Carmen Enid Martínez

Solubility of lead, zinc and copper added to mineral soils.

Elevated levels of heavy metals in soils are a result of industrial activities, atmospheric deposition, and the land application of sewage sludges and industrial by-products. Their persistence in the soil environment has created interest in the possible changes in solubility. In this study, total dissolved concentrations of Pb, Zn, and Cu were monitored in seven metal-amended soils (a calcareous and six acid mineral soils). Single metal solutions were added to soils and equilibrated (aged) for 40 days. During the 40 days the soil was allowed to air-dry and was rewetted in cycles of about 5 days. At the end of this reaction period, metal solubility was measured (by atomic absorption spectrometry and direct current plasma spectrometry) at the initial soil pH and at decreased pH values which were induced by addition of small aliquots of acid. As expected, solubility of added Pb, Zn, and Cu increased with a decrease in pH. Furthermore, the results showed that the solubility relationship with pH was similar in all non-calcareous soils. This suggests that metal solubility may be controlled by similar soil components, presumably involving soil characteristics such as pH, organic matter content, and soil mineralogy. For each metal, an approximate pH value was found at which solubility deviated from the solubility of metals when they occur in soils at typical (natural) values. This pH was about (pH+/-0.2): 5.2 for Pb, 6.2 for Zn, and 5.5 for Cu. Thus, pH values below these thresholds may enhance metal mobility, biological availability and toxicity in soils. Metals dissolved at higher pH in the calcareous soil (18.8 g kg(-1) inorganic carbon, initial pH 8.2). In a calcareous soil, a significant fraction of these metals react with carbonates, and decreased pH results in much higher metal dissolution. Yet, metal solubility in soils is not determined by the formation and dissolution of single metal compounds.

Spectroscopic (XANES/XRF) characterization of contaminant manganese cycling in a temperate watershed

Many soils around the globe are contaminated with metals due to inputs from anthropogenic activities; however, the long-term processes that retain these metals in soils or flush them into river systems remain unclear. Soils at the Susquehanna/Shale Hills Critical Zone Observatory, a headwater catchment in central Pennsylvania, USA, are enriched in manganese due to past atmospheric deposition from industrial sources. To investigate how Mn is retained in the catchment, we evaluated the spatial distribution and speciation of Mn in the soil–plant system using X-ray fluorescence and X-ray Absorption Near Edge Structure spectroscopies. Weathered soils near the land surface were enriched in both amorphous and crystalline Mn(III/IV)-oxides, presumably derived from biogenic precipitation and atmospheric deposition, respectively. In contrast, mineral soils near the soil–bedrock interface contained Mn(II) in clays and crystalline Mn(III/IV)-oxides that formed as Mn(II) was leached from the parent shale and oxidized. Roots, stems, and foliar tissue were dominated by organic-bound and aqueous Mn(II); however, a small portion of foliar Mn was concentrated as organic-bound Mn(III) in dark spots that denote Mn toxicity. During decomposition of leaves and roots, soluble Mn(II) stored in vegetation was rapidly oxidized and immobilized as mixed-valence Mn-oxides. We propose that considerable uptake of Mn by certain plant species combined with rapid oxidation of Mn during organic matter decomposition contributes to long-term retention in soils and may slow removal of Mn contamination from watersheds.

Kinetic and Conformational Insights of Protein Adsorption onto Montmorillonite Revealed Using in Situ ATR-FTIR/2D-COS

