Effects of Natural Organic Matter on the Dissolution Kinetics and Bioavailability of Metal Oxide Nanoparticles

The rapid development of nanotechnology and wider applications of engineered nanomaterials (ENMs) in the last few decades have generated concerns regarding their environmental and health risks. After release into the environment, ENMs undergo aggregation, transformation, and, for metal-based nanomaterials, dissolution processes, which together determine their fate, bioavailability and toxicity to living organisms in the ecosystems. The rates of these processes are dependent on nanomaterial characteristics as well as complex environmental factors, including natural organic matter (NOM). As a ubiquitous component of aquatic systems, NOM plays a key role in the aggregation, dissolution and transformation of metal-based nanomaterials and colloids in aquatic environments.

The goal of this dissertation work is to investigate how NOM fractions with different chemical and molecular properties affect the dissolution kinetics of metal oxide ENMs, such as zinc oxide (ZnO) and copper oxide (CuO) nanoparticles (NPs), and consequently their bioavailability to aquatic vertebrate, with Gulf killifish (Fundulus grandis) embryos as model organisms.

ZnO NPs are known to dissolve at relatively fast rates, and the rate of dissolution is influenced by water chemistry, including the presence of Zn-chelating ligands. A challenge, however, remains in quantifying the dissolution of ZnO NPs, particularly for time scales that are short enough to determine rates. This dissertation assessed the application of anodic stripping voltammetry (ASV) with a hanging mercury drop electrode to directly measure the concentration of dissolved Zn in ZnO NP suspensions, without separation of the ZnO NPs from the aqueous phase. Dissolved zinc concentration measured by ASV ([Zn]ASV) was compared with that measured by inductively coupled plasma mass spectrometry (ICP-MS) after ultracentrifugation ([Zn]ICP-MS), for four types of ZnO NPs with different coatings and primary particle diameters. For small ZnO NPs (4-5 nm), [Zn]ASV was 20% higher than [Zn]ICP-MS, suggesting that these small NPs contributed to the voltammetric measurement. For larger ZnO NPs (approximately 20 nm), [Zn]ASV was (79±19)% of [Zn]ICP-MS, despite the high concentrations of ZnO NPs in suspension, suggesting that ASV can be used to accurately measure the dissolution kinetics of ZnO NPs of this primary particle size.

Using the ASV technique to directly measure dissolved zinc concentration, we examined the effects of 16 different NOM isolates on the dissolution kinetics of ZnO NPs in buffered potassium chloride solution. The observed dissolution rate constants (kobs) and dissolved zinc concentrations at equilibrium increased linearly with NOM concentration (from 0 to 40 mg-C L-1) for Suwannee River humic acid (SRHA), Suwannee River fulvic acid and Pony Lake fulvic acid. When dissolution rates were compared for the 16 NOM isolates, kobs was positively correlated with certain properties of NOM, including specific ultraviolet absorbance (SUVA), aromatic and carbonyl carbon contents, and molecular weight. Dissolution rate constants were negatively correlated to hydrogen/carbon ratio and aliphatic carbon content. The observed correlations indicate that aromatic carbon content is a key factor in determining the rate of NOM-promoted dissolution of ZnO NPs. NOM isolates with higher SUVA were also more effective at enhancing the colloidal stability of the NPs; however, the NOM-promoted dissolution was likely due to enhanced interactions between surface metal ions and NOM rather than smaller aggregate size.

Based on the above results, we designed experiments to quantitatively link the dissolution kinetics and bioavailability of CuO NPs to Gulf killifish embryos under the influence of NOM. The CuO NPs dissolved to varying degrees and at different rates in diluted 5‰ artificial seawater buffered to different pH (6.3-7.5), with or without selected NOM isolates at various concentrations (0.1-10 mg-C L-1). NOM isolates with higher SUVA and aromatic carbon content (such as SRHA) were more effective at promoting the dissolution of CuO NPs, as with ZnO NPs, especially at higher NOM concentrations. On the other hand, the presence of NOM decreased the bioavailability of dissolved Cu ions, with the uptake rate constant negatively correlated to dissolved organic carbon concentration ([DOC]) multiplied by SUVA, a combined parameter indicative of aromatic carbon concentration in the media. When the embryos were exposed to CuO NP suspension, changes in their Cu content were due to the uptake of both dissolved Cu ions and nanoparticulate CuO. The uptake rate constant of nanoparticulate CuO was also negatively correlated to [DOC]×SUVA, in a fashion roughly proportional to changes in dissolved Cu uptake rate constant. Thus, the ratio of uptake rate constants from dissolved Cu and nanoparticulate CuO (ranging from 12 to 22, on average 17±4) were insensitive to NOM type or concentration. Instead, the relative contributions of these two Cu forms were largely determined by the percentage of CuO NP that was dissolved.

