Investigating the synthesis of ettringite and monosulfate analogue phases and their use as waste remediation materials
Abstract
Pollution from industrial sources into freshwater supplies is a growing global problem
with 80 % of all wastewater produced being released into the environment with no
treatment. This poses a risk to both human and animal health, as well as affecting the
environment and economy. This research will focus on non-organic pollutants, which
cannot simply be destroyed and are not biodegradable. The main way these types of
wastes are dealt with are by stabilisation and solidification processes where the
contaminants are either converted to non-hazardous forms or encapsulated in solid
materials to reduce their mobility.
The work presented in this thesis will focus on using the mineral ettringite
(Ca6[Al(OH)6]2(SO4)3.26H2O) to solidify hazardous waste ions. Ettringite is an
important hydration product of Portland cement and also occurs naturally, first being
discovered in 1874. It has a well characterised structure crystallising in the space
group P31c. The structure consists of cation columns with composition
Ca3[Al(OH)6.12H2O]3+, that run parallel to the c axis, with the sulfate anions and
remaining water molecules in channels running parallel to these columns. The crystal
structure of ettringite has been shown to accommodate other cations, including Fe3+
and Cr3+, on the Al3+ sites, and other oxyanions, including CO3
2- and NO3
-
, in place of
the SO4
2- in the channels.
The ability of the structure to accommodate a range of different ions, gives it the
potential for use as a waste remediation material. To test its viability, the synthesis
of analogue phases was investigated, along with the time dependence of the
formation of these phases.
Ettringite analogue phases containing chromate and selenate
(Ca6[Al(OH)6]2(XO4)3.26H2O, X=Cr6+, Se6+) were successfully synthesised using the
saccharate method. These particular phases were targeted as hexavalent chromium
and selenium are hazardous pollutants which affect public freshwater as a result of
industrial runoff. With successful syntheses of the chromate and selenate ettringite
analogue phases confirmed, a reaction was set up to emulate the remediation
process of removing selenium or chromium from solutions, by incorporation into an
ettringite analogue phase. The time dependence of the reaction was studied by
taking samples at different time points to follow the evolution of the reaction
products and removal of the pollutant ions from the solution.
Tricalcium aluminate (Ca3Al2O6, C3A) was chosen as the start material and was added
to solutions of varying concentrations of chromate or selenate. For chromate
containing solutions, the chromate ettringite analogue phase was only formed in
concentrated conditions (0.2 M K2CrO4 solution), with weaker solutions (0.01 – 0.1 M
K2CrO4 solutions) producing a related phase known as monochromate
(Ca4Al2O6(CrO4).xH2O). All tests resulted in the successful removal of hexavalent
chromium from solution and solidification of it into the solid phase.
For selenate containing solutions, the selenate ettringite analogue phase was formed
in all concentrations of solutions (0.02 – 0.2 M Na2SeO4 solutions) when the reaction
time was less than 24 hours, with conversion of these phases to the related
monoselenate phase (Ca4Al2O6(SeO4).xH2O) at reaction times greater than this. The
end result was that selenium was removed from solution and stabilised in a solid
phase for all tested concentrations.
Solid solution behaviour between the ettringite end member and analogue phase end
members was investigated to determine the level of incorporation of the hazardous
ions CrO4
2- and SeO4
2- when in the presence of SO4
2-. The chromate-sulfate ettringite
series (Ca6[Al(OH)6]2(CrO4)x(SO4)3-x.26H2O) was found to only exhibit solid solution
behaviour when x < 0.44, and any attempts to incorporate more chromate resulted
in the production of the two end members. The selenate-sulfate ettringite series
(Ca6[Al(OH)6]2(SeO4)x(SO4)3-x.26H2O) was found to behave as a complete solid
solution for a full range of compositions 0 < x < 3. The solid solution investigation
results can inform future remediation strategies if competing sulfur is present in the
waste system.
Ettringite analogue phases containing iron and gallium (Ca6[X(OH)6]2(SO4)3.26H2O,
X=Fe, Ga) were successfully synthesised and the ideal conditions for their formation
were investigated. The inclusion of sugar in the synthesis was found to be necessary
to produce an ettringite phase with no secondary impurity phases present. The
gallium analogue phase could form in 10 minutes, with the iron analogue phase
requiring a 24 hour reaction time. The solid solution behaviour between aluminium
ettringite and gallium ettringite end members (Ca6[AlxGa1-x(OH)6]2(SO4)3.26H2O, x =
0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0) was investigated and the series was
found to form completely. These results demonstrated that gallium could be partially
incorporated into the ettringite structure with aluminium also present.
From the research presented and discussed in this thesis, the overall conclusions
were that the precipitation of ettringite analogue phases and other related materials
could be used to remove hazardous ions from water and stabilise them in a solid
phase.