Modelling of sorption of trace elements in an agricultural soil impacted by mining activities
Date
2017
Authors
Mosai, Alseno Kagiso
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Abstract
The development of the economy of South Africa and many other countries has been highly dependent on mining industries. Minerals such as gold, platinum, diamond and many others have been mined and continue to be mined. Despite the importance of these minerals, their processing comes with social and environmental problems. During the processing of these minerals, trace elements such as copper, chromium, nickel, mercury, uranium, molybdenum and many others are released as wastes into the environment either, directly or indirectly. The release of the elements into the soil is of concern due to the possibility of groundwater system contamination.
The presence of these elements in the groundwater system poses serious challenges to the wellbeing of life forms, due to their toxicity, when they exceed threshold limits. From the processing plants, these elements could be released onto the soil, and mobilise to groundwater, increasing the already existing environmental crisis due to water pollution. Once these elements are in the water, access to living organisms becomes easier through the food chain. Some of these elements are not biodegradable and thus persist in the environment as well as in the bodies of living organisms. They can cause serious health problems because of their toxicity effect. In humans, these elements can be carcinogenic, and also cause chronic disorders, kidney failures, defects in infants, bone and vascular diseases which could also be lethal.
It is therefore of importance that these elements are neither bioavailable nor bioaccessible to living organisms. When these elements are mobile in the soil, the probability of reaching groundwater increases. Water, an important natural resource should always be protected from such pollutants. The demand for unpolluted water has been rising every year in the world due to increasing population, extended droughts and improper disposal.
This research was dedicated to determining the behaviour of elements in an agricultural soil impacted by mining activities. Agricultural soils are sometimes exposed to pollutants that could originate from dust fallout or precipitation; fertilisers and manure; pesticides; and water used for irrigation. Understanding the
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processes that control the distribution of these pollutants in agricultural soils is an important risk assessment measure, considering that such pollutants have the potential of being taken up by crops and vegetables or transported to groundwater.
In this study, a soil on a farm that grows vegetables for commercial purpose. Cabbage, spinach, carrots and potatoes are some of the vegetables grown on the plot and sold to markets in Pretoria and Johannesburg. The plot is in the vicinity of smelting operations in the North West Province. The mobility of trace elements in the soil can be controlled, depending on the type and properties of soil. Hence in this research, the ability of the soil to adsorb elements entering the soil is studied.
The batch experimental work was performed to determine the effect of pH, initial concentration (5 - 100 mg/L), competing ions (Fe3+, Ca2+, Co2+, Mg2+, K+, Ni2+ and Zn2+), fertilisers (ammonium nitrate, ammonium phosphate and calcium chloride) and plant exudates (acetic acid, citric acid and oxalic acid as well as ethylenediaminetetraacetic acid (EDTA) which is often used as proxy organic ligand (found in manure)) on the adsorption of cadmium (Cd), copper (Cu) and chromium (Cr) onto an agricultural soil. The PHREEQC geochemical modelling code was used to complement experimental methods in predicting processes and to further assess the leaching behaviour of the elements. Powder X-ray diffraction (PXRD) and X-ray fluorescence (XRF) were used to determine the mineralization of the soil. The structural features of the soil were determined using Fourier Transform Infrared spectroscopy (FTIR) and the element content was determined using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). The point of zero charge (PZC) of the soil was found to be 8.3 and the cation exchange capacity (CEC) of 51.6 meq/ 100g.
In the absence of fertilisers and plant exudates, the soil exhibited a similar high adsorption for elements at all initial concentrations by all the elements. Most (> 90%) of the elements were adsorbed within the first 3 minutes of contact with the soil. Langmuir, Freundlich and Dubinin-Radushkevich adsorption isotherms were used to describe the experimental data for the elements. Kinetic rates were modelled using pseudo first-order and pseudo second-order equations. Pseudo
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second-order gave the best fit for all the elements (R2 >0.999) indicating chemisorption. The effect of pH on Cd and Cu was insignificant however, the adsorption of Cr decreased with pH. The presence of competing ions decreased the adsorption of cadmium more than that of the other analyte elements. The soil was generally effective in adsorbing and retaining the elements. However, the retention was highly dependent on elemental speciation and prevailing conditions e.g. pH (as in the case of Cu and Cr). Such changes in conditions would have implications for groundwater quality.