Protein adsorption onto clay minerals is a process with wide-ranging impacts on the environmental cycling of nutrients and contaminants. This process is influenced by kinetic and conformational factors that are often challenging to probe in situ. This study represents an in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopic investigation of the adsorption of a model protein (bovine serum albumin (BSA)) onto a clay mineral (montmorillonite) at four concentrations (1.50, 3.75, 7.50, and 15.0 μM) under environmentally relevant conditions. At all concentrations probed, FTIR spectra show that BSA readily adsorbs onto montmorillonite. Adsorption kinetics follow an Elovich model, suggesting that primary limitations on adsorption rates are surface-related heterogeneous energetic restrictions associated with protein rearrangement and lateral protein-protein interaction. BSA adsorption onto montmorillonite fits the Langmuir model, yielding K = 5.97 × 10(5) M(-1). Deconvolution and curve fitting of the amide I band at the end of the adsorption process (∼120 min) shows a large extent of BSA unfolding upon adsorption at 1.50 μM, with extended chains and turns increasing at the expense of α-helices. At higher concentrations/surface coverages, BSA unfolding is less pronounced and a more compact structure is assumed. Two-dimensional correlation spectroscopic (2D-COS) analysis reveals three different pathways corresponding to adsorbed conformations. At 1.50 μM, adsorption increases extended chains, followed by a loss in α-helices and a subsequent increase in turns. At 3.75 μM, extended chains decrease and then aggregated strands increase and side chains decrease, followed by a decrease in turns. With 7.50 and 15.0 μM BSA, the loss of side-chain vibrations is followed by an increase in aggregated strands and a subsequent decrease in turns and extended chains. Overall, the BSA concentration and resultant surface coverage have a profound impact on the dynamics of BSA adsorption onto montmorillonite. These results enhance our understanding of the molecular-level protein dynamics and stabilization of organic matter at mineral surfaces.

Highly charged swelling mica reduces Cu bioavailability in Cu-contaminated soils.

This is the first test of a highly charged swelling micas (Na-2-mica) ability to reduce the plant-absorbed Cu in Cu-contaminated soils from Chile. Perennial ryegrass (Lolium perenne L.) was grown in two acid soils (Sector 2: pH 4.2, total Cu = 172 mg Cu kg(-1) and Sector 3: pH 4.2, total Cu = 112 mg Cu kg(-1)) amended with 0.5% and 1% (w/w) mica, and 1% (w/w) montmorillonite. At 10 weeks of growth, both mica treatments decreased the shoot Cu of ryegrass grown in Sector 2 producing shoot Cu concentrations above 21-22 mg Cu kg(-1) (the phytotoxicity threshold for that species), yet the mica treatments did not reduce shoot Cu concentrations when grown in Sector 3, which were at a typical level. The mica treatments improved shoot growth in Sector 3 by reducing free and extractable Cu to low enough levels where other nutrients could compete for plant absorption and translocation. In addition, the mica treatments improved root growth in both soils, and the 1% mica treatment reduced root Cu in both soils. This swelling mica warrants further testing of its ability to assist re-vegetation and reduce Cu bioavailability in Cu-contaminated surface soils.

Importance of dynamic soil properties in metal retention: an example from long-term Cu partitioning and redistribution studies using model systems.

The effect of initial conditions and reaction pathways in the long term solid-solution partitioning and solid-phase distribution of Cu among ferrihydrite, leaf compost (LC), and montmorillonite (K-SWy2) were established using compartmentalized batch reactors by varying the sequence of mixing of the sorbents. Copper was allowed to react with a single solid phase for 30 days (1st equilibration) before introducing the other two solid phases and equilibration for 8 additional months (2nd equilibration). The systems were labeled Fe-Ox, Organic, or Smectitic reflecting the single initial solid phase present during the first equilibration. Total dissolved Cu and total Cu in individual solid phases were determined as a function of time during the first and second equilibrations. Results showed that different initial conditions elicited different dynamic responses where the generation of dissolved organic carbon (DOC) and diffusion of colloidal ferrihydrite seemed to influence the long-term partitioning and distribution of Cu. Trends in total dissolved Cu for the systems at the end of the first equilibration were Fe-Ox > Organic > Smectitic, while at the end of the second equilibration the organic system was the least effective in the removal of Cu from solution (Organic > Fe-Ox ≈ Smectitic). Furthermore, our results indicated Cu redistribution toward organic matter and montmorillonite, with small amounts of Cu retained by ferrihydrite. These results are attributed to reaction pathways where the formation of soluble Cu-organic complexes and colloidal Cu-ferrihydrite, and their subsequent reaction with the solids present in the systems, were operative. The experiments reported herein show dynamic properties dictate Cu reaction pathways in multiphase-multicomponent systems and might help to explain unexpected higher mobility of metals after soil remediation.

