Abstract
Activated carbons prepared from cashew nut shells and modified by grafting polyethylenimine onto the surface were tested for removal of Cr (VI). The removal efficiency of carbons without and with polyethylenimine decreased with an increase in pH, with maximum efficiency found at pH 2. The average maximum adsorption capacities of carbons were calculated to be 340 ± 20 mg/g and 320 ± 20 mg/g for unmodified and modified carbons, respectively. Surface characterization of carbons revealed that C–O functionalities are actively involved in both polyethylenimine grafting and Cr (VI) removal. Moreover, lactone groups and amides, formed by polyethylenimine grafting, seemingly undergo acid hydrolysis with formation of phenol and carboxylic groups. Considering that Cr (III) is the only form of chromium found on the surface of both carbons, the reduction mechanism is deduced as the predominant one. Here Cr (VI), majorly present as HCrO4¯, is attracted to the positively charged carbon surface, reduced to Cr (III) by phenol groups, and adsorbed inside the pores. The mechanism of Cr (VI) removal appears to be similar for unmodified and modified carbons, where the smaller adsorption capacity of the latter one can be related to steric hindrance and pore inaccessibility.
Highlights
Chromium water pollution is a serious environmental issue and is usually a result of human activities due to a poor industrial storage and inadequate waste disposal
The maximum in removal efficiency is detected at solution pH of ~2, which is in agreement with the results obtained by other researchers [16,19,22,28,41]
It is suggested that HCrO4 ̄, which is the predominant form of Cr (VI) at pH 2, is electrostatically attracted to the positively charged carbon surface, causing an increase in its removal by the activated carbon
Summary
Chromium water pollution is a serious environmental issue and is usually a result of human activities due to a poor industrial storage and inadequate waste disposal. The forms of chromium that most commonly occur in natural waters are trivalent chromium, Cr (III), and hexavalent chromium, Cr (VI). The former one is found in many food products and is essential in human dietary [1], whereas the latter one occurs naturally in rocks and soil, or is produced by industrial processes, such as tanning, metal plating, and welding. Cr (VI) is a more toxic form of chromium, which is non-biodegradable, and classified as a known carcinogen by the Environmental Protection Agency (EPA).
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