Irene M.C. Lo

Magnetic nanoparticles: Essential factors for sustainable environmental applications

In recent years, there has been an increasing use of engineered magnetic nanoparticles for remediation and water treatments, leading to elevated public concerns. To this end, it is necessary to enhance the understanding of how these magnetic nanoparticles react with contaminants and interact with the surrounding environment during applications. This review aims to provide a holistic overview of current knowledge of magnetic nanoparticles in environmental applications, emphasizing studies of zero-valent iron (nZVI), magnetite (Fe3O4) and maghemite (γ-Fe2O3) nanoparticles. Contaminant removal mechanisms by magnetic nanoparticles are presented, along with factors affecting the ability of contaminant desorption. Factors influencing the recovery of magnetic nanoparticles are outlined, describing the challenges of magnetic particle collection. The aggregation of magnetic nanoparticles is described, and methods for enhancing stability are summarized. Moreover, the toxicological effects owing to magnetic nanoparticles are discussed. It is possible that magnetic nanoparticles can be applied sustainably after detailed consideration of these discussed factors.

The limitations of applying zero-valent iron technology in contaminants sequestration and the corresponding countermeasures: the development in zero-valent iron technology in the last two decades (1994-2014).

Over the past 20 years, zero-valent iron (ZVI) has been extensively applied for the remediation/treatment of groundwater and wastewater contaminated with various organic and inorganic pollutants. Based on the intrinsic properties of ZVI and the reactions that occur in the process of contaminants sequestration by ZVI, this review summarizes the limitations of ZVI technology and the countermeasures developed in the past two decades (1994-2014). The major limitations of ZVI include low reactivity due to its intrinsic passive layer, narrow working pH, reactivity loss with time due to the precipitation of metal hydroxides and metal carbonates, low selectivity for the target contaminant especially under oxic conditions, limited efficacy for treatment of some refractory contaminants and passivity of ZVI arising from certain contaminants. The countermeasures can be divided into seven categories: pretreatment of pristine ZVI to remove passive layer, fabrication of nano-sized ZVI to increase the surface area, synthesis of ZVI-based bimetals taking advantage of the catalytic ability of the noble metal, employing physical methods to enhance the performance of ZVI, coupling ZVI with other adsorptive materials and chemically enhanced ZVI technology, as well as methods to recover the reactivity of aged ZVI. The key to improving the rate of contaminants removal by ZVI and broadening the applicable pH range is to enhance ZVI corrosion and to enhance the mass transfer of the reactants including oxygen and H(+) to the ZVI surface. The characteristics of the ideal technology are proposed and the future research needs for ZVI technology are suggested accordingly.

Heavy metal speciation and leaching behaviors in cement based solidified/stabilized waste materials

A circuit board printing factory sludge containing high concentrations of copper, zinc and lead was stabilized and solidified (S/S) with different portions of ordinary Portland cement (OPC) and pulverized fly ash (PFA). The chemical speciation and leaching behavior of heavy metals in these cement-based waste materials were studied by different sequential extraction procedures, standard toxicity characteristic leaching procedure (TCLP) and progressive TCLP tests. The sequential extraction results showed that more than 80% of Cu, Pb and Zn were associated with Fraction 2 (weak acid soluble, extracted with 1M NaOAc at pH 5.0 with a solid to liquid ratio of 1:60). This indicated that the heavy metals could exist in the S/S matrix as metal hydrated phases or metal hydroxides precipitating on the surface of calcium silicate hydrates (C-S-H), PFA and sludge particles. The progressive TCLP test results and MINTEQA2 calculation also showed the importance of Cu and Zn oxides during the leaching process. The leaching behaviors of these metals in the S/S waste materials were mainly controlled by the alkaline nature and acid buffering capacity of the S/S matrix. During the progressive TCLP tests, the alkaline conditions and acid buffering capacity of the matrix decreased with the dissolution of calcium hydroxide and C-S-H, therefore, the leaching of heavy metals in the S/S waste materials increased. The leaching of heavy metals in the S/S materials can be considered as a pH dependent and corresponding metal hydroxide solubility controlled process.