Overall, this dissertation elucidated the important role that dissolved NOM plays in affecting the environmental fate and bioavailability of soluble metal-based nanomaterials. This dissertation work identified aromatic carbon content and its indicator SUVA as key NOM properties that influence the dissolution, aggregation and biouptake kinetics of metal oxide NPs and highlighted dissolution rate as a useful functional assay for assessing the relative contributions of dissolved and nanoparticulate forms to metal bioavailability. Findings of this dissertation work will be helpful for predicting the environmental risks of engineered nanomaterials.

Reckoning With Apartheid: The Conundrum of Working Through the Past, An Introduction

From April 26-29, 1994, South Africa held its first universal, democratic elections. Witnessed by the world, South Africans of all races waited patiently in line to cast their ballots, signaling the official and symbolic birth of the “new” South Africa. The subsequent years, marked initially with euphoric hopes for racial healing enabled by institutional processes such as the Truth and Reconciliation Commission (TRC), have instead, most recently, inspired deep concern about epidemic levels of HIV/AIDS, violent crime, state corruption, and unbridled market reforms directed at everything from property to bodies to babies. Now, seemingly beleaguered state officials deploy the mantra “TINA” (There Is No Alternative [to neoliberal development]) to fend off criticism of growing income and wealth disparities. To coincide, more or less, with the anniversary of 1994—less to commemorate than to signal something about the trajectory of the past twenty years—we are proposing an interdisciplinary, special theme section of Comparative Studies in South Asia, Africa, and the Middle East (CSSAAME) entitled “The Haunted Present: Reckoning After Apartheid” (tentative title). The special theme section is framed around questions of reckoning in the double sense of both a moral and practical accounting for historical injury alongside the challenges and failures of the no-longer “new” South Africa. Against accounts depicting the liberation era as non-violent and peaceable, more nuanced analysis we argue suggests not only that South Africa’s “revolution” was marked by both collective and individual violence—on the part of the state and the liberation movements—but that reckoning with the present demands of scholars, the media, and cultural commentators that they begin to grapple more fully with the dimensions and different figurations of South Africa’s violent colonial history. Indeed, violence and reckoning appear as two central forces in contemporary South African political, economic, and social life. In response, we are driven to pose the following questions: In the post-apartheid period, what forms of (individual, structural) violence have come to bear on South African life? How does this violence reckon with apartheid and its legacies? Does it in fact reckon with the past? How can we or should we think about violence as a response to the (failed?) reckoning of state initiatives like the TRC? What has enabled or enables aesthetic forms—literature, photography, plastic arts, and other modes of expressive culture—to respond to the difficulties of South Africa’s ongoing transition? What, in fact, would a practice or ethic of reckoning defined in the following way look like? ˈrekəniNG/ noun: • the action or process of calculating or estimating something: last year was not, by any reckoning, a particularly good one; the system of time reckoning in Babylon • a person’s view, opinion, or judgment: by ancient reckoning, bacteria are plants • archaic, a bill or account, or its settlement • the avenging or punishing of past mistakes or misdeeds: the fear of being brought to reckoning there will be a terrible reckoning (Oxford English Dictionary) Looking back on the period, just before 1994, is sobering indeed. At the time, many saw in the energies and courage of those fighting for liberation the possibilities of a post-racial, post-conflict society. Yet as much as the new was ushered in, old apartheid forms lingered. Recalling Nadine Gordimer’s invocation of Gramsci’s “morbid symptoms” more and more it seems “the old is dying and the new cannot be born” (Gramsci cited in Gordimer 1982). And even as the new began to emerge other forces—both internal and external to South Africa—redefined the conditions for transformation. The so-called “new” South Africa, as Jennifer Wenzel has argued, was really more than anything “the changing face of old oppressions” (Wenzel 2009:159). The implications for our special theme section of CSSAAME are many. We begin by exploring the gender, race, and class dimensions of contemporary South African life by way of its literatures, histories, and politics, its reversion to custom, the claims of ancestors on the living, in brief, the various cultural expressive modes in which contemporary South Africa reckons with its past and in so doing accounts, day by day, for the ways in which the present can be lived, pragmatically. This moves us some distance from the exercise in “truth and reconciliation” of the earlier post-transition years to consider more fully the nature of post-conflict, the suturing of old enmities in the present, and the ways of resolving those lingering suspicions both ordinary and the stuff of the dark night of the soul (Nelson 2009:xv).

