The kinetic curves of sediment adsorption on TBBPA are displayed in Fig. 2. The adsorption of sediment on TBBPA in the first 2 h was a fast adsorption process, and the amount of TBBPA adsorbed reached 61.4% of the maximum adsorption amount at 2 h. After the fast adsorption step, desorption started to play a crucial role. Thus, the amount of adsorbed TBBPA changed slowly, and the adsorption reached apparent equilibrium at approximately 48 h. This result revealed that fast adsorption plays the key role in the adsorption process. The fast adsorption of TBBPA may be attributed to the adsorption of solutes on the surface of the sediment mineral medium (Huang et al. 1996) or to the distribution to the sediment organic matter (SOM) solute region (Weber et al. 1996), whereas the slow adsorption is attributed to the gradual diffusion of TBBPA into the SOM matrix and sediment micropores (Pignatello et al. 1996; Sun et al. 2008). Based on the results, the adsorption time was selected as 48 h adsorption and desorption in all samples for the equilibrium time experiment.
Isothermal adsorption results
The Langmuir and Freundlich adsorption models were used to analyse the isotherm adsorption characteristics of TBBPA in Weihe River sediments. The fitting diagram is displayed in Fig. 3, and the fitting parameters are listed in Table 2.
Langmuir model can be expressed as follows:
\({Q}_{e}={Q}_{m}{K}_{L}{C}_{e}/(1+{K}_{L}{C}_{e})\) (2)
where Qe is the equilibrium adsorption capacity of the flame retardant in the sediment (mg·kg− 1); Ce is the equilibrium concentration of the flame retardant (mg·L− 1); KL is the Langmuir coefficient; and Qm is the maximum adsorption capacity (mg·kg− 1). The adsorption nature of the Langmuir model can be explained by the dimensionless constant RL. When RL = 0, the adsorption is irreversible; when 0 < RL < 1, the adsorption can proceed; when RL = 1, the adsorption conforms to linear adsorption; when RL > 1, adsorption cannot be performed [18].
Freundlich model is expressed as follows:
\({Q}_{e}={K}_{f}{C}_{e}^{n}\) (3)
where Qe is the equilibrium adsorption capacity of the flame retardant in the sediment (mg·kg− 1); Ce is the equilibrium concentration of the flame retardant (mg·L− 1); Kf is the adsorption equilibrium constant; and n is the nonlinear index. The size of n can represent the adsorption strength of the adsorbent, that is, the larger the value of n is, the more difficult it is to adsorb, and the smaller the value of n is, the easier it is to adsorb.
Table 2 lists the simulation results and parameters of the adsorption isotherm curve. The correlation coefficients R2 of the two adsorption isotherm models are both greater than 0.9, which indicates that both the Langmuir model and Freundlich models can perform an excellent mathematical fit, but the R2 of the Freundlich model is greater. The residuals are smaller, so the Freundlich model fits better in comparison. The RL value of TBBPA in the Langmuir model is 0.5310, which indicates that TBBPA can be adsorbed by sediments. In the Freundlich model, n is 0.7249. Weihe sediments exhibit a strong adsorption capacity for TBBPA, but the KF value is only 27.909 (mg·kg− 1)/(µg·kg− 1), which is compared with similar sediments for HBCD. The adsorption capacity is small (Wang et al. 2020). Figure 3 reveals that as the initial concentration of TBBPA increases from 0.01 to 0.4 mg·L− 1, the equilibrium adsorption concentration increases from 0.252 to 14.7 mg·L− 1.
Table 2
Isotherm parameters for TBBPA sorption on sediment


Langmuir model fitting parameters

Freundlich model fitting parameters


Qm(mg·kg1)

KL

R2

RSS/dof

RL

KF

n

R2

RSS/dof

TBBPA

34.1309

1.7665

0.9859

2.3414

0.5310

27.9090

0.7249

0.9921

0.1878
