Provided for non-commercial research and educational use only. Not for reproduction or distribution or commercial use.
This article was originally published by IWA Publishing. IWA Publishing recognizes the retention of the right by the author(s) to photocopy or make single electronic copies of the paper for their own personal use, including for their own classroom use, or the personal use of colleagues, provided the copies are not offered for sale and are not distributed in a systematic way outside of their employing institution. Please note that you are not permitted to post the IWA Publishing PDF version of your paper on your own website or your institution’s website or repository. Please direct any queries regarding use or permissions to
[email protected]
1439
© IWA Publishing 2012 Water Science & Technology
|
66.7
|
2012
Metal removal via particulate material in a lowland river system J. G. Webster-Brown, T. J. Dee and A. F. Hegan
ABSTRACT Twelve month surveys of acid-soluble and dissolved trace metal concentrations in the lower Waikato River (in 1998/9 and 2005/6) showed abnormally low particulate Fe, Mn, Cu, Pb and Zn concentrations and mass flux in autumn, when the suspended particulate material (SPM) had a relatively high diatom and organic carbon content, and low Fe and Al content. Dissolved Mn, Cu and Zn concentrations also decreased in autumn, while dissolved Fe and Pb concentrations were unaffected. While SPM settlement under the low river flow conditions present in autumn can explain the removal of particulate metals, it does not explain dissolved metal removal. SPM-metal interaction was therefore investigated using seasonal monitoring data, experimental adsorption studies, sequential extraction and geochemical modelling. Pb binding to SPM occurred predominantly via Fe-oxide surfaces, and could be reliably predicted using surface complexation
J. G. Webster-Brown (corresponding author) Waterways Centre for Freshwater Management, University of Canterbury, Private Bag 4800, Christchurch, New Zealand E-mail:
[email protected] T. J. Dee Celtic Technologies Ltd, Cardiff, United Kingdom A. F. Hegan SEAES, University of Manchester, Manchester, United Kingdom
adsorption modelling. Dissolved Mn concentrations were controlled by the solubility of Mn oxide, but enhanced removal during autumn could be attributed to uptake by diatoms. Zn and Cu were also adsorbed on Fe-oxide in the SPM, but removal from the water column in autumn appeared augmented by Zn adsorption onto Mn-oxide, and Cu adsorption onto the organic extracellular surfaces of the diatoms. Key words
| adsorption, diatoms, iron oxide, SPM, Waikato River
INTRODUCTION The chemical behaviour of urban trace metal pollutants such as Cu, Pb and Zn affects their toxicity and accumulation in aquatic environments. Minerals, diatoms and other components of suspended particulate material (SPM) in the water columns of rivers, lakes and estuaries have the ability to bind dissolved trace metals, thereby reducing metal toxicity and influencing environmental fate. There are two principal agencies of dissolved metal removal by SPM; adsorption onto mineral or organic surfaces and biological uptake, particularly by diatoms and algae (e.g., Dzombak & Morel ; Warren & Zimmerman ; Tessier et al. ). These removal processes need to be understood and quantified before reliable predictions can be made concerning the extent to which metal toxicity will be reduced, and/or metals will accumulate in the sediments, in a natural aquatic system. However the variable nature of SPM, within and between waterways, means that this process cannot be reliably predicted using basic partitioning coefficients doi: 10.2166/wst.2012.313
derived for a particular or generic SPM and water chemistry. Nor can single surface adsorption models be applied with confidence (e.g., Muller & Sigg ; Wang et al. ). This study aimed to determine the mechanisms of SPM – dissolved Cu, Pb and Zn interaction in a lowland river experiencing sudden dramatic fluctuations in trace metal concentrations in autumn, which were suspected to relate to changing SPM conditions. Water quality monitoring data, experimentally determined Cu, Pb and Zn adsorption onto SPM, sequential extraction of metals from the SPM, and surface complexation modelling have been used to identify important metal-SPM binding processes. An improved understanding of the relationship between trace element solubility and the nature of the SPM will have application to other river and estuary systems, where SPM composition varies naturally (e.g., seasonally) or is influenced by anthropogenic change.
1440
J. G. Webster-Brown et al.
|
Metal removal in a lowland river
METHODS The Waikato River is the longest river in New Zealand (approximately 350 km) and drops over 365 m in elevation between its source at Lake Taupo and the estuary at Port Waikato (Figure 1). Annual average river flow increases from approximately 200 to 450 m3/s over this distance, but is largely artificially controlled by electricity generation requirements, as there are hydroelectricity dams and lakes between Lake Taupo and Hamilton city. The sampling sites used in this study are shown in Figure 1. All of the sites shown on Figure 1 were originally sampled in early summer (November 1998) and in winter (July 1999), and site 3 was sampled monthly between Nov 1998 and Oct 1999, from the Tuakau road bridge across the river. Due to apparent anomalies in the 1998/99 data at site 3, this site was resampled monthly between Dec 2005 and Nov 2006, which confirmed the presence of anomalous autumnal fluctuations in metal concentrations (Dee ). Adsorption studies (Hegan ) and geochemical modelling were undertaken to define the extent of SPMmetal interaction at this time of year, and compared this to that of other seasons.
Water Science & Technology
|
The catchment of the Waikato River in the central North Island of New Zealand. Data from sites 3–15 provide context for this study, but the emphasis is on the lower Waikato River as represented at site 3 (Tuakau bridge). Sites 1 and 2 are not shown as they were located in the estuary and not used in this study.
66.7
|
2012
Conductivity, pH and temperature were measured in situ using portable field meters, and water samples were collected for major ion, trace metal and SPM analysis. Two 50 ml water samples were collected at each site for trace metal analysis; one sample was filtered through a 0.45 μm Millipore cellulose membrane, then acidified to pH < 2 using 8N Aristar HNO3, for dissolved metal analysis. The other sample was collected without filtration, and acidified as above, for acid-soluble metal concentration. These samples were analysed by inductively coupled plasmamass spectrometry (ICP-MS) with the following detection limits (dl); Fe 10 μg L1, Mn 1.0 μg L1, Cu 0.1 μg L1, Pb 0.1 μg L1 and Zn 1.0 μg L1. Analytical errors were consistently
