X-ray photoelectron spectroscopy (XPS) is a powerful surface-sensitive analytical technique widely used to investigate the chemical composition and electronic states of environmental and geochemical samples. Its ability to provide quantitative information about elemental composition, chemical bonding, and oxidation states makes it particularly valuable for studying complex natural matrices such as mineral surfaces, nanoparticles, and biochar. The technique is especially useful for probing surface speciation of heavy metals, interactions between organic matter and mineral phases, and redox processes in environmental systems.

One of the primary applications of XPS in environmental science is the study of heavy metal speciation on mineral surfaces. Heavy metals such as arsenic, lead, and chromium often adsorb onto mineral phases like iron oxides, clays, and aluminosilicates in soils and sediments. XPS can distinguish between different oxidation states of these metals, which is critical for understanding their mobility and toxicity. For example, arsenic can exist as As(III) or As(V), with As(III) being more toxic and mobile in the environment. XPS identifies these species through shifts in binding energy, where As(III) typically exhibits a peak near 44.3 eV for As 3d, while As(V) appears around 45.2 eV. Similarly, chromium speciation between Cr(III) and Cr(VI) is discernible, with Cr(VI) showing a higher binding energy due to its more oxidized state.

Organic matter interactions with mineral surfaces are another key area where XPS provides valuable insights. Natural organic matter (NOM), including humic and fulvic acids, often coats mineral particles in soils and sediments, influencing contaminant binding and transport. XPS can detect carbon functional groups such as C-C/C-H, C-O, C=O, and O-C=O, which help characterize the nature of organic-mineral interactions. For instance, carboxylate groups (O-C=O) are frequently involved in metal complexation, and their presence can be confirmed through high-resolution C 1s spectra. Additionally, nitrogen speciation in organic matter, such as amine or amide groups, can be resolved, providing clues about organic matter degradation and nutrient cycling.

Redox processes play a crucial role in environmental systems, and XPS is particularly effective in identifying redox-sensitive elements. Iron is a common element studied due to its abundance and role in electron transfer reactions. XPS can differentiate between Fe(II) and Fe(III) based on Fe 2p spectra, where Fe(II) typically shows a satellite peak at higher binding energy than Fe(III). Manganese oxides, which participate in redox reactions involving organic and inorganic species, also exhibit distinct Mn 2p spectra that reflect Mn(II), Mn(III), and Mn(IV) oxidation states. These measurements are essential for understanding electron transfer mechanisms in soils and sediments.

Biochar, a carbon-rich material produced from biomass pyrolysis, has gained attention for its role in carbon sequestration and contaminant remediation. XPS is instrumental in characterizing biochar surfaces, particularly in identifying oxygen-containing functional groups that influence metal sorption. The O 1s spectrum often reveals contributions from carbonyl, hydroxyl, and carboxyl groups, which are critical for binding heavy metals. Additionally, XPS can detect inorganic species such as phosphates or carbonates that may form on biochar surfaces during environmental aging.

Despite its advantages, XPS analysis of environmental samples presents several challenges. Sample preparation is critical, as improper handling can alter surface chemistry. Air-sensitive samples, such as reduced iron minerals, must be transferred under inert conditions to prevent oxidation. Powdered samples require uniform mounting to avoid charging effects, often achieved by pressing onto indium foil or conductive tape. Heterogeneous natural matrices, such as soils containing mixed mineralogy and organic matter, may produce overlapping peaks that complicate data interpretation. Spectral deconvolution using appropriate fitting parameters is necessary to resolve contributions from different chemical states.

Interpreting XPS data from complex environmental samples requires careful consideration of potential interferences. For example, adventitious carbon contamination is common and must be accounted for when analyzing carbon spectra. Charging effects in insulating samples can shift binding energies, necessitating charge correction using an internal reference such as the C 1s peak from aliphatic carbon at 284.8 eV. Additionally, the probing depth of XPS is limited to approximately 10 nm, meaning subsurface phases may not be detected, and surface coatings may dominate the signal.

Quantitative analysis with XPS relies on sensitivity factors and assumes homogeneous composition within the analyzed volume. However, environmental samples are often heterogeneous, requiring complementary techniques such as scanning electron microscopy (SEM) or synchrotron-based methods for comprehensive characterization. Despite these limitations, XPS remains indispensable for surface analysis due to its ability to provide chemical state information that bulk techniques cannot offer.

In summary, XPS is a vital tool for investigating surface processes in environmental and geochemical systems. Its capacity to identify heavy metal speciation, organic-mineral interactions, and redox states makes it invaluable for understanding contaminant behavior, nutrient cycling, and biogeochemical transformations. While challenges exist in sample preparation and data interpretation, advances in instrumentation and analytical methods continue to enhance its applicability to natural matrices. By providing detailed surface chemistry insights, XPS contributes significantly to environmental science and geochemistry research.