Due to their remarkable adsorptive qualities, low cost, and wide availability, iron oxides and hydroxides have attracted a lot of interest as efficient materials for the removal of heavy metals from wastewater. The mechanisms by which iron-based minerals, such as ferrihydrite, magnetite, haematite and goethite interact with heavy metals through ion exchange, co-precipitation, and adsorption are covered in detail in this chapter. In order to maximise metal uptake efficiency, the importance of surface chemistry, particle size and mineral crystallinity is emphasised. This study critically examines developments in iron oxide functionalisation and modification to improve their capacity and selectivity for particular metals, including cadmium, lead and arsenic. While there are naturally occurring iron oxides/hydroxides, it is possible to synthesise them, coupling the synthesis with surface modifications, and these are usually monitored or verified using easily accessible instruments such as X-Ray Diffraction (XRD), Scanning Electron Microscope (SEM) and Fourier Transform Infrared Spectroscopy (FTIR). This integration of the iron oxides/hydroxides with materials in nanotechnology and polymers has been beneficial for the removal of both heavy metals and other pollutants, such as organic ones, which highlights the versatility of the materials. The ability of the materials to be regenerated for further use makes them attractive in the wastewater treatment industry and in terms of environmental beneficiation. This chapter, therefore, provides a comprehensive analysis of iron oxides and hydroxides, highlighting their crucial role in developing sustainable wastewater treatment systems and reducing heavy metal pollution.

The growing interest in reprocessing mine tailings for gold recovery requires a suitable quantification method that is accurate, rapid, and not harsh to the environment. Acid digestion is often used to determination of gold; however, it often faces the challenge of incomplete digestion due to the presence of minerals such as quartz, and homogeneity is compromised due to small sample masses, which can result in low bias. This study investigated a shorter acid digestion method employing reverse aqua regia, both in the presence and absence of hydrofluoric acid. Before digestion, the sample was subjected to gold depot analysis, which showed that 78% was free-milling gold and that only 0.8% was associated with pyrite, increasing the chances of accurate quantifications. Furthermore, the size screening test showed that most of the gold could be recovered on the − 38 μm screen. This proposed method provided good linearity (5–100 µg. L^− 1) and low detection limits (0.139–0.183 µg.kg^− 1). The concentrations obtained by the acid digestion was 0.258 g.t^− 1 with the recoveries ranging between 80% and 82%, which fit the criteria set. The method also worked well for the certified reference materials (CRM), AMIS 610 (accurate value = 0.068 g.t^− 1) and AMIS 646 (accurate value = 0.166 g.t^− 1), which are of a similar matrix and are also lower in grade compared to the sample. The method was also evaluated for uncertainty (± value) using the bottom-up approach, and the expanded uncertainty (k = 2) was reported to be 0.258 ± 0.092 g.t^− 1, which was comparable to that offered by the fire assay with the ICP‒OES finish, which was 0.28 ± 0.10 g.t^− 1. This implies that the acid digestion method is suitable for quantifying gold from mine tailings without large uncertainties.

In this study, a South African mine tailing sample was characterized for base metals using X-Ray fluorescence, alkaline fusion, and acid digestion to subsequently evaluate the uncertainties involved. This was based on the importance of characterization data in subsequent methods and obtaining confidence in such methods. It was determined that instrument repeatability and reproducibility contributed most to the overall uncertainty. At a coverage factor of 2 (k=2), the measurement result for iron through alkaline fusion, XRF, and acid digestion was found to be 3.476 ± 0.026%, 3.835 ± 0.023%, and 3.741 ± 0.020%, respectively, and that of arsenic were 88 ± 11 mg kg-1 and 85.5 ± 8.3 mg kg-1 for fusion and acid digestion, respectively. Based on the calculated expanded uncertainties which are at an average of 0.8% of the measurement results, the three methods can be associated with high statistical confidence and therefore eases decision-making regarding subsequent analytical methods.