Natural organic matter in the environment

Natural organic matter refers to a complex mixture of different organic compounds and is formed in both terrestrial and aquatic environments through chemical and microbial decomposition of plant litter as well as animal and microbial biomass.^84,85 Due to the immense number of different chemical constituents of NOM, it is not practically feasible to characterize it simply based on individual compounds. Indeed, the formation mechanisms and its long-term stability are under ongoing debate.⁸⁶ For example, there still exists some ambiguity on whether NOM is chemically a true macromolecular entity⁸⁷ or just consists of self-assembled aggregates of small compounds held together by relatively weak noncovalent interactions

A B

^E

2

C

Introduction

(primarily electrostatic and hydrogen (H) bonding) thereby mimicking large macromolecules (Lehmann and Kleber⁸⁶ and references therein).

In general, multiple environmental dynamics are affecting the decomposition of NOM such as time, climate, topography, minerals as well as parent material, and the microbial community available, and thus control the degree of its accumulation in the environment. These factors result in a range of soil NOM contents from ≤1 % in the mineral horizons of some tropical and sandy soils to nearly 100

weight-% in ombrotrophic peat bogs, where the decomposition of organic material is slowed down under anoxic and often cold conditions.^85,88 The average elemental composition of NOM reveals that it is primarily composed of carbon (C) (55-57%), O (34-36%), H (4-6%), nitrogen (N) (0.9-3%), and S (0.4-1.8%).⁸⁴ Despite NOM being a complex mixture with unknown exact composition, it possesses unique combinations of carboxylic, phenolic, esteric, quinone, aliphatic hydroxylic, amino, nitroso, and thiol functional groups, as well as nonpolar aliphatic and aromatic structural units.^84,89 An example for a model “humic compound”, to demonstrate possible units and functional groups of NOM, is depicted in Figure 4.

Figure 4: Model “humic compound” which represents many functional groups of natural organic matter (slightly modified) as presented by Sundararajanet al.⁹⁰ Copyright (2020), with permission from Royal Society of Chemistry.

There are several inherent properties to NOM which gives it a central role in aqueous and terrestrial biogeochemistry. Primarily due to its ubiquitous quinone functional groups, NOM is redox-active⁹¹ and can influence the biogeochemistry of trace metal(loid)s and other (organic) contaminants.^92-95 Further, it is known to act as microbial electron shuttle in reductive dissolution processes^95,96 and can for example be used by microorganisms as a terminal electron acceptor (in reversible redox cycles) during anaerobic

Introduction

respiration.^95,97,98

Moreover, NOM possesses variable charges which originate mostly from deprotonated carboxylic (R-COOH) and phenolic (R-OH) groups (Brønsted acids) with proton dissociation constants (pKa) of ~3 to ~5 and ~7 to ~10, respectively. To a lesser extent, also sulfhydryl (thiol) groups (R-SH, pKa~5 to ~11) contribute to the negative charge of NOM. Next to acidic groups, NOM features base groups like the amino group (R-NH2) which are able to accept protons and consequently are positively charged. However, the concentration of these groups is low compared to the acid groups, which results in an overall net-negative charge of NOM at circumneutral pH.^48,99

As a consequence of their net negative charge, NOM can influence the (trans)formation, structure and reactivity of many minerals. Dissolved NOM can, for example, effect the formation (biomineralization) of Fe and Mn minerals through complexation and solubilization of the metal ions and by sorption to the mineral surface, generally causing the formation of less crystalline minerals.^100-102 Mineral-organic associations can lower the pHPZC of particularly Fe minerals and therefore directly influence nutrient and contaminant mobility.^48,103,104 Additionally, by conveying negative charge to mineral surfaces, NOM can stabilize mineral colloids,^105,106 and thus facilitate their transport in the environment. Further, dissolved NOM competes especially with anionic (contaminant) species for sorption sites on minerals, contributing to their overall higher mobility.^107-109 Strong mineral-organic associations or NOM occlusion during mineral formation prevent biodegradation and mineralization of C and are recognized as critically important to the long-term stabilization of organic C in soils and marine sediments.^86,110,111

The mostly anionic (Coulomb) interactions of NOM with variable charges from mineral surfaces, as described before, can be generalized for metal cations by the concept of “hard” and “soft” acids and bases.

Therein, “hard” applies to species which have a high charge density and are weakly polarizable, whereas

“soft” species have a lower charge density and are strongly polarizable. These characteristics have implications for binding site strength, and also for competition among metals. For example, Ca(II), which is abundant in natural waters, may compete effectively with Cd(II) at carboxylic sites, but hardly at thiol sites.⁹⁹ However, the binding preferences explained by this concept are not exclusive as shown for Cd(II) binding on humic acid (HA) under varying redox conditions.¹¹²

Moreover, complexation of NOM with redox-active cations can stabilize them against reduction/oxidation.

For example, through binding of carboxylic or phenolic groups to Fe(III),¹¹³ the redox potential of the Fe(II)/Fe(III) redox couple is lowered¹¹⁴ and consequentially Fe in NOM-rich environments is stabilized in its oxidized redox state over a broad range of redox conditions. This mechanism can explain the occurrence of Fe(III) in reduced, water-saturated histosols.^115,116 It is further worth noting that the high affinity between metals and certain functionalities of dissolved as well as solid NOM can result in a complex interplay in the environment. Mehlhornet al.¹¹⁷, for example, investigated a NOM and redox gradient at a mofette site and found copper (Cu) immobilized and mainly bound to solid NOM at the anoxic center of the mofette

Introduction

and at an oxic reference soil. Under suboxic conditions in between these two extremes, pore water Cu concentration, however, was elevated due to the increased complexation of Cu with dissolved NOM.

Among the non-metallic minor components of NOM (N, P, and S), S is exceptional. Additionally to the assimilation by plants and microbes, it can be incorporated into the NOM structure during early diagenesis, which is known to occur in many environmental systems such as peatlands, wetlands, deep groundwaters, as well as marine, estuarine, and lake sediments.^118,119 The exact mechanisms of diagenetic S incorporation are still under debate, but generally require functional groups containing π-bonds (Hoffmannet al.¹²⁰ and references therein). The reaction of dissolved sulfide (for example originating from microbial sulfate reduction) with solid and dissolved NOM under anoxic conditions results in the formation of surface associated S(0)-species and various organic S functional groups, mainly thiols as well as organic disulfides, or S heterocycles.^120-122 Therefore, depending on the prevalent redox conditions, reduced organic S constitutes about 10-30% of total S in soils, whereas it can increase up to∼75% in NOM-rich soils.^123-125 The formation of As and Sb complexes with S- and also O-containing functional groups of dissolved as well as solid NOM influences the metalloids` cycling in the environment and will therefore be introduced in more detail in the following part.