Soil geochemical factors regulate Cd accumulation by metal hyperaccumulating Noccaea caerulescens (J. Presl & C. Presl) F.K. Mey in field-contaminated soils

Cadmium contamination in soil is a substantial global problem, and of significant concern due to high food-chain transfer. Cadmium hyperaccumulators are of particular interest because of their ability to tolerate and take up significant amounts of heavy metal pollution from soils. One particular plant, Noccaea caerulescens (formerly, Thlaspi caerulescens), has been extensively studied in terms of its capacity to accumulate heavy metals (specifically Zn and Cd), though these studies have primarily utilized hydroponic and metal-spiked model soil systems. We studied Cd and nutrient uptake by two N. caerulescens ecotypes, Prayon (Zn-only hyperaccumulator) and Ganges (Zn- and Cd-hyperaccumulator) in four long-term field-contaminated soils. Our data suggest that individual soil properties such as total soil Cd, Zn:Cd molar ratio, or soil pH do not accurately predict Cd uptake by hyperaccumulating plants. Additionally, total Cd uptake by the hyperaccumulating Ganges ecotype was substantially less than its physiological capacity, which is likely due to Cd-containing solid phases (primarily iron oxides) and pH that play an important role in regulating and limiting Cd solubility. Increased P accumulation in the Ganges leaves, and greater plant Fe accumulation from Cd-containing soils suggests that rhizosphere alterations via proton, and potentially organic acid, secretion may also play a role in nutrient and Cd acquisition by the plant roots. The current study highlights the role that soil geochemical factors play in influencing Cd uptake by hyperaccumulating plants. While these plants may have high physiological potential to accumulate metals from contaminated soils, individual soil geochemical factors and the plant-soil interactions in that soil will dictate the actual amount of phytoextractable metal. This underlines the need for site-specific understanding of metal-containing solid phases and geochemical properties of soils before undertaking phytoextraction efforts.

Structures and mechanisms in clay nanopore trapping of structurally-different fluoroquinolone antimicrobials

Smectite clay nanoparticles are implicated in the retention of antimicrobials within soils and sediments; these clays are also inspected as drug carriers in physiological systems. Cation exchange is considered the primary adsorption mechanism of antimicrobials within smectite nanopores. However, a dual role of acid-base chemistry and adsorptive structures is speculated by recent studies. Using the prototypical smectite clay montmorillonite, we employed a combination of X-ray diffraction (XRD), nuclear magnetic resonance, attenuated total reflectance-Fourier transform infrared spectroscopy, and molecular dynamics simulations to investigate the interlayer nanopore trapping of two structurally-different fluoroquinolone (FQ) antimicrobials with similar acid-base chemistry: ciprofloxacin (a first-generation FQ) and moxifloxacin (a third-generation FQ). Greater sorption at pH 5.0 than at pH 7.0 for both FQs was consistent with cation-exchange of positively-charged species. However, the clay exhibited a near twofold higher sorption capacity for moxifloxacin than for ciprofloxacin. This difference was shown by the XRD data to be accompanied by enhanced trapping of moxifloxacin within the clay interlayers. Using the XRD-determined nanopore sizes, we performed molecular dynamics simulations of thermodynamically-favorable model adsorbates, which revealed that ciprofloxacin was adsorbed parallel to the clay surface but moxifloxacin adopted a tilted conformation across the nanopore. These conformations resulted in more slowly-exchanged than quickly-exchanged Na complexes with ciprofloxacin compared with moxifloxacin. These different Na populations were also captured by 23Na nuclear magnetic resonance. Furthermore, the simulated adsorbates uncovered different complexation interactions that were corroborated by infrared spectroscopy. Therefore, beyond acid-base chemistry, our findings imply that distinct adsorbate structures control antimicrobial trapping within clay nanopores, which can promote persistence in environmental matrices and stable delivery in biological systems.

