## 4 Discussion  
The sorption characteristics of CMs were closely related to their physicochemical properties. According to the \(n\) values, the sorption isotherms of BC850 and AC almost had good nonlinearity, because they had more micro-pores and higher BC content, but the sorption isotherms of \(p\),\(p^{\prime}\)-DDE on AC appeared to be slightly linear, suggesting that the linear partition was involved in the sorption of \(p\),\(p^{\prime}\)-DDE on AC [26]. Comparing the \(n\) values of SC with BC450, the nonlinearity of lindane and \(p\),\(p^{\prime}\)-DDE sorption isotherms on SC was much stronger than that of BC450. Pore-filling was found to account for a significant proportion of the nonlinearity sorption process of HCHs and DDTs by CMs [27; 28; 29; 30; 31]. As such, the explanation for why the sorption of SC had a better nonlinearity could be related to its porosity, because BC450 had a very low level of porosity due to its incomplete pyrolysis, while SC contained more microporous structures [24; 32]. The porosity could be an important factor affecting the nonlinearity of CMs. The \(K_{\mathrm{d}}\) values showed that the sorption capacity of lindane and \(p\),\(p^{\prime}\)-DDE by AC and BC850 was 1.8-38.0 times larger than that of SC. Comparing the sorption capacity of BC450 and SC, they were similar, though the SC sorption was slightly superior to that of BC450. The sorption capacities of CMs were closely related to their content of BC [33; 34]. Previous studies showed that BC had a strong sorption capacity and could play a leading role in the entire sorption of natural solids despite its content being very low; when the proportion of BC in TOC was only 23%, the contribution rate to the entire sorption could reach 52% [35; 36; 37]. Furthermore, the correlation analysis between \(f_{\mathrm{CM}}\) and \(K_{\mathrm{d}}\) in this study showed that the two had good correlation (\(r>0.85\)). Thus, it was speculated that the difference of \(f_{\mathrm{CM}}\) could be the reason why the sorption capacity of SC was stronger than that of BC450. This shows that \(f_{\mathrm{CM}}\) is an important factor affecting the sorption capacity of CMs. The sorption capacity of BC850 and AC was at least 11-21 times higher than that of natural sediments, while SC and BC450 were only 4-11 times higher than that of natural sediments. Adding a small amount of BC850 or AC to natural sediments could significantly increase the proportion of BC in TOC; by contrast, adding SC and BC450 could gradually increase said proportion of BC in TOC, meaning that they were more suitable to investigate the effects of CMs content on the sorption of lindane and \(p\),\(p^{\prime}\)-DDE by the sediments. Comparing SC with BC450, SC had higher \(f_{\mathrm{CM}}\) and more pores, therefore, it was chosen to investigate the effects of CMs on the sorption of lindane and \(p\),\(p^{\prime}\)-DDE by the sediments. As the contents of SC increased in the sediments, the \(n\) values of lindane sorption on the latter decreased from 0.889 to 0.492, and the \(n\) values of \(p\),\(p^{\prime}\)-DDE decreased from 0.907 to 0.476, suggesting that the nonlinearity of the sorption became stronger. The same situation was observed in previous studies [35; 36], indicating that the content of SC was an important factor affecting the nonlinearity of the sorption of lindane and \(p\),\(p^{\prime}\)-DDE by the sediments. By comparing the \(K_{\mathrm{d}}\) values of the sediments with different contents of SC, the sorption capacity of lindane sorption on the sediments with 0.5-9.0% SC was 1.8-7.5 times larger than that of natural sediments, and the sorption capacity of \(p\),\(p^{\prime}\)-DDE sorption on the sediments with 0.5-9% SC was 1.9-9.4 times larger than that of natural sediments, respectively.