Humic-like acids from hydrochars: Study of the metal complexation properties compared with humic acids from anthropogenic soils using PARAFAC and time-resolved fluorescence

Abstract

Humic acids (HA) play an important role in the distribution, toxicity, and bioavailability of metals in the environment. Humic-like acids (HLA) that simulate geochemical processes can be prepared by NaOH aqueous extraction from hydrochars produced by hydrothermal carbonization (HTC). HLA can exhibit properties such as those found in HA from soils, which are known for their ability to interact with inorganic and organic compounds. The molecular characteristics of HLA and HA help to explain the relationship between their molecular features and their interaction with metallic species.The aim of this study is to assess the molecular features of HA extracted from Terra Mulata (TM) and HLA from hydrochars as well as their interaction with metals by using Cu(II) ions as a model. The results from 13 C NMR, elemental analysis, FTIR, and UV-Vis showed that HA are composed mostly of aromatic structures and oxygenated functional groups, whereas HLA showed a mutual contribution of aromatic and aliphatic structures as main constituents. The interactions of HA and HLA with Cu(II) ions were evaluated through fluorescence quenching, in which the density of complexing sites per gram of carbon for interaction was higher for HLA than for HA. Furthermore, the HLA showed similar values for stability constants, and higher than those found for other types of HA in the literature. In addition, the average lifetime in both humic extracts appeared to be independent of the copper addition, indicating that the main mechanism of interaction was static quenching with a non-fluorescent groundstate complex formation. Therefore, the HLA showed the ability to interact with Cu(II) ions, which suggests that their application can provide a new approach for remediation of contaminated areas.

Fulvic acids from Amazonian anthropogenic soils: Insight into the molecular composition and copper binding properties using fluorescence techniques

“…Amazonian anthropogenic soils, also called Terra Mulata (TM) soils are found in some Amazon regions. They are dark colored soils, with a high nutrient content and high fertility (dos Santos et al, 2020;Oliveira et al, 2018;Lucheta et al, 2017;Fraser et al, 2011;Cunha et al, 2009;Glaser et al, 2000), contrasting with other Amazonian soils which are highly acidic, with low fertility and cation exchange capacity (Soares et al, 2005). Their formation is still unclear, but researches have pointed out that, human activity in the pre-Columbian past has resulted in a large organic matter and nutrient accumulation over the years (Clement et al, 2015;Novotny et al, 2009).…”

“…Their formation is still unclear, but researches have pointed out that, human activity in the pre-Columbian past has resulted in a large organic matter and nutrient accumulation over the years (Clement et al, 2015;Novotny et al, 2009). Therefore, such soils have been considered as a model for sustainable agriculture (dos Santos et al, 2020;Novotny et al, 2009).…”

“…According to its solubility in aqueous medium, HS is usually grouped in three fractions: humic acids (HA), fulvic acids (FA) and humin (HU). In the literature several studies are reported, with the most diverse purposes, involving HA from soils, due to its great agronomic importance (dos Santos et al, 2020;Bento et al, 2020;Canellas and Façanha, 2004;da Silva et al, 2020;Nebbioso and Piccolo, 2011;Novotny et al, 2009;Senesi et al, 2003;Tadini et al, 2020;Vergnoux et al, 2011). FA also have a great influence on soil functionality, however, the amount of studies regarding FA from soils in the literature is less compared to HA.…”

“…Fluorescence excitation-emission matrix (EEM) coupled to parallel factor analysis (PARAFAC) has proved to be a powerful technique to investigate the binding properties between organic matter with metallic species, and this due to the high sensitivity, low cost and low sample volume (dos Santos et al, 2020;Tadini et al, 2020;Yuan et al, 2015;Mounier et al, 2011;Stedmon and Bro, 2008). Further, because different chemical species have different fluorescence lifetimes, it is also possible to study the fluorescence quenching mechanism through time-resolved fluorescence (TRF).…”

“…Thereby, the possible mechanisms includes dynamic or static quenching (dos Santos et al, 2020;Nouhi et al, 2018).…”