Increase of the concentration of dissolved copper in drinking water systems due to flow-induced nanoparticle release from surface corrosion by-products
Research highlights
► Hydrodynamics determine the release of Cu corrosion by-product nanoparticles ► Detached Cu corrosion by-product nanoparticles were identified as malachite ► Copper release models have to consider hydrodynamics within the pipes
Introduction
For 200 years copper pipes have been used for domestic water services around the world [1]. Nowadays it is the most used piping material in household drinking water systems because its high corrosion resistance [2]. There are considerable experiences and scientific understanding on the properties of this noble material [3]. Since the first copper pipe, there have been a large number of manufacturing developments to obtain better material performances in order to decrease the concentration of copper released into the tap water.
Although copper is an essential metal for the human diet, in some cases the ingestion of copper and long-term overexposure can generate acute and chronic health effects including gastrointestinal diseases and liver damage [4]. The World Health Organization (WHO) recommends 2 mg/L as a maximum concentration value for drinking water [5]. This value is based on gastrointestinal epidemiologic studies conducted on populations under controlled exposures. Several of those studies used copper sulfate salts to add dissolved copper into the drinking water [6], [7], [8]. As a result, over the last decade, research on drinking water supply systems and copper pipeline corrosion has focused on determining and modeling the processes that control the release of copper into the water [9], [10], [11], [12]. In fact, based on experimental measurements, models have been developed to estimate the concentration of soluble copper released into the tap water [13], [14].
In general, the release of soluble copper into the water is controlled by three processes: (1) an electrochemical process that involves two half-reactions, one anodic (metallic copper oxidation) and one cathodic (dissolved oxygen reduction) [15]; (2) a scale formation process associated with thermodynamic equilibrium conditions, affected by pH, dissolved oxygen (DO), temperature, and presence of ions [3], [16], [17]; and (3) a dissolution of solid corrosion by-products and release of dissolved copper into the bulk water [3], [18].
Current knowledge of copper corrosion supports the theory that, for new pipe systems, soluble corrosion by-product release into water is controlled by the solubility of cupric hydroxide (Cu(OH)2(s))[10], [19]. For aged pipe systems, the transition to less soluble and stable phases is catalyzed by the presence of anions in water [20], [21]. In cold and low mineral waters cupric hydroxide ages to tenorite (CuO(s)), however, in water with high dissolved inorganic carbon concentration, ages to malachite (Cu(OH)2·CuCO3(s)) [3]. Malachite dominates the solid phase speciation of Cu(II) for pHs between 5 and 9 [22].
Fluid flow can also affect corrosion by changing chemical and mechanical conditions at the metal–liquid interface. Flow-induced corrosion has been traditionally classified in four different types: mass-transport-controlled corrosion, phase-transport-controlled corrosion, erosion–corrosion, and cavitation corrosion [23], [24], [25]. Mass-transport-controlled corrosion is related to the increased rate of mass-transport due to the flow velocity profile which contributes to increase the amount of corrosive species reaching the metal surface, or alternatively, by enhancing the removal of dissolved corrosion by-products from the solid phase. Phase-transport-controlled corrosion occurs when a liquid phase containing the corrosion agent gets in contact with the metal surface. Erosion–corrosion is associated to the mechanical removal of protective layers from the metal surface by high-velocity turbulent flows through shear stresses applied on solid boundaries. Cavitation corrosion takes place when liquid pressure drops below the vapour pressure, generating an implosion of gaseous bubbles that creates impulsive forces capable of removing material from the solid phase [18], [25], [26]. In addition, there is evidence that mechanical removal of nanoparticles from the metal surface can also occur even for low velocity flows. This process seems to be similar to the so-called erosion–corrosion, but acting at a smaller scale where shear stresses might be capable of sloughing micro and nanoparticles from corrosion by-products [18], [25], [26]. Similarly, fluid flow can enhance concentration gradients that facilitate desorption of labile copper bonded to organic moieties attached to the metallic surface [18]. Therefore, for domestic pipe systems where flow velocities are rather low, it is important to identify the mechanisms that control the release of dissolved and particulate copper into tap water, especially because the health implications of particulate copper are poorly characterized.