The effect of plant exudates and EDTA was studied and the results showed that low molecular weight organic acids (LMWOAs) viz acetic acid (AA), citric acid (CA) and oxalic acid (OA) and EDTA significantly (p < 0.05) decreased the adsorption capacity of the elements onto the agricultural soil. AA had the least effect on the adsorption capacity of the elements whereas OA and EDTA strongly prevented the adsorption of the elements. Moreover, some of the elements which were already in the soil including those which were not under study such as Ca and Mg were desorbed from the soil by OA and EDTA. Thus, the mobility of the elements was increased by the presence of plant exudates, increasing groundwater contamination and consequently threatening the health of living organisms.
Agrochemicals such as fertilisers, stabilizers and pesticides are constantly applied to agricultural soils to improve the fertility of the soil for better crop production however; their presence may affect the mobility and bioavailability of elements in the soil. The effect of ammonium nitrate and ammonium phosphate as well as calcium chloride on the adsorption of Cd, Cu and Cr onto an agricultural soil was studied. The effects of initial concentrations of the elements (5 – 50 mg/L), concentrations of fertilisers (0.01 – 0.1 mol/L) and pH (3 - 8) on the adsorption of Cd, Cu and Cr were studied. The initial concentration of the elements and the concentration of fertilisers had no significant effect (p > 0.05) on the adsorption capacities of Cu and Cr at pH 5. But, ammonium nitrate and calcium chloride decreased the adsorption capacity of Cd. The adsorption of Cd onto the soil was reduced as the concentration of fertilisers increased. The adsorption of Cd was lower than that of Cu and Cr at all pH values. The agricultural soil was found to
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be an effective adsorbent in preventing the mobility of Cu and Cr in the presence of fertilisers but not for Cd whose adsorption was significantly affected by the presence of ammonium nitrate and calcium chloride.
A continuous flow fixed-bed column script with specified conditions simulating the natural environment was utilised in PHREEQC for column studies. The geochemical computer model PHREEQC can simulate solute transport in soil surfaces. The effect of initial concentration (100 and 300 mg/L) of the elements, column bed depth (5 and 10 cm) and pH (3, 5, 7 and 10) were considered in this study. The adsorption capacity was affected by initial concentration of the elements since the breakthrough curves at higher analyte concentrations were reached at lower pore volumes than at low concentrations. This can be attributed to the fast occupation of active sites of the soil at higher concentrations. The results from PHREEQC indicated that the conditions used would lead to the oxidation of Cr3+ to Cr6+ leading to the formation of HCrO4- and Cr2O72- which were not favoured for adsorption by soil surfaces due to high solubility. This could have potential implications on the quality of groundwater in regions with similar conditions. Thus, the leaching of Cr6+ onto the agricultural soil will be high in areas where remediation techniques are not applied. The changing of bed depth from 5 to 10 cm did not have an effect on the adsorption of the elements. The ability of the soil surfaces to adsorb Cd and Cu even at lower bed depth implies that the soil will be effective in preventing the leaching of the elements to groundwater due to strong surface interactions of the elements with the soil. The results from PHREEQC showed that the adsorption of Cd and Cr onto the soil surface was not affected by pH. The results for Cr were contradicting with those obtained from laboratory experiments which could be due to the conditions used in PHREEQC. The change in the speciation of Cu at basic conditions decreased the ability of Cu adsorption onto the soil surfaces. The Cu2+ was converted to Cu(OH)2 which were large in size and thus only a small amount could be adsorbed since the other adsorption sites were covered by the large species.
This research had notable outputs in the form of publications which will form an important repository of information.
Description
A dissertation submitted to the Faculty of Science,
University of the Witwatersrand in fulfilment of the requirements for the degree of Master of Science
Johannesburg 2017.
Keywords
Citation
Mosai, Alseno Kagiso (2017) Modelling of sorption of trace elements in an agricultural soil impactedby mining activities, University of the Witwatersrand, Johannesburg, <http://hdl.handle.net/10539/23522>