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Orange peel is used as an adsorbent for the removal of chromium from wastewater because it contains cellulose, hemicelluloses, pectin (galacturonic acid) and lignin (Feng et al. 2011). These components also have various coordinating functional groups including carboxylic and phenolic acid groups which can bind heavy metals. Orange peel is an attractive adsorbent because of its availability and low cost (Feng et al. 2011). Marín et al. (2010) studied the role of three major functional groups (amine, carboxyl and hydroxyl) on chromium removal where the bioadsorbent (orange peel) was chemically modified by esterification, acetylation and methylation in order to selectively block the functional groups. Thus, esterification decreased removal capacity, which indicates that the carboxylic groups present in the adsorbent are important for chromium removal and that the amine and hydroxyl groups have a negligible effect. The maximum adsorption capacity reported by these researchers was 40.56 mg/g. Lugo-Lugo et al. (2012) biosorbed chromium on pre-treated orange peel in both single (presence of chromium only) and binary mixtures (presence of chromium along with iron). It was observed that in the binary mixture, removal of chromium was interfered with by the presence of iron as more than one heavy metal in the mixture may increase, decrease or may not affect removal performance of the adsorbent. The removal per cent and adsorption capacity obtained in single phase (presence of chromium only) was 51% and 4.79 mg/g while for the binary system (presence of chromium along with iron) it was 79% and 7.60 mg/g. López-Téllez et al. (2011) removed chromium by preparing a composite that incorporates iron nanoparticles into orange peel pith. It was found that for this composite the percentage removal and adsorption capacity were 71% and 5.37 mg/g, respectively, as compared to raw orange peel, i.e., 34% and 1.90 mg/g, respectively. Table 9 summarises the reported use of modified orange peel waste as an adsorbent for chromium removal from wastewater.

Table 9

Chromium removal using modified orange peel waste as an adsorbent

AdsorbentMetal concentration (mg/L)Optimum pHBest model fitContact time (min)Adsorbent dose (g/L)Adsorbent capacity (mg/g)Removal per cent (%)References
Modified orange peel 0–500 Langmuir 4,320 4.0 40.56 82% Marín et al. (2010)  
Pre-treated orange peel 10 Langmuir model 260 10.0 4.79, 7.60 51%, 79% Lugo-Lugo et al. (2012)  
Composite of iron nanoparticles into orange peel pith 10–50 Langmuir 60 5.0 1.90, 5.37 34%, 71% López-Téllez et al. (2011)  
AdsorbentMetal concentration (mg/L)Optimum pHBest model fitContact time (min)Adsorbent dose (g/L)Adsorbent capacity (mg/g)Removal per cent (%)References
Modified orange peel 0–500 Langmuir 4,320 4.0 40.56 82% Marín et al. (2010)  
Pre-treated orange peel 10 Langmuir model 260 10.0 4.79, 7.60 51%, 79% Lugo-Lugo et al. (2012)  
Composite of iron nanoparticles into orange peel pith 10–50 Langmuir 60 5.0 1.90, 5.37 34%, 71% López-Téllez et al. (2011)  

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