Ironing Out Genes in the Environment: An Experimental Study of the DNA–Goethite Interface

DNA fate in soil plays an important role in the cycling of genetic information in the environment. Adsorption onto mineral surfaces has great impact on this function. This study probes the kinetics, equilibrium behavior and bonding mechanisms associated with adsorption of DNA onto goethite, a common soil mineral. Surface sensitive ATR-FTIR and XPS approaches are applied to directly characterize the DNA-goethite interface. Adsorption kinetics follow a pseudo-first-order model, suggesting adsorption rate is surface limited. Adsorption rate constants increase with DNA concentration, ranging from 3.29 × 10-3 to 3.55 × 10-1 min-1. Equilibrium adsorption, as monitored by ATR-FTIR and XPS, follows the Langmuir model, with a high affinity of DNA for goethite observed (K = 1.25 × 103 and 9.48 × 102 mL/mg for ATR-FTIR and XPS, respectively). ATR-FTIR and XPS characterization of the structure of surface adsorbed DNA demonstrates inner-sphere coordination between backbone phosphate groups of DNA and goethite. Furthermore, adsorbed DNA retains a B-form, suggesting the DNA helix adsorbs on goethite without degradation or alteration to helical structure, despite binding of backbone phosphate groups. This work advances our understanding of the environmental behavior of DNA by characterizing the mechanism of adsorption onto a prominent soil mineral.

Microscale Investigations of Soil Heterogeneity: Impacts on Zinc Retention and Uptake in Zinc-Contaminated Soils

Metal contaminants in soils can persist for millennia, causing lasting negative impacts on local ecosystems. Long-term contaminant bioavailability is related to soil pH and to the strength and stability of solid-phase associations. We combined physical density separation with synchrotron-based microspectroscopy to reduce solid-phase complexity and to study Zn speciation in field-contaminated soils. We also investigated Zn uptake in two Zn-hyperaccumulating ecotypes of (Ganges and Prayon). Soils were either moderately contaminated (500-800 mg Zn kg via contaminated biosolids application) or grossly enriched (26,000 mg Zn kg via geogenic enrichment). Soils were separated using sodium polytungstate into three fractions: light fraction (LF) (<1.6 g cm), medium fraction (MF) (1.6-2.8 g cm), and heavy fraction (HF) (>2.8 g cm). Approximately 45% of the total Zn was associated with MF in biosolids-contaminated soils. From these data, we infer redistribution to the MF after biosolids application because Zn in biosolids is principally associated with HF and LF. Our results suggest that increasing proportions of HF-associated Zn in soils may be related to greater relative Zn removal by Zn hyperaccumulating plants. Using density fractions enabled assessment of Zn speciation on a microscale despite incomplete fractionation. Analyzing both density fractions and whole soils revealed certain phases (e.g., ZnS, Zn coprecipitated with Fe oxides) that were not obvious in all analyses, indicating multiple views of the same soils enable a more complete understanding of Zn speciation.

Dissolution of mixed amorphous-crystalline Cd-containing Fe coprecipitates in the presence of common organic ligands

Environmental context Common soil minerals, which often contain trace metals in contaminated environments, are thought to limit metal-related risks in the environment. We studied the stability of these contaminated minerals and found that the presence of plant-derived organic compounds can alter contaminant availability in such environments. Understanding how soluble organics may change trace metal availability helps to predict risk and potentially remediate such environments more efficiently. Abstract Trace metals are common impurities in Fe oxides in soils and can be liberated by organic compounds in the soil. Impurities can also alter and potentially destabilise mineral structures by increasing amorphous or nanocrystalline components. Two Fe oxides found in smelter-contaminated soils, goethite (α-FeOOH) and franklinite (ZnFe2O4), were synthesised as coprecipitates with Cd and subjected to dissolution using oxalic acid, citric acid, cysteine and histidine. Substantial quantities of amorphous- (10–60% of total) and surface-associated Cd (5–70% of total) were present in all coprecipitates. Histidine and oxalic acid consistently enhanced, while cysteine inhibited, Cd release from all coprecipitated minerals. Mixed amorphous–crystalline mineral aggregations are common in natural soils, and must be further studied to understand their role in contaminant availability. In addition, Fe oxides, often considered sinks for toxic metals, may be less effective at reducing contaminant mobility and bioavailability in biologically active soils.