According to our current knowledge there is not a study that simulates the dissolution of copper particles in the gastric fluid, however a recent work of lead contamination in drinking water systems shows that gastric fluid can easily dissolve lead particles, with the associated health impact of soluble lead released into the human body [27]. On the other hand, Taylor et al. [28] found through a modeling study that corrosion and dissolution potentials of copper nanoparticles are dependent on the size and shape of the particle. Thus, the typical thermodynamic values calculated for metals and minerals (corrosion by-products) are not necessarily accurate for small particles.
The standard measurement of dissolved copper is made by filtering the water sample through a membrane with a pore-size of 0.45 μm [29]. However, a passivating film of copper carbonate hydroxides, such as malachite, growing over the metallic surface is formed by the aggregation of structures with a size less than 0.2 μm that could be detached into the water due to flow [30]. Thus, the standard definition of dissolved copper includes both soluble cuprous and cupric species, together with particulated copper. To avoid this inaccuracy, operational definitions have been used to describe the size distribution for particulate species in drinking water [31], [32]. McNeill and Edwards [31] divide dissolved copper into colloidal (0.1 μm < [Cu] < 0.45 μm) and soluble copper ([Cu] < 0.1 μm). Using this definition, (nanoparticles) particles < 0.1 μm could be confounded with soluble copper (Table 1). Interestingly, even though the release of particulate copper has been reported [33] and its detachment can be linked with the hydrodynamic conditions of the piping system [18], the effect of flow-stagnation events on the detachment of micro and nanoparticles of copper corrosion by-products has been poorly considered.
Our work focuses on the effect of hydrodynamic conditions on the detachment of copper corrosion by-product nanoparticles under abiotic conditions. This paper presents evidence of the detachment of nano and micro copper carbonate hydroxide structures formed on the inner surface of copper pipes, induced by the shear stress produced by the fluid flow, which increases the concentration of dissolved copper in water.
Section snippets
Materials and methods
Flushing experiments were conducted using a single-pass laboratory system, which consisted of a 1 m long copper pipe with an internal diameter of 1.95 cm and 0.3 L of volume, preceded by a PVC pipe and a Cole-Parmer model N° 7553-75 peristaltic pump connected to a water tank. The copper pipes were preconditioned in a three steps protocol: (1) the pipes were filled with NaOH (0.1 M) to dissolve all oxides present on the inner surface of the pipe. (2) After two minutes with sodium hydroxide, the
Results
The results of the flushing experiments are organized in three sections: (1) copper release measurements, depicted by curves of copper mass versus the volume of water extracted from the tested pipes; (2) surface analyses that include a detailed multi-method characterization of the pipes before and after the flushing experiments and; (3) particle observations, where the results of micro and nanoparticles captured by the sequential filtering procedure are shown.
Discussion
For non-reactive surfaces, a plug-flow analysis would be sufficient to characterize the release of copper during the flushing event. This non-reactive characteristic would be noted as a sudden increase followed by an early stabilization in the mass of copper released (Fig. 2). However, for pipes coated with a reactive film of solid corrosion by-products, the ideal plug-flow assumption is not adequate to describe the release of copper into the water, and the effect of particle detachment must be
Conclusions
This work is the first effort aimed at characterizing the effect of hydrodynamic conditions on the release of copper corrosion by-product nanoparticle into drinking water systems.
Flushing experiments conducted under laminar and transition to turbulent conditions show that even if the wall shear stress produced by the flow is one order of magnitude smaller than the erosion–corrosion threshold values reported by Efird [25] mechanical detachment of nanoparticles could occur. Thus, corrosion
Acknowledgements
This research was funded by CONICYT Grant 24080013/2008 and FONDECYT Project 1080578/2008.
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