Cu, Pb, and Zn Sorption to Biogenic Iron (Oxyhydr)Oxides Formed in Circumneutral Environments

Andrew Whitaker,Owen Duckworth

doi:10.3390/soilsystems2020018

Abstract

The transportation and immobilization of potentially toxic metals in near-surface environments may be partially controlled by sorption processes at the solid-water interface. Myriad studies have shown that iron (oxyhydr)oxides have large sorption capacities and form strong surface complexes with metal ions. Biogenic iron (oxyhydr)oxides (BIOS) form at redox gradients where dissolved ferrous iron encounters oxygenated conditions, allowing bacteria to outcompete abiotic Fe oxidation. This process produces biominerals with distinct surface and structural properties (incorporation of cell-derived organic matter, poor crystallinity, and small particle sizes) that may alter their metal-binding affinity and sorption processes. To better understand metal binding by BIOS, Cu, Pb, and Zn, sorption rate and isotherm studies were conducted with synthetic two-line ferrihydrite and BIOS. Additionally, X-ray absorption spectroscopy and total scattering were used to elucidate the BIOS mineral structure and metal ion surface structures. On a mass normalization basis, BIOS sorbed approximately 8, 4, and 2 times more Cu, Pb, and Zn, respectively, than 2LFh over similar dissolved concentrations. Spectroscopic analyses revealed poorly crystalline structures and small coherent scattering domain sizes for BIOS. Additionally, extended X-ray absorption fine-structure spectroscopy revealed Cu, Pb, and Zn sorbed to BIOS via inner-sphere complexes, similar to 2LFh. These results suggest that, in metal contaminated environments, BIOS are more efficient in metal binding than their synthetic counterparts.

Highlights

Iron oxides, hydroxides, and oxyhydroxides ( referred to as “ironoxides”for simplicity) are ubiquitous minerals [1,2], that can partially control the environmental fate and transportation of metals, such as Cu, Pb, and Zn [2,3,4,5,6,7,8,9,10,11,12,13,14,15,16]
Our results suggest that Biogenic iron (oxyhydr)oxides (BIOS) has an enhanced sorption reactivity when compared to 2LFh, consistent with previous work [34,38,47,107] that reported maximum sorbed concentrations on BIOS
At pH = 6 and similar aqueous metal concentrations, Cu, Pb, and Zn sorption onto BIOS was approximately 8, 4, and 2 times more than 2LFh, respectively. These large differences between BIOS and 2LFh sorption were attributed to the reduced ordering (Supplementary Materials Figure S2, Figure 2 and Table 1), small coherent scattering domain (CSD) (Figure 3), and presence of organic C (Supplementary Materials Table S3) within the BIOS, resulting in a larger sorption capacity [38,83,84]; the 2LFh point of zero charge and sorbent preparation contributed to the large sorption differences as well

Summary

Introduction

Hydroxides, and oxyhydroxides ( referred to as “iron (oxyhydr)oxides”for simplicity) are ubiquitous minerals [1,2], that can partially control the environmental fate and transportation of metals, such as Cu, Pb, and Zn [2,3,4,5,6,7,8,9,10,11,12,13,14,15,16]. Hydroxides, and oxyhydroxides ( referred to as “iron (oxyhydr)oxides”. Sediments, and surface water, the oxidation of Fe can occur via abiotic and biotic processes [17], leading to the formation of iron (oxyhydr)oxides. Several studies have shown that BIOS mineralogy closely resembles that of two-line ferrihydrite (2LFh) [8,32,33,34], a poorly crystalline iron oxyhydroxide [1,2] with small mineral domain sizes, poorly ordered structures, and high specific surface areas [32,34,35].

Methods

Results

Conclusion