Reckoning With Apartheid: The Conundrum of Working Through the Past

From April 26-29, 1994, South Africa held its first universal, democratic elections. Witnessed by the world, South Africans of all races waited patiently in line to cast their ballots, signaling the official and symbolic birth of the “new” South Africa. The subsequent years, marked initially with euphoric hopes for racial healing enabled by institutional processes such as the Truth and Reconciliation Commission (TRC), have instead, most recently, inspired deep concern about epidemic levels of HIV/AIDS, violent crime, state corruption, and unbridled market reforms directed at everything from property to bodies to babies. Now, seemingly beleaguered state officials deploy the mantra “TINA” (There Is No Alternative [to neoliberal development]) to fend off criticism of growing income and wealth disparities. To coincide, more or less, with the anniversary of 1994—less to commemorate than to signal something about the trajectory of the past twenty years—we are proposing an interdisciplinary, special theme section of Comparative Studies in South Asia, Africa, and the Middle East (CSSAAME) entitled “The Haunted Present: Reckoning After Apartheid” (tentative title). The special theme section is framed around questions of reckoning in the double sense of both a moral and practical accounting for historical injury alongside the challenges and failures of the no-longer “new” South Africa. Against accounts depicting the liberation era as non-violent and peaceable, more nuanced analysis we argue suggests not only that South Africa’s “revolution” was marked by both collective and individual violence—on the part of the state and the liberation movements—but that reckoning with the present demands of scholars, the media, and cultural commentators that they begin to grapple more fully with the dimensions and different figurations of South Africa’s violent colonial history. Indeed, violence and reckoning appear as two central forces in contemporary South African political, economic, and social life. In response, we are driven to pose the following questions: In the post-apartheid period, what forms of (individual, structural) violence have come to bear on South African life? How does this violence reckon with apartheid and its legacies? Does it in fact reckon with the past? How can we or should we think about violence as a response to the (failed?) reckoning of state initiatives like the TRC? What has enabled or enables aesthetic forms—literature, photography, plastic arts, and other modes of expressive culture—to respond to the difficulties of South Africa’s ongoing transition? What, in fact, would a practice or ethic of reckoning defined in the following way look like? ˈrekəniNG/ noun: • the action or process of calculating or estimating something: last year was not, by any reckoning, a particularly good one; the system of time reckoning in Babylon • a person’s view, opinion, or judgment: by ancient reckoning, bacteria are plants • archaic, a bill or account, or its settlement • the avenging or punishing of past mistakes or misdeeds: the fear of being brought to reckoning there will be a terrible reckoning (Oxford English Dictionary) Looking back on the period, just before 1994, is sobering indeed. At the time, many saw in the energies and courage of those fighting for liberation the possibilities of a post-racial, post-conflict society. Yet as much as the new was ushered in, old apartheid forms lingered. Recalling Nadine Gordimer’s invocation of Gramsci’s “morbid symptoms” more and more it seems “the old is dying and the new cannot be born” (Gramsci cited in Gordimer 1982). And even as the new began to emerge other forces—both internal and external to South Africa—redefined the conditions for transformation. The so-called “new” South Africa, as Jennifer Wenzel has argued, was really more than anything “the changing face of old oppressions” (Wenzel 2009:159). The implications for our special theme section of CSSAAME are many. We begin by exploring the gender, race, and class dimensions of contemporary South African life by way of its literatures, histories, and politics, its reversion to custom, the claims of ancestors on the living, in brief, the various cultural expressive modes in which contemporary South Africa reckons with its past and in so doing accounts, day by day, for the ways in which the present can be lived, pragmatically. This moves us some distance from the exercise in “truth and reconciliation” of the earlier post-transition years to consider more fully the nature of post-conflict, the suturing of old enmities in the present, and the ways of resolving those lingering suspicions both ordinary and the stuff of the dark night of the soul (Nelson 2009:xv).