Heavy Metals and Other Ions Removal

Various methods have been developed for the remediation of contaminated soils and waters from heavy metals, i.e., thermal, biological, physical and chemical treatments. Among all the other methods, adsorption is highly selective, more efficient, easy to operate and low cost method (Zhu et al., 2009).

One of the most dangerous water pollutants is hexavalent chromium, Cr (VI), which is widespread in industrial wastewaters. A low cost aquatic plant residue, Trapa natans L., was used as raw material for the preparation of Fe-modified activated carbon (THAC-Fe) (Liu et al., 2010). Activated carbon was tested for its ability to adsorb Cr(VI) from aqueous solutions. Iron addition to activated carbons increased the adsorption ability with the maximum adsorption capacity to reach 11.83 mg/g Cr(VI). Kinetic data fitted well to pseudo-second order equation while equilibrium data followed the Temkin and Freundlich models. The entire adsorption process was controlled by external mass transfer and intraparticle diffusion. Similar studies examined the removal of nickel from aqueous solutions by activated carbons derived from doum-palm seed coat (El-Sadaawy and Abdelwahab, 2014) and zinc, cadmium and copper by activated carbons derived from almond shells, olive stones and peach stones (Ferro-Garcia et al., 1988). Batch experiments were conducted investigating the role of solution pH, initial nickel concentration, adsorbent dose and contact time. Kinetic data fitted well to pseudo-second order equation while equilibrium data followed the Freundlich model. The maximum nickel adsorption capacity was 13.51 mg/g (El-Sadaawy and Abdelwahab, 2014). According to Ferro-Garcia et al., different parameters were examined such as solution pH and extent of heavy metal adsorption in the presence of CL-, CN-, SCN-, EDTA. Chemical structure and porous texture are the main parameters which influenced the adsorption of zinc, cadmium and copper by activated carbons. Other studies (Baccar et al., 2009) concerning the uptake of Cu(II) ions from activated carbons derived from olive waste cakes and a chemical modification using KMnO4 as oxidant (KMnO4-modified carbons) and unmodified activated carbons showed high adsorption capacity equal to 35.3 and 12 mg/g for modified and unmodified carbons, respectively.

Although the production of activated carbons consists of pyrolysis at high temperatures and physical or chemical activation, biosorbents, which derived from agricultural wastes, need little processing so as to increase their adsorption ability. Removal of copper(II) from aqueous solutions by chemically - treated tomato waste (Solanum lycopersicum) (Yargi? et al., 2014) or by peanut hull (Zhu et al., 2009) or by coconut shell, neem leaves, hyacinth roots, rice straw, rice bran and rice husk (Singha and Das, 2013) were investigated. The Cu(II) removal was pH dependent to all cases indicating that the optimum pH for adsorption was equal to 8 (Yargi? et al., 2014), 5.5 (Zhu et al., 2009) and 6 (Singha and Das, 2013), respectively. Kinetic data were best described by pseudo-second order equation to all cases. The maximum biosorbent capacity was equal to 22.37 mg/g for 125 mg/L copper solution (Yargi? et al., 2014), 21.25 mg/g (Zhu et al., 2009) and 19.89, 17.49, 21.80, 18.35, 20.98, 17.87 mg/g for coconut shell, neem leaves, hyacinth roots, rice straw, rice bran and rice husk, respectively (Singha and Das, 2013). Another study (Brown et al., 2000) assessed the removal of copper(II), cadmium(II), zinc(II) and molybdenum(II) from wastewaters by peanut hulls and hull pellets indicating that almost 90% of metals removal occurred at the first 20 min of contact.

The removal of other ions except of heavy metals by adsorbents, which were prepared from agricultural wastes, were also reported in literature. Dried orange juice residue (DOJR) (Paudyal et al., 2013) or Citrus limonum (lemon) leaf (Tomar et al., 2014) was converted to a promising adsorbent for fluoride ions from water. Different metal loaded DOJR were produced for the adsorption of fluoride ion traces from wastewaters leading to maximum adsorption capacities with values between 0.67 and 1.43 mg/g depending on the metal, i.e., Zr(II), Ce(IV), Al(III) loaded DOJR. Adsorption of fluoride ion from aqueous solutions on Citrus limonum (lemon) leaf led to 70% maximum defluoridation capacity with optimal pH equal to 2.

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