Occurrence and behaviour of dissolved, nano-particulate and micro-particulate iron in waste waters and treatment systems: New insights from electrochemical analysis

Abstract

Cyclic-, Differential Pulse- and Steady-state Microdisc Voltammetry (CV, DPV, SMV) techniques have been used to quantify the occurrence and fate of dissolved Fe(ii)/Fe(iii), nano-particulate and micro-particulate iron over a 12 month period in a series of net-acidic and net-alkaline coal mine drainages and passive treatment systems. Total iron in the mine waters is typically 10-100 mg L(-1), with values up to 2100 mg L(-1). Between 30 and 80% of the total iron occurs as solid phase, of which 20 to 80% is nano-particulate. Nano-particulate iron comprises 20 to 70% of the nominally "dissolved" (i.e. <0.45 μm) iron. Since coagulation and sedimentation are the only processes required to remove solid phase iron, these data have important implications for the generation or consumption of acidity during water treatment. In most waters, the majority of truly dissolved iron occurs as Fe(ii) (average 64 ± 22%). Activities of Fe(ii) do not correlate with pH and geochemical modelling shows that no Fe(ii) mineral is supersaturated. Removal of Fe(ii) must proceed via oxidation and hydrolysis. Except in waters with pH < 4.4, activities of Fe(iii) are strongly and negatively correlated with pH. Geochemical modelling suggests that the activity of Fe(iii) is controlled by the solubility of hydrous ferric oxides and oxyhydroxysulfates, supported by scanning and transmission electron microscopic analysis of solids. Nevertheless, the waters are generally supersaturated with respect to ferrihydrite and schwertmannite, and are not at redox equilibrium, indicating the key role of oxidation and hydrolysis kinetics on water treatment. Typically 70-100% of iron is retained in the treatment systems. Oxidation, hydrolysis, precipitation, coagulation and sedimentation occur in all treatment systems and - independent of water chemistry and the type of treatment system - hydroxides and oxyhydroxysulfates are the main iron sinks. The electrochemical data thus reveal the rationale for incomplete iron retention in individual systems and can thus inform future design criteria. The successful application of this low cost and rapid electrochemical method demonstrates its significant potential for real-time, on-site monitoring of iron-enriched waters and may in future substitute traditional analytical methods.

Sedimentation Kinetics of Hydrous Ferric Oxides in Ferruginous, Circumneutral Mine Water

“…The low electrophoretic mobility observed for both ferric solids between pH 6 and 8 indicates that the particles display little surface charge at circumneutral pH and thus naturally tend toward aggregation. These observations are consistent with literature reports where the IEP of synthetic ferric (oxyhydr)­oxides is commonly reported in the neutral or alkaline range, whereas the IEP of naturally precipitated hydrous ferric oxides from mining environments was reported in the slightly acidic range. ,, The median hydrodynamic particle diameters of natural ochre and synthetic ferrihydrite at pH 6–8 are found at 2–3 and 1–4 μm, respectively, perfectly falling into the particle size range outlined in the introduction. ,− Characteristics of the different ferric solids used in this study are compiled in Table .…”

“…Hence, aggregation of freshly precipitated, colloidal hydrous ferric oxides results in proceeding growth toward a critical aggregate size that facilitates effective sedimentation . The critical window governing transition from dispersed colloids to effectively settleable particles is to be expected in the upper nanometer and lower micrometer range. , This estimate is substantiated by the examination of artificial hydrous ferric oxides from controlled precipitation experiments ,, and ochreous sediments from various mine sites, where ferric aggregates predominantly occurred between 0.2 and 10 μm. ,− This is in accordance with a recent study by Chikanda et al, reporting effective gravitational separation of hydrous ferric oxides in a drain and pond system once the ferric aggregates considerably exceeded 0.3 μm. Moreover, the size range is in accordance with early sizing approaches for settling ponds in the mining industry, which recommended an overflow rate of 1 × 10 –5 m/s based on the estimated settling time of shale-sized particles with a diameter of ≈4 μm .…”

“…Above and below this range, the simplified first-order approach will inevitably tend to progressively underestimate and overestimate iron removal, respectively. In passive mine water treatment systems, the concentration of particulate hydrous ferric oxides is usually low because ferric solids are only gradually generated upon ferrous iron oxidation and concomitantly removed through sedimentation . What is more, low iron levels are ultimately most relevant for sizing purposes to ensure reliable compliance with typical discharge limits in the order of 1–3 mg/L total iron.…”

Naturally occurring iron oxide nanoparticles: morphology, surface chemistry and environmental stability

“…48 The ferrihydrite found in abandoned coal mine drainages (rich in iron and sulfate) had similar morphologies and aggregates into large spheres ($100 nm). 53 The 2-line ferrihydrite nanocrystals collected from an acid mine drainage (pH ¼ 4.4) were mainly sub-spherical to pseudo-hexagonal nanoparticles, 2-5 nm in diameter, and heavily aggregated to form electronically dense and irregular-shaped masses. 29 These environments are characterized by slightly alkaline to strongly acidic pH, cold (5-7 C) to low-temperature ($80 C, as near hydrothermal venting uids) water, a range of ions, in addition to a rich Fe(II) supply, supersaturation of oxygen (from microaerobic to aerobic), and frequently, biological activities.…”

Optimising Operational Reliability and Performance in Aerobic Passive Mine Water Treatment: the Multistage Westfield Pilot Plant

“…A third sample was collected for total organic and inorganic carbon (TOC, TIC) measurements. Samples for analysis of Fe as the primary contaminant were collected twice weekly at all monitoring points and subjected to spectrophotometric analysis of dissolved ferrous and ferric as well as particulate and total Fe as described by Matthies et al (2012) and Opitz et al (2020). Acidity was calculated from analytical results and corrected for CO 2 using PHREEQC broadly following Kirby and Cravotta (2005).…”