Research articlePhosphate adsorption kinetics and equilibria on natural iron and manganese oxide composites
Graphical abstract
Natural Mn-rich limonite, denoted “LM”, and those for an acid treated version, denoted “LAT”, were used to prepare naturally complex Fe- and Mn-oxide composite materials to examine phosphate adsorption/desorption process. The saturated adsorption capacity on LAT was almost double that on LM, but the rate of achieving saturation on LM was higher than on LAT. Resulting thermodynamic quantities implied that adsorption is exergonic but driven by entropy that outweighs an underlying endothermic process, and that the net driving force decreases with increasing pH on LM but not on LAT. The collective results confirm that phosphate uptake and strong binding is selectively controlled by the Fe-oxide fraction. And another important conclusion is that the role of Mn-oxide fraction in the composite cannot be ignored because it clearly can severely limit the Fe-oxide uptake capacity.
Introduction
Because phosphorus (P) is an essential nutrient to maintain the metabolism of organisms (over 0.6% by weight of plants and animals)(Peacock, 2020), it is important to understand mechanisms controlling its distribution in soils, sediments and water bodies. For example, excessive phosphorus content caused by discharge of wastewater may deteriorate ecosystems and lead to serious environmental pollution(Klapper, 1991; Zhou et al., 2001). According to previous study(Qin et al., 2010), worldwide about 3–4 million tons of P2O5 migrates from soil into groundwater every year. Over the past few decades this problem has attracted global interest because of its implications for human survival and development(Chen and Graedel, 2016; Elser, 2012; Roy, 2017). However, given the chemical and physical complexities of environmental media, uncertainties remain regarding factors controlling the relative partitioning of P between discrete particles, organic matter, and various mineral sorbents, which ultimately controls its migration and fixation.
Pollutant adsorption is easily happened and a simple and efficient way to get rid of high risk of toxicity(Aljeboree et al., 2022; Cheng et al., 2022; Hui et al., 2021; Sun et al., 2021; Wang et al., 2020; Wu et al., 2022), which come into a hot topic in wastewater treatment(Jing et al., 2022; Liao et al., 2021; N et al., 2022; Sun et al., 2022; Wu et al., 2022; Zhang et al., 2022; Zhang et al., 2023). Similarly, adsorption/desorption of phosphate is a major process governing P fate and transport. Generally, the total P in soils and sediments can be divided into orthophosphate and organo-phosphorus forms, which have interconversion pathways under certain conditions(Missong et al., 2018; Xu et al., 2017). Studies have shown that although phosphate can occur as discrete compounds with iron, aluminum and calcium(Penn and Camberato, 2019), absorbed orthophosphate is the main inorganic P species(Baumann et al., 2020). Consequently, phosphate adsorption on various minerals has been widely studied, which clearly shows selective affinities for certain phases(Bar-Yosef et al., 1988; Edzwald et al., 1976; Gérard, 2016; Jara et al., 2006; Luengo et al., 2006; Rajan and Perrott, 1975; Sø et al., 2011; Wang et al., 2013a, 2013b; Yan et al., 2017a, 2017b). Colloids and nanoparticles of metal-(oxy)(hydr)oxides and phyllosilicates are prominent in this regard because they are abundant and have large specific surface area with high affinity surface sites(Missong et al., 2018). Fe- and Mn-oxides in particular have a high capacity for P sequestration(Gasparatos et al., 2019) and are among the most abundant pedogenic components, especially in agricultural soils with redoximorphic features(Jiang et al., 2013; Staff, 2014; Tsao et al., 2011).
