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Modified Langmuir and Freundlich models were utilized in this study to describe the sediment isotherms. Figure 5 presents the fitting of sorption equations with the modified models. The figure indicates that the equilibrium sorption capacity of the sediment in 2013 was larger than that in 2011. D3 exhibited the largest equilibrium sorption capacity in the two years. Table 2 presents the fitting results. The correlation coefficient, R2 (0.85 < R2 < 0.99), verifies that the modified Freundlich and Langmuir models can effectively describe the sorption isotherms. However, the overall data were better fitted by the modified Langmuir model than by the modified Freundlich model. Table 2 also provides a good estimate of theoretical sorption maxima Qm and sorption affinity parameter K. The Langmuir sorption maxima and the sorption affinity parameter ranged from 0.578 mg g−1 to 0.865 mg g−1 and from 0.057 L mg−1 to 0.365 L mg−1, respectively. D3 in 2011 exhibited the maximum sorption and minimum K. The calculated results indicated that D4 had the highest EPC0 and NAP, whereas D1 had the lowest EPC0 and NAP (Table 2).
Table 2

Modified Langmuir and Freundlich model parameters for phosphorus sorption

    Modified Langmuir:
Modified Freundlich:
Fitting results
Calculated results
Fitting results
Calculated results
SampleYearCe0 (mg L−1)Qe0 (mg g−1)K (L mg−1)Qm (mg g−1)R2EPC0 (mg L−1)NAP (mg g−1)Kp (L g−1)nKf (L g−1)R2EPC0 (mg L−1)NAP (mg g−1)Kp (L g−1)
D1 2011 0.017 0.001 0.072 0.753 0.9966 0.032 0.002 0.054 0.471 0.102 0.9863 0.019 0.016 0.837 
2013 0.017 0.001 0.111 0.844 0.9961 0.025 0.002 0.094 0.391 0.180 0.9867 0.018 0.037 2.106 
D2 2011 0.140 0.007 0.125 0.664 0.9808 0.228 0.018 0.081 0.301 0.225 0.9841 0.168 0.131 0.782 
2013 0.275 0.014 0.157 0.740 0.9851 0.406 0.044 0.109 0.185 0.527 0.9770 0.328 0.428 1.308 
D3 2011 0.119 0.006 0.057 0.865 0.9831 0.243 0.012 0.049 0.478 0.110 0.9915 0.160 0.046 0.287 
2013 0.171 0.009 0.365 0.836 0.9630 0.203 0.058 0.284 0.074 1.810 0.9506 0.184 1.598 8.686 
D4 2011 0.089 0.004 0.148 0.578 0.9749 0.137 0.011 0.084 0.386 0.124 0.9629 0.109 0.053 0.485 
2013 0.459 0.023 0.234 0.813 0.9112 0.612 0.102 0.167 0.094 1.417 0.8511 0.552 1.340 2.428 
D5 2011 0.068 0.003 0.092 0.644 0.9801 0.127 0.007 0.058 0.370 0.142 0.9894 0.081 0.056 0.692 
2013 0.101 0.005 0.264 0.763 0.9882 0.127 0.025 0.195 0.351 0.213 0.9855 0.117 0.100 0.859 
    Modified Langmuir:
Modified Freundlich:
Fitting results
Calculated results
Fitting results
Calculated results
SampleYearCe0 (mg L−1)Qe0 (mg g−1)K (L mg−1)Qm (mg g−1)R2EPC0 (mg L−1)NAP (mg g−1)Kp (L g−1)nKf (L g−1)R2EPC0 (mg L−1)NAP (mg g−1)Kp (L g−1)
D1 2011 0.017 0.001 0.072 0.753 0.9966 0.032 0.002 0.054 0.471 0.102 0.9863 0.019 0.016 0.837 
2013 0.017 0.001 0.111 0.844 0.9961 0.025 0.002 0.094 0.391 0.180 0.9867 0.018 0.037 2.106 
D2 2011 0.140 0.007 0.125 0.664 0.9808 0.228 0.018 0.081 0.301 0.225 0.9841 0.168 0.131 0.782 
2013 0.275 0.014 0.157 0.740 0.9851 0.406 0.044 0.109 0.185 0.527 0.9770 0.328 0.428 1.308 
D3 2011 0.119 0.006 0.057 0.865 0.9831 0.243 0.012 0.049 0.478 0.110 0.9915 0.160 0.046 0.287 
2013 0.171 0.009 0.365 0.836 0.9630 0.203 0.058 0.284 0.074 1.810 0.9506 0.184 1.598 8.686 
D4 2011 0.089 0.004 0.148 0.578 0.9749 0.137 0.011 0.084 0.386 0.124 0.9629 0.109 0.053 0.485 
2013 0.459 0.023 0.234 0.813 0.9112 0.612 0.102 0.167 0.094 1.417 0.8511 0.552 1.340 2.428 
D5 2011 0.068 0.003 0.092 0.644 0.9801 0.127 0.007 0.058 0.370 0.142 0.9894 0.081 0.056 0.692 
2013 0.101 0.005 0.264 0.763 0.9882 0.127 0.025 0.195 0.351 0.213 0.9855 0.117 0.100 0.859 
Figure 5

Sorption isotherms of phosphorus on five sediment samples obtained from Dongting Lake, China, in 2011 (a) and 2013 (b), fitted with modified Langmuir and Freundlich models.

Figure 5

Sorption isotherms of phosphorus on five sediment samples obtained from Dongting Lake, China, in 2011 (a) and 2013 (b), fitted with modified Langmuir and Freundlich models.

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