 7, 8 and 9 had a higher adsorption capacity compared to the soil samples from the mine
area. The soils had a higher affinity for copper than nickel and zinc. The adsorption capacity of the
bottom soils was higher than for the top soils.
Table 3 Adsorption capacity of top and bottom soils
Sampling
points
Adsorption capacity (mg/g)
Top soil
Bottom soil
Cu
Ni
Zn
Cu
Ni
Zn
1
2.0302
1.5929
0.0000
2.4215
1.9730
0.0839
2
3.8515
1.8033
0.0000
3.6846
1.4974
0.0000
6
8.0450
3.0901
1.7723
9.2118
4.8350
3.5411
7
17.0081
3.5654
5.0376
19.8845
7.0922
11.4323
8
14.5404
4.0340
4.6615
19.6250
6.6929
9.9608
9
13.9350
2.9908
3.7840
18.6574
3.9756
9.6111
Discussion
The presence of clay minerals in soils may affect both the vertical and the horizontal mobility of the
metals in the effluent by reducing the permeability and increasing the CEC values. The higher
permeability of soils in the mine areas may be due to the disturbance of the soil during mining activity
but mostly to the lower content in clay minerals. In this study the soil samples were collected only up
to 1 m depth, which makes it difficult to predict the vertical mobility of metals up to the water table;
however, with stagnant water in the ponds around the mine areas, the abundance of acid rain water and
the higher content of metals in the soil, such properties (low clay content and higher permeability) of
the soils is are conducive to potential contamination of ground water in relatively short period of time
(Bhattacharya et al., 2012).
The impact of the soil properties on the retention of metals was confirmed by the determination of the
adsorption capacity of the different soils. The soil samples around the mining area with lower clay
content had a lower adsorption capacity (< 4 mg Cu/g soil, < 2 mg Ni/g soil and < 1 mg Zn/g soil)
compared to other soils with a higher clay content (up to 19.9 mg Cu/g soil, 7.1 mg Ni/g soil and 11.4
mg Zn/g soil). A similar trend was observed among the top and the bottom soils, the latter having a
higher adsorption capacity. Although the lower adsorption capacity of soils around mine areas may be
ascribed to a lower content of clay minerals, it is important to mention that the higher concentration of
metals may play a role of poisoning, competing with absorbates for binding sites on soils’ surface
(Sheikhhosseini et al., 2013). The implication on surface water was the high concentration in water of
metals (nickel and zinc) with lower binding affinity to soils 1 and 2 at mine sites, and a decrease of the
concentration of these metals in water downstream where soils (7, 8 and 9) had higher binding capacity.
The progressive reduction of metal concentration in water downstream was related to the adsorption
capacity of the soils, and it therefore ensues that the soils in the effluent may have played a considerable
role in the alleviation of the concentration of nickel and zinc in water. Although, the surface water at
points 4 and 6 downstream still content relatively high concentration of nickel above the value (< 0.15
mg/L for Ni) recommended by the South African National Standard (SABS, 2005), the concentration
of zinc was within acceptable range (< 5 mg/l for Zn) in downstream effluent.
Conclusion
The cost implication of waste water treatment is often a limitation factor to the commitment of some
companies to abide to environmental regulation, therefore discharging effluents with abnormal
concentrations of metals into the water system. The ability of the receiving basin in the mining areas
near Potchefstroom (South Africa) to naturally mitigate the level of metal pollutants was determined to
estimate the risk of pollution of surface water, impairment of ecosystem and human risk. The clay
Commented [RV11]: Could increased dilution play a role as
well? Other factors?

content of the soil was found to improve the metal adsorption capacity and reduce the permeability of
the soil. This may have contributed to an improvement of water quality along the effluent basin,
resulting in less polluted water in the residential areas.
Acknowledgments
The authors are grateful of the contributions of Ms N. Maloyi, Mr F. Thaimo and Mr J. Makhafola from
the University of Johannesburg, and J Hendriks from the North-West University.
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Commented [RV12]: See previous comment. Your database is
very small and you don’t take into account other factors, such as
flow and seasonality. What about other industrial activities that
may be contributing metals, such as agriculture? Although your
conclusion may be appropriate, you need to acknowledge such
potential weaknesses in your argument if you do not correct for
other factors.