PhD (Environmental Sciences) / Department of Ecology and Resource Management / The presence of arsenic in groundwater has drawn worldwide attention from researchers and public
health officials due to its effects on human health such as, cancer, skin thickening, neurological
disorders, muscular weakness, loss of appetite and nausea. World Health Organisation (WHO) has
set the limit of 10 μg/L for arsenic in drinking water in trying to reduce the effects of arsenic. This
was further adopted by South African National Standard (SANS). The present study aims at
evaluating arsenic concentration in selected groundwater sources around Greater Giyani
Municipality in Limpopo Province and further synthesize clay based adsorbents for arsenic
removal using Fe3+ and Mn2+ oxides and hexadecylammonium bromide (HDTMA-Br) cationic
surfactant as modifying agents.
The first section of the work presented the hydrogeochemical characteristics of groundwater in the
Greater Giyani Municipality. The results showed that the pH of the samples ranges from neutral
to weakly alkaline. The dominance of major anionic and cationic species was found to be in the
order: HCO3
->Cl->SO4
2->NO3
- and Na+>Mg2+>Ca2+>K+>Si4+, respectively. Hydrogeochemical
facies identified in the study area include CaHCO3 (90%) and mixed CaNaHCO3 (10%) which
shows the dominance of water-rock interaction. About 60% of the tested samples contains arsenic
concentration above 10 μg/L as recommended by SANS and WHO. Concentration of arsenic was
found to be ranging between 0.1 to 172.53 μg/L with the average of 32.21 μg/L.
In the second part of this work, arsenic removal efficiency of locally available smectite rich and
kaolin clay was evaluated. Results showed that the percentage As(V) removal by kaolin clay was
optimum at pH 2 while the percentage As(III) removal was greater than 60% at pH 2 to 12. For
smectite rich clay soils, the percentage of As(III) and As(V) removal was found to be optimum at
pH between 6 and 8. The adsorption isotherm data for As(III) and As(V) removal by both clays
fitted better to Freundlich isotherm. Adsorption of both species of arsenic onto the clay mineral
occurred via electrostatic attraction and ion exchange mechanisms. Both clay soils could be
regenerated twice using Na2CO3 as a regenerant. Kaolin clay showed a better performance and was
selected for further modification.
In the third section of this work, Fe-Mn bimetal oxide modified kaolin clay was successfully
synthesized by precipitating Fe3+ and Mn2+ metal oxides to the interlayer surface of kaolin clay.
Modification of kaolin clay increased the surface area from 19.2 m2/g to 29.8 m2/g and further
v
decreased the pore diameter from 9.54 to 8.5 nm. The adsorption data fitted to the pseudo second
order of reaction kinetics indicating that adsorption of As(III) and As(V) occurred via
chemisorption. The adsorption isotherm data was described by Langmuir isotherm models
showing a maximum As(III) and As(V) adsorption capacities of 2.16 and 1.56 mg/g, respectively
at a temperature of 289 K. Synthesized adsorbent was successfully reused for 6 adsorptiondesorption
cycles using K2SO4 as a regenerant. Column experiments showed that maximum
breakthrough volume of ≈2 L could be treated after 6 hours using 5 g adsorbent dosage.
Furthermore, the concentration of Fe and Mn were within the WHO permissible limit.
In the fourth part of the work kaolin clay was functionalized with hexadecyltrimethylamonium
bromide (HDTMA-Br) cationic surfactant and its application in arsenic removal from groundwater
was investigated. The results revealed that adsorption of As(III) and As(V) is optimum at pH range
4-8. The maximum As(III) and As(V) adsorption capacities were found 2.33 and 2.88 mg/g,
respectively after 60 min contact time. Pseudo first order model of reaction kinetics described the
adsorption data for As(V) better while pseudo second order model described As(III) adsorption
data. The adsorption isotherm data for As(III) and As(V) fitted well to Langmuir model indicating
that adsorption of both species occurred on a mono-layered surface. Adsorption thermodynamics
model revealed that adsorption of As(III) and As(V) was spontaneous and exothermic. The
As(III)/As(V) adsorption mechanism was ascribed to electrostatic attraction and ion exchange.
The regeneration study showed that synthesized adsorbent can be used for up to 5 times.
In the firth part of the work inorgano-organo modified kaolin clay was successfully synthesized
through intercalation of Fe3+ and Mn2+ metal oxides and HDTMA-Br surfactant onto the
interlayers of the clay mineral. The batch experiments showed that As(III) removal was optimum
at pH range of 4-6, while the As(V) removal was optimum at pH range 4-8. The adsorption data
for both species of arsenic showed a better fit to pseudo second order of reaction kinetics which
suggest that the dominant mechanism of adsorption was chemisorption. The isotherm studies
showed better fit to Langmuir isotherm model as compared to Freundlich model. The maximum
adsorption capacity As(III) and As(V) at room temperature as determined by Langmuir model
were found to be 7.99 mg/g and 7.32 mg/g, respectively. The thermodynamic studies for sorption
of As(III) and As(V) showed negative value of ΔGᴼ and ΔHᴼ indicating that adsorption process
occurred spontaneously and is exothermic in nature. The regeneration study showed that the
vi
inorgano-organo modified kaolin clay can be reused for up 7 adsorption-regeneration cycles using
0.01 M HCl as a regenerant. Thomas kinetic model and Yoon-Nelson model showed that the rate
of adsorption increases with increasing flow rate and initial concentration and decreases with
increasing of the bed mass.
In conclusions, adsorbents synthesized from this work showed a better performance as compared
to other adsorbents available in the literature. Among the synthesized adsorbents, inorgano-organo
modified clay showed highest adsorption capacity as compared to surfactant functionalized and
Fe-Mn bimetal oxides modified kaolin clay. However, all adsorbents were recommended for use
in arsenic remediation from groundwater. The following recommendations were made following
the findings from this study: 1) routine monitoring of arsenic in groundwater of Greater Giyani
Municipality, 2) evaluating the possible link between arsenic exposure and arsenic related diseases
within Giyani in order to find the extent of the problem in order to establish the population at risk,
3) The toxicity assessment for HDTMA-Br modified kaolin clay should be carried out, 4) Materials
developed in the present study should be modeled and tested at the point of use for arsenic removal,
and lastly, 5) this study further encourage the development of other arsenic removal materials that
can be used at household level. / NRF
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:univen/oai:univendspace.univen.ac.za:11602/1288 |
| Date | 16 May 2019 |
| Creators | Mudzielwana, Rabelani |
| Contributors | Gitari, W. M., Ndungu, P. |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | 1 online resource (xiv, 166 leaves : Color illustrations, color maps) |
| Rights | University of Venda |
Page generated in 0.0029 seconds