Arsenic (As) is abundant in the environment and can be
found in
both organic (e.g., methylated) and inorganic (e.g., arsenate and
arsenite) forms. The source of As in the environment is attributed
to both natural reactions and anthropogenic activities. As can also
be released naturally to groundwater through As-bearing minerals including
arsenopyrites, realgar, and orpiment. Similarly, agricultural and
industrial activities have elevated As levels in groundwater. High
levels of As in groundwater pose serious health risks and have been
regulated in many developed and developing countries. In particular,
the presence of inorganic forms of As in drinking water sources gained
widespread attention due to their cellular and enzyme disruption activities.
The research community has primarily focused on reviewing the natural
occurrence and mobilization of As. Yet, As originating from anthropogenic
activities, its mobility, and potential treatment techniques have
not been covered. This review summarizes the origin, geochemistry,
occurrence, mobilization, microbial interaction of natural and anthropogenic-As,
and common remediation technologies for As removal from groundwater.
In addition, As remediation methods are critically evaluated in terms
of practical applicability at drinking water treatment plants, knowledge
gaps, and future research needs. Finally, perspectives on As removal
technologies and associated implementation limitations in developing
countries and small communities are discussed.
Field electron emission cathodes were constructed from knitted fabrics comprised entirely of carbon nanotube (CNT) fibers. The fabrics consisted of a top layer array of ∼2 mm high looped structures and a bottom layer that was 1 mm thick with a flat underlying surface. Field emission (FE) experiments were performed on 25.4 mm diameter CNT fabric cathodes in both direct current (DC) and pulsed voltage (PV) modes, and the results were compared to those obtained from a CNT film cathode. The DC measurements were performed at a maximum voltage of 1.5 kV. The CNT fabric cathode emitted 20 mA, which was an 8× increase over the emission current from the CNT film cathode. The DC results were analyzed using the corrected form of the Fowler–Nordheim FE theory initially developed by Murphy and Good, which allows for the determination of the formal emission area and effective gap-field enhancement factor. The PV experiments resulted in Ampere level emission currents from both CNT fabric and CNT film cathodes. For a 25 kV, 500 ns voltage pulse, the CNT fabric cathode emitted 4 A, which was 2× more current than the CNT film cathode. Scanning electron microscopy imaging after PV testing revealed that the fibers remained intact after >5000 pulses. These results indicate that knitted CNT fabrics offer a promising approach for developing large area, conformable, robust FE cathodes for vacuum electronic devices.
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