(1) free from Cd
2+ ions at pH lower than 6, (2) Cd(OH)
+ at pH between 6.5 and 8.5,
and (3) precipitation as hydroxides expect to occur at pH higher than 9 [42]. For
heavy metals that exist as anions in acidic condition, such as Cr(VI), the highest
adsorption capacity occurs at very acidic pH. Conventionally, the Cr(VI) species at
very low pH identify as HCrO 4
À , Cr 2 O 7
2À , and CrO 4
2À
. For this reason, the
adsorption of Cr(VI) by using wheat bran is the uppermost at solution pH 2.2,
where Cr(VI) is simultaneously reduced to Cr(III) [52].
3.2 Effect of Initial Concentration
The initial concentration of heavy metals provides the driving force for adsorption
process. High concentration difference results in greater adsorption capacity but
reduced efficiency of the adsorbent. This phenomenon frequently causes controversy
in selecting a suitable treatment process for decontamination of heavy metals as
mostly the threshold limit for toxic heavy metals is very low, either at ppm or ppb
level. The World Health Organization (WHO) provides drinking water quality
standard which includes a permissible limit for each of the heavy metals such as a
maximum limit of 0.01 ppm for Pb, 0.003 ppm for Cd, and 0.05 ppm for Cr(VI) [57].
Research work from Shrestha et al. [45] using lignite and coconut shell-based
activated carbon found that the adsorption capacity increased from 0.22 mg/g to
1.96 mg/g when the initial concentration increased from 10 to 50 mg/L; nonetheless,
the percentage removal declined from 88% to 78%, by reason of limited active sites
available for adsorption as the number of adsorbates competing for the sites are
increasing. Ullah et al. [58] and Mahajan and Sud [38] disclosed similar remarks
from their laboratory studies which recycled sugarcane bagasse to remove Cr
(VI) and Arachis hypogaea to remove Cd(II) and Ni(II), respectively.
3.3 Effect of Adsorbent Dosage
Addition of more adsorbent in the adsorption process gives the effect of booming the
adsorption site availability. On the contrary to the initial concentration effect, the
process efficiency inclines, and the adsorption capacity declines as the adsorbent
dosage increases. The more significant amount of available adsorption sites increase,
the number of adsorbate molecules interacting, resulting in higher percentage
removal of heavy metal ions.
Zhong et al. [55] demonstrated this comprehensive observation in Cu(II) and Cr
(VI) removal using wheatgrass-derived adsorbent. In the study, the increase of
sorbent dosage up to 2 g/L resulted in achieving 86.44% and 92.19% removal
capacity, respectively. The adsorption capacities, howbeit, declined from 45 mg/g
to 22 mg/g for Cu(II) and from 230 mg/g to 50 mg/g for Cr(VI). The study also found
that increasing the adsorbent dosage further did not increase the percentage removal
9 Agricultural Waste-Derived Adsorbents for Decontamination of Heavy Metals
379
2+ ions at pH lower than 6, (2) Cd(OH)
+ at pH between 6.5 and 8.5,
and (3) precipitation as hydroxides expect to occur at pH higher than 9 [42]. For
heavy metals that exist as anions in acidic condition, such as Cr(VI), the highest
adsorption capacity occurs at very acidic pH. Conventionally, the Cr(VI) species at
very low pH identify as HCrO 4
À , Cr 2 O 7
2À , and CrO 4
2À
. For this reason, the
adsorption of Cr(VI) by using wheat bran is the uppermost at solution pH 2.2,
where Cr(VI) is simultaneously reduced to Cr(III) [52].
3.2 Effect of Initial Concentration
The initial concentration of heavy metals provides the driving force for adsorption
process. High concentration difference results in greater adsorption capacity but
reduced efficiency of the adsorbent. This phenomenon frequently causes controversy
in selecting a suitable treatment process for decontamination of heavy metals as
mostly the threshold limit for toxic heavy metals is very low, either at ppm or ppb
level. The World Health Organization (WHO) provides drinking water quality
standard which includes a permissible limit for each of the heavy metals such as a
maximum limit of 0.01 ppm for Pb, 0.003 ppm for Cd, and 0.05 ppm for Cr(VI) [57].
Research work from Shrestha et al. [45] using lignite and coconut shell-based
activated carbon found that the adsorption capacity increased from 0.22 mg/g to
1.96 mg/g when the initial concentration increased from 10 to 50 mg/L; nonetheless,
the percentage removal declined from 88% to 78%, by reason of limited active sites
available for adsorption as the number of adsorbates competing for the sites are
increasing. Ullah et al. [58] and Mahajan and Sud [38] disclosed similar remarks
from their laboratory studies which recycled sugarcane bagasse to remove Cr
(VI) and Arachis hypogaea to remove Cd(II) and Ni(II), respectively.
3.3 Effect of Adsorbent Dosage
Addition of more adsorbent in the adsorption process gives the effect of booming the
adsorption site availability. On the contrary to the initial concentration effect, the
process efficiency inclines, and the adsorption capacity declines as the adsorbent
dosage increases. The more significant amount of available adsorption sites increase,
the number of adsorbate molecules interacting, resulting in higher percentage
removal of heavy metal ions.
Zhong et al. [55] demonstrated this comprehensive observation in Cu(II) and Cr
(VI) removal using wheatgrass-derived adsorbent. In the study, the increase of
sorbent dosage up to 2 g/L resulted in achieving 86.44% and 92.19% removal
capacity, respectively. The adsorption capacities, howbeit, declined from 45 mg/g
to 22 mg/g for Cu(II) and from 230 mg/g to 50 mg/g for Cr(VI). The study also found
that increasing the adsorbent dosage further did not increase the percentage removal
9 Agricultural Waste-Derived Adsorbents for Decontamination of Heavy Metals
379