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Based on the results of the fitting procedure (Figure 5(c) and Table 3), GAC has a qmax value of 320 mg g–1. It is worth noting that the value of qmax calculated in this work is much higher than the value of 71 mg g–1 (Tao & Fletcher 2013) and 15 mg g–1 (Busquets et al. 2014) obtained using other GACs. This is likely to be primarily associated with the higher specific surface area (and hence with the higher availability of adsorption sites) of the GAC used for our experiments (774 m2 g–1) compared to that used in the earlier studies (560 and 500 m2 g–1, respectively). However, the observed increase in the adsorption capacity is not strictly proportional to the increase in the specific surface area, suggesting that other factors, such as the PZC, may also play a relevant role in the adsorption process. The PZC indicates the pH at which a surface has a net neutral charge. Although metaldehyde is a neutral compound, its oxygen atoms bear a negative partial charge that may strongly influence the adsorption behaviour of the pesticide depending on the surface charge of the adsorbent. Surfaces with low PZC normally exhibit a negative charge at environmental pH in contrast to those having high PZC, which are positively charged over a wider pH range. Based on the above considerations, metaldehyde is expected to be preferentially adsorbed on GAC with higher PZC. In line with this, it was found that the GAC used in the present work has a PZC value of 8.7 (see Figure 6), a value sufficiently high to assure a high metaldehyde adsorption efficiency (Busquets et al. 2014).
Table 3

Thermodynamic parameters as determined by the fitting procedure

ModelKL (L mg–1)KF (mg1–N g–1 LN)KLF (LN mg–N)Nqmax (mg g–1)R2
Langmuir isotherm 270 ± 30    220 ± 10 0.962 
Freundlich isotherm  1,800 ± 30  0.51 ± 0.10  0.989 
Langmuir–Freundlich isotherm   (1.7 ± 0.4) × 10−3 0.69 ± 0.06 (320 ± 70) 0.980 
ModelKL (L mg–1)KF (mg1–N g–1 LN)KLF (LN mg–N)Nqmax (mg g–1)R2
Langmuir isotherm 270 ± 30    220 ± 10 0.962 
Freundlich isotherm  1,800 ± 30  0.51 ± 0.10  0.989 
Langmuir–Freundlich isotherm   (1.7 ± 0.4) × 10−3 0.69 ± 0.06 (320 ± 70) 0.980 
Figure 6

Determination of PZC for GAC.

Figure 6

Determination of PZC for GAC.

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