Subsurface Iron and Arsenic Removal

1 Introduction
to be investigated. In order to do so, knowledge needs
to be gained regarding the subsurface processes that
determine the site-specific efficacy of SIR/SAR.
This thesis
Arsenic contamination of shallow tube well drinking
water in Bangladesh is an urgent developmental and
health problem (British Geological Survey/DPHE,
2001; World Health Organization, 2001; Smith et al.,
2002), disproportionately affecting the rural poor,
i.e., those most reliant on this source of drinking
water. Current arsenic mitigation solutions, including
(household) arsenic removal options, do not always
provide a sustainable alternative for safe drinking
water in rural Bangladesh. A novel HWTS combines
the removal of iron and arsenic by utilizing the existing
hand pump infrastructure. This new technology shows
great potential, but for safe implementation there is the
need to address urgent research questions regarding
the production of sufficient water quality and water
quantity. The problem description of this thesis has
been formulated as follows:
Subsurface Iron and Arsenic Removal is a
promising safe water solution, however, there is the
need for better understanding of the (subsurface)
processes determining the sustainable operation
in diverse geochemical settings.
This problem has been addressed by identifying research
questions within three main knowledge gaps: (1) Fe 2+ /
As(III) immobilization processes, (2) site-specific
effectiveness, and (3) safe and sustainable application.
The knowledge gaps include research topics that are
within and outside the scope of this thesis, as has been
summarized below.
Fe 2+ /As(III) immobilization processes
Although much work has already been done on the
oxidation and adsorption kinetics of iron and arsenic,
the rapidly changing redox conditions during injectionabstraction
cycles make SIR/SAR a unique application.
A combination of Fe 2+ oxidation, precipitation,
adsorption and exchange may occur during injectionabstraction
cycles, resulting in the immobilization
of iron. Adsorptive-catalytic oxidation and cation
exchange have been proposed to dominate the
subsurface iron removal process (van Beek, 1985; Appelo
et al., 1999), with arsenic removal being a beneficiary
side effect. However, to target effective arsenic removal
it is desirable to understand the behaviour of this
constituent during the adsorptive-catalytic oxidation
mechanism, including the speciation of arsenic. In this
thesis the competition of iron oxidizing bacteria for the
same dissolved oxygen as the chemical Fe 2+ oxidation
reaction will not be addressed. This does, however, not
mean that it is assumed that IOB communities play an
insignificant role during SIR, as the observed presence
of Gallionella spp. in subsurface treated groundwater
13
1