Key Takeaways & Executive Findings
- •• • Adsorption capacity of ADOM onto ferrihydrite peaked at 21.59 mg C/g at pH 7.0, with a decline above pH 7.0 due to electrostatic repulsion, indicating optimal pH for carbon sequestration in near-neutral waters. • • Selective fractionation intensified with pH from 3.0 to 9.0, favoring high-aromaticity, high-molecular-weight CDOM and humified, autochthonous protein-like FDOM, which may reduce bioavailability of these reactive components. • • Non-linear increase in adsorption with initial DOC concentration (2–100 mg C/L) revealed preferential uptake of low-aromaticity, high-molecular-weight CDOM and low-humification protein-like FDOM, suggesting concentration-dependent molecular sorting. • • The observed fractionation aligns with an 'onion model' of multilayer adsorption, implying that ferrihydrite can dynamically regulate ADOM composition, impacting carbon turnover and pollutant mobility in eutrophic systems.
Abstract
The escalating eutrophication of aquatic systems has intensified algal blooms, leading to substantial release and accumulation of algal-derived dissolved organic matter (ADOM), which profoundly influences carbon cycling and pollutant transport. Iron minerals, particularly ferrihydrite, are recognized as critical mediators of DOM sequestration, yet the adsorption fractionation of ADOM under varying environmental conditions remains poorly understood. This study systematically investigated the effects of pH (2.0–10.0) and initial dissolved organic carbon (DOC) concentration (2–100 mg C/L) on the adsorption capacity and selectivity of ADOM onto ferrihydrite, employing UV-Vis spectroscopy and excitation-emission matrix fluorescence with parallel factor analysis (EEM-PARAFAC). Results demonstrated that adsorption capacity increased with pH from 2.0 to 7.0, reaching a maximum of 21.59 mg C/g at pH 7.0, followed by a decline at pH > 7.0 due to enhanced electrostatic repulsion. Within the environmentally relevant pH range of 3.0–9.0, selective fractionation intensified with increasing pH, favoring highly aromatic, high-molecular-weight chromophoric DOM (CDOM) and protein-like/aromatic amino acid fluorescent DOM (FDOM) with high humification and autochthonous characteristics. With increasing initial DOC concentration, adsorption exhibited non-linear growth, with preferential uptake of low-aromaticity, high-molecular-weight CDOM and protein-like FDOM of lower humification and stronger autochthonous features. These findings elucidate that ferrihydrite can effectively sequester reactive ADOM components via pH- and concentration-dependent selective adsorption, potentially altering DOM composition and reactivity in eutrophic waters, thereby providing fundamental data for understanding iron mineral-mediated internal carbon sequestration.
1. Introduction
Eutrophication-driven algal blooms release copious amounts of algal-derived dissolved organic matter (ADOM), a complex mixture of proteins, polysaccharides, and humic-like substances, which profoundly alters aquatic carbon cycling and the fate of contaminants. Iron (hydr)oxides, particularly the poorly crystalline ferrihydrite, are ubiquitous in sediments and suspended particles, and their adsorption of DOM is a key abiotic pathway for organic carbon preservation. However, conventional adsorption studies have largely focused on terrestrial humic substances, leaving a critical gap in understanding how ADOM—with its distinct molecular signatures—interacts with ferrihydrite under dynamic pH and concentration regimes typical of eutrophic lakes. This lack of mechanistic insight hampers predictive modeling of carbon sequestration and the mobility of co-transported pollutants in such impacted systems.
This study directly addresses this bottleneck by systematically varying pH (2.0–10.0) and initial DOC concentration (2–100 mg C/L) to quantify both adsorption capacity and selective fractionation of ADOM onto ferrihydrite. By integrating UV-Vis and EEM-PARAFAC, the research deciphers how specific molecular fractions—aromatic vs. aliphatic, high vs. low molecular weight—are preferentially sequestered under varying conditions. The findings reveal that ferrihydrite acts as a molecular sieve, with pH and concentration governing the extent and selectivity of adsorption, thereby influencing the bioavailability and reactivity of ADOM. This work provides essential empirical parameters for refining carbon cycle models and assessing the environmental fate of ADOM in eutrophic waters.
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ZHANG Lirong, LI Jingnan, ZHAO Pan, SONG Na, WANG Qunhui (2026). Effects of pH and Initial DOC Concentration on Ferrihydrite-Mediated Adsorption of Algal-Derived Dissolved Organic Matter under Eutrophication. Journal of Environmental Engineering Technology. https://doi.org/10.13205/j.hjgc.202605013
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Frequently Asked Questions
What is the optimal pH for maximum ADOM adsorption onto ferrihydrite, and what are the implications for natural eutrophic lakes?
The maximum adsorption capacity of 21.59 mg C/g was observed at pH 7.0, with a decline at pH > 7.0 due to electrostatic repulsion. In eutrophic lakes, pH often fluctuates between 7 and 9 during algal blooms, suggesting that adsorption efficiency may be reduced under alkaline conditions, potentially increasing the bioavailability of ADOM.
How does the initial DOC concentration influence the selectivity of ADOM fractionation, and what does this imply for carbon sequestration under bloom conditions?
With increasing initial DOC concentration (2–100 mg C/L), adsorption increased non-linearly, and the fractionation shifted towards low-aromaticity, high-molecular-weight CDOM and low-humification protein-like FDOM. This suggests that at high ADOM loadings, ferrihydrite preferentially removes less reactive components, potentially leaving more recalcitrant fractions in solution, which may affect long-term carbon storage.
What are the limitations of this study regarding the stability of adsorbed ADOM and the potential for release under changing environmental conditions?
The study did not conduct desorption experiments, so the stability of adsorbed ADOM and its potential release upon pH or ionic strength changes remain unknown. Additionally, pH adjustment introduced uncontrolled ionic strength variations, confounding the isolated pH effect. Future work should incorporate desorption assays and controlled ionic strength gradients to assess the risk of ADOM remobilization.
How does the 'onion model' explain the observed concentration-dependent adsorption behavior, and what are its implications for modeling DOM dynamics?
The 'onion model' describes multilayer adsorption where initial layers are formed by high-affinity components, and subsequent layers involve weaker interactions. The observed non-linear adsorption and selective fractionation with increasing DOC concentration align with this model, suggesting that at high concentrations, competitive effects and surface saturation alter the composition of adsorbed DOM, which must be considered in predictive models of carbon cycling.
What future research directions are suggested to overcome the limitations of this study and further elucidate ADOM adsorption mechanisms?
Future studies should employ adsorption isotherm models and high-resolution mass spectrometry to quantify the affinity and capacity of individual components. Additionally, desorption experiments under varying pH and ionic strength, and experiments using natural water matrices, are needed to better simulate real-world conditions and assess the environmental fate of ADOM.
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