Because natural Fe- and Mn-oxides in soils and sediments tend to occur in co-association, many studies have been devoted to understanding the basis for their selective uptake and retention of phosphate. Studies of trace elements in natural Fe–Mn concretions tend to indicate a positive correlation between P and Fe suggesting P is mainly associated with Fe-oxides(Couture et al., 2018; Latrille et al., 2001; Neaman et al., 2004, 2008). Likewise, examination of iron and manganese plaques on the surface of roots suggest selective P binding to the surface of Fe plaques(Batty et al., 2002; Christensen et al., 1998). Additional studies have showed that Fe-oxide nanoparticles play a key effect in P cycling(Jiang et al., 2015; Xu et al., 2017), and then affect the P bioavailability in soils. Such observations have motivated more detailed P uptake studies using model systems of isolated Mn-oxides(Mustafa et al., 2006, 2008), and Fe-(hydro)oxides(Chitrakar et al., 2006; Gérard, 2016), results of which generally indicate a higher P adsorption capacity on Fe-(hydro)oxides. This has been attributed to P loading on Fe-(hydro)oxides being controlled by chemical adsorption(Kim et al., 2011; Luengo et al., 2006) versus physical adsorption on Mn-oxides(Mustafa et al., 2006, 2008). However, the detailed P uptake knowledge gained from isolated phases has not yet been reconnected back to the observed behavior of more realistic co-associated Fe- and Mn-oxides. In particular, because natural Fe- and Mn-oxides have complex composition, crystallinity and morphology, the extent of connection between laboratory studies on model systems and realistic Fe- and Mn-oxides remains unclear(Parida and Mohanty, 1998).
The present study is motivated to help fill this knowledge gap. Here we report examination of P uptake on natural samples of limonite as a representation of composite Fe- and Mn-oxide particulates. Although P incorporation in these materials during precipitation or recrystallization is one important uptake mechanism (Gasparatos et al., 2019), here we focus on adsorption onto their surfaces from solution. To address the question regarding the relative importance of Fe-versus Mn-oxides in these samples, we compare the behavior of the natural composite to one treated with a chemical extractant selective for the Mn-oxide fraction. Our P uptake comparison on these two samples is based on solution analytics and modeling, combined with solids characterization by zeta potential measurements, FT-IR and XPS spectroscopies, and TEM/STEM imaging. Uptake equilibria and kinetics are presented in terms of the relative contributions of the Fe-versus Mn-oxide fractions. The findings help connect model system behavior to realistic natural nanomaterials thus strengthening the basis for predicting P migration and transformation in complex soils and sediments.
Section snippets
Sample selection and processing
The natural Fe- and Mn-oxides used in the experiments were in the form of a selected natural limonite sample, collected from the Yeshan district of Tongling City, Anhui Province, China. This sample originated from the weathering of Mn-riched silicate minerals (such as ilvaite) and detailed geological background is available in the literature(Chen et al., 2018). This raw sample was subjected to minimal processing for our laboratory experiments. First, the limonite was crushed and sieved using a
Physicochemical properties of LM and LAT solids
The physical and chemical properties of the LM sample are described in detail in Chen et al., 2017), 2018(Chen et al., 2017, 2018). Briefly, in terms of oxide equivalents, LM is composed of Fe2O3 (58.6 wt%), MnO (16.0 wt%), Al2O3 (1.9 wt%), SiO2 (6.5 wt%), ZnO (0.8 wt%) and an ignition loss fraction (14.9 wt% H2O). Organic carbon was below detection by FT-IR. In terms of mineralogic composition, the LM sample consists dominantly of goethite (35.3%), poorly crystallized or amorphous Fe phases
Conclusions
Our results indicate that in complex natural composites of Fe and Mn-oxides, phosphate uptake is controlled primarily by the Fe-oxide fraction but that Mn-oxides play an opposing role that undermines the ability to predict phosphate uptake simply on the basis of the Fe-oxide fraction present alone. While consistent with related studies on model systems suggesting that phosphate retention is proportional to the Fe-oxide concentration, at the same time our findings call into question the
Credit author statement
Ping Chen: Conceptualization, Investigation, Methodology, Formal analysis, Writing − original draft, Preparation. Yuefei Zhou: Conceptualization, Writing − review & editing. Qiaoqin Xie: Writing − review & editing. Tianhu Chen: Conceptualization, Supervision, Writing − review & editing. Haibo Liu: Writing − review & editing. Sichuang Xue: Characterization, Methodology. Xuehua Zou: Investigation, Funding acquisition. Lin Wei: Characterization, Methodology. Liang Xu: Investigation,
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The present research was supported financially by the National Science Foundation of China (41572029, 41672038, 42102029). XZ, SCX, and KMR acknowledge support from the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division through its Geosciences Program at Pacific Northwest National Laboratory (PNNL). A portion of the work was performed using the Environmental and Molecular Sciences Laboratory (EMSL), a
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