EDTA Extraction of Heavy Metals from Different Soil Fractions and Synthetic Soils

A laboratory-prepared contaminated soil was partitioned into four fractions, namely carbonate, Fe/Mn oxides, organic matter and clay mineral, according to the form in which the heavy metal bound with soil constituents. Individual contaminated soil fractions and synthetic soils were prepared for the study of soil extraction using ethylenediaminetetraacetic acid (EDTA). The effect of contact time and EDTA concentration were evaluated for both individual soil fractions and synthetic soils. The extraction reached equilibrium rapidly, after about 30 min. A 0.01 M EDTA solution was less effective than a 0.05 M or a 0.10 M EDTA. EDTA was proved to be effective for metal removal from the four individual soil fractions and synthetic soils. In general, approximately 90% of metals were removed from synthetic soils by 0.10 M EDTA. EDTA extraction of Pb from a contaminated carbonate fraction was thought to be affected by the formation of lead carbonates. A simple equation based on the sum of the released heavy metal from the individual components is used to check if there are interactions among the different soil components when mixed. The estimated values agreed well with the experimentally measured results only for the 0.10 M EDTA system.

Effects of humic acid on arsenic(V) removal by zero-valent iron from groundwater with special references to corrosion products analyses.

The effects of humic acid (HA) on As(V) removal by zero-valent iron (Fe(0)) from groundwater, associated with corrosion products analyses, were investigated using batch experiments. It was found that arsenic was rapidly removed from groundwater possibly due to its adsorption and co-precipitation with the corrosion products of Fe(0). The removal rate of arsenic by Fe(0) was inhibited in the presence of HA probably because of the formation of soluble Fe-humate in groundwater which hindered the production of iron precipitates. A longer reaction time was then required for arsenic removal. Such an influence of HA on arsenic removal increased with increasing HA concentration from 5 to 25mgL(-1). The binding capacity of HA for dissolved Fe was estimated to be about 0.75mg Femg(-1) HA. When the complexation of HA with dissolved Fe was saturated, further corrosion of Fe(0) would produce precipitates, which significantly accelerated the removal of arsenic from groundwater via adsorption and co-precipitation with the corrosion products. Iron (hydr)oxides such as maghemite, lepidocrocite, and magnetite were characterized by XRD analyses as the corrosion products, while As(V) was found on the surface of these corrosion products as detected by fourier transform infrared spectrometry and X-ray photoelectron spectroscopy.

Fate of As(V)-treated nano zero-valent iron: Determination of arsenic desorption potential under varying environmental conditions by phosphate extraction

Nano zero-valent iron (NZVI) offers a promising approach for arsenic remediation, but the spent NZVI with elevated arsenic content could arouse safety concerns. This study investigated the fate of As(V)-treated NZVI (As-NZVI), by examining the desorption potential of As under varying conditions. The desorption kinetics of As from As-NZVI as induced by phosphate was well described by a biphasic rate model. The effects of As(V)/NZVI mass ratio, pH, and aging time on arsenic desorption from As-NZVI by phosphate were investigated. Less arsenic desorption was observed at lower pH or higher As(V)/NZVI mass ratio, where stronger complexes (bidentate) formed between As(V) and NZVI corrosion products as indicated by FTIR analysis. Compared with the fresh As-NZVI, the amount of phosphate-extractable As significantly decreased in As-NZVI aged for 30 or 60 days. The results of the sequential extraction experiments demonstrated that a larger fraction of As was sorbed in the crystalline phases after aging, making it less susceptible to phosphate displacement. However, at pH 9, a slightly higher proportion of phosphate-extractable As was observed in the 60-day sample than in the 30-day sample. XPS results revealed the transformation of As(V) to more easily desorbed As(III) during aging and a higher As(III)/As(V) ratio in the 60-day sample at pH 9, which might have resulted in the higher desorption.

Removal and redistribution of metals from contaminated soils by a sequential extraction method

The fate of heavy metals in a contaminated soil is dependent on both the total amount of metals and the chemical forms in which they exist. A widely applied sequential extraction method was used to study the metal speciation in soils. Sequential extraction for metal-contaminated individual soil components and synthetic soils was performed. The experimental results show that the specific reagents were effective for the extraction of metals from the corresponding individual soil components; but the impact of non-corresponding reagents on other soil components was also significant. The chemical forms in which metals exist in the synthetic soils were complicated and different. Metals in soils might be released and redistributed during the sequential extraction process, but knowledge about this behavior and the extent of metal redistribution is ambiguous.

Humic acid aggregation in zero-valent iron systems and its effects on trichloroethylene removal.

