Cr(VI) and Cr(III) Release From Different Layers of Cast Iron Corrosion Scales in Drinking Water Distribution Systems: The Impact of pH, Temperature, Sulfate, and Chloride

Chromium accumulated from source water and pipeline lining materials in corrosion scales could potentially be released into bulk water in drinking water distribution systems (DWDS). Chromium behaviors between corrosion scale phase and the surrounding water phase vary spatially in different layers and temporally in different DWDS running periods. In this study, corrosion scales sampled from actual DWDS were first characterized by SEM, XRD, XRF, and the modified BCR three-step sequential extraction procedure. Then scales were divided into the outer and inner layers for subsequent analysis. Static accumulation and release experiments were performed with Cr(VI) and Cr(III) on two distinct scale layers to systematically assess the influence of pH, temperature, sulfate, and chloride. The release behaviors of Cr(VI) under the co-effect of multiple factors were investigated in orthogonal experiments. Results showed that in the outer and inner layers of corrosion scales, chromium exhibited differences in accumulation and release behaviors, with the outer layer accumulating less and releasing more. The mechanisms of chromium retention based on different iron (oxyhydr)oxides were discussed.

Fundamental insights into the stabilisation and chemical degradation of the corrosion product scales

Carbonate stability plays a crucial role in clarifying the evolution and protection of the naturally formed corrosion scales on the steel surface in the application of geothermal production. In this paper, the stability of the corrosion scales from both micro and atomic level are studied via a combination of electrochemistry, surface analysis and first-principle calculation. The chemical and physical characterisation of various iron–calcium mixed carbonates are experimentally analysed and results are compared with the first-principle calculation. In the presence of Ca2+, the preferential loss of Ca during the dissolution experiments was observed, the interactions within the crystal weaken where Ca2+ co-precipitation, confirmed by a faster degradation rate for Ca0.51Fe0.49CO3 than FeCO3. This work reveals the degradation and protection performance of the naturally formed carbonate layers and provides insights into understanding the corrosion product stability and chemical breakdown of the corrosion scales.

Adsorption and (induced) desorption of Cd(II) from the corrosion scales of water distribution pipes, following a deliberate contamination event

Intrusion of toxic heavy-metal cations into water-distribution systems (WDS) may cause severe adverse health-effects on large populations, along with an undesirable psychological impact. The corrosion (scale) layer, that invariably develops on the pipes’ inner walls, is capable of adsorbing a significant mass of metal-cations and releasing them thereafter via diffusion to the water once operation is resumed, thereby causing a secondary contamination event. To overcome this, the contaminant should be completely removed, in a controlled fashion, from both the aqueous and scale phases, with minimum damage to the pipe's physical stature. This study determined the range of the Cd(II) adsorption capacity of corrosion-scales and quantified alternative treatments for desorbing it, using an assortment of metal water-pipes, extracted from the WDS. Batch, water-recirculation and flow-through experiments were conducted to determine the extent of Cd(II) adsorption and the best way to desorb it. Corrosion-scales showed substantial Cd(II)-absorption capacity (up to 0.75 mg Cd(II)/g scale) with an approximately linear relation between the aqueous Cd(II) concentration and the adsorbed mass. Desorption experiments included dosages of various acids. Sequential rinsing (eight pipe-volumes) by pH3 solution was found to be the best approach, releasing close to ∼100% of the adsorbed Cd(II), with only a minor effect on the pipes’ integrity.