The influence of natural organic matter on contaminant removal by Fe(0) systems has been of increasing concern. Recent studies have shown that, in addition to direct sorption on the Fe(0) surfaces, humic acid complexation with dissolved iron released from corrosion results in the formation of colloids and aggregates in solution that may affect contaminant removal. High-pressure size-exclusion chromatographic analyses revealed increasing molecular weights of dissolved humic acids with reaction time. Humic acid aggregation occurred across a wide range of molecular weight fractions. Fourier transform infrared spectroscopic analysis of humic acid aggregates suggested the presence of inner-sphere complexation involving different oxygen-containing functional groups; hydrophobic interactions also probably contributed to aggregation as the humic acid of more aromatic and hydrophobic character was aggregated at a faster rate. Because of multiple underlying processes, a variety of cross-correlated physicochemical properties of humic acids contributed to their aggregation. The presence of humic acid aggregates provided an additional hydrophobic domain for partitioning that enhanced trichloroethylene removal, although steric blocking of the Fe(0) surfaces may inhibit its reduction to some extent. Comparable effects were demonstrated for various types of humic acids.

Heavy metal extraction from an artificially contaminated sandy soil under EDDS deficiency: significance of humic acid and chelant mixture.

Biodegradable EDDS ([S,S]-ethylenediaminedisuccinic acid) has been suggested for enhancing heavy metal extraction from contaminated soils. Recent studies showed that Zn and Pb are less effectively extracted due to metal exchange and re-adsorption onto the soil surfaces, especially for EDDS-deficiency conditions. This study therefore investigated the influence of dissolved organic matter and the co-presence of EDTA (ethylene-diamine-tetraacetic acid) on metal extraction from an artificially contaminated sandy soil under deficient amount of chelants in batch kinetics experiments. The addition of 10 and 20mgL(-1) of humic acid as dissolved organic matter (DOC) suppressed metal extraction by EDDS, probably resulting from the competition of adsorbed humic acid for heavy metals and adsorption of metal-humate complexes onto the soil surfaces. The effects were most significant for Pb because of greater extent of metal exchange of PbEDDS and high affinity towards organic matter. Thus, one should be cautious when there is a high content of organic matter in soils or groundwater. On the other hand, compared to individual additions of EDDS or EDTA, the equimolar EDDS and EDTA mixture exhibited significantly higher Pb extraction without notable Pb re-adsorption. The synergistic performance of the EDDS and EDTA mixture probably resulted from the change of chemical speciation and thus less competition among Cu, Zn and Pb for each chelant. These findings suggest further investigation into an optimum chemistry of the chelant mixture taking into account the effectiveness and associated environmental impact.

Influences of humic acid, bicarbonate and calcium on Cr(VI) reductive removal by zero-valent iron.

The influences of various geochemical constituents, such as humic acid, HCO(3)(-), and Ca(2+), on Cr(VI) removal by zero-valent iron (Fe(0)) were investigated in a batch setting. The collective impacts of humic acid, HCO(3)(-), and Ca(2+) on the Cr(VI) reduction process by Fe(0) appeared to significantly differ from their individual impacts. Humic acid introduced a marginal influence on Fe(0) reactivity toward Cr(VI) reduction, whereas HCO(3)(-) greatly enhanced Cr(VI) removal by maintaining the solution pH near neutral. The Cr(VI) reduction rate constants (k(obs)) were increased by 37.8% and 78.3%, respectively, with 2 mM and 6 mM HCO(3)(-) in solutions where humic acid and Ca(2+) were absent. Singly present Ca(2+) did not show a significant impact to Cr(VI) reduction. However, probably due to the formation of passivating CaCO(3), further addition of Ca(2+) to HCO(3)(-) containing solutions resulted in a decrease of k(obs) compared to solutions containing HCO(3)(-) alone. Ca(2+) enhanced humic acid adsorption led to a minor decrease of Cr(VI) reduction rates. In Ca(2+)-free solutions, humic acid increased the amount of total dissolved iron to 25 mg/l due to the formation of soluble Fe-humate complexes and stably dispersed fine Fe (oxy)hydroxide colloids, which appeared to suppress iron precipitation. In contrast, the coexistence of humic acid and Ca(2+) significantly promoted the aggregation of Fe (oxy)hydroxides, with which humic acid co-aggregated and co-precipitated. These aggregates would progressively be deposited on Fe(0) surfaces and impose long-term impacts on the permeability of PRBs.