磷是水体富营养化的主要控制因素, 在外源磷输入逐步得到控制后, 作为内源磷主要来源的表层沉积物对黄河水体水质的影响作用不容忽视。 掌握表层沉积物中磷的积累程度以及沉积物-水界面磷的交换能力, 对区域内水环境的治理和磷负荷的调控具有重要的意义。 选择黄河甘宁蒙段作为研究区域, 分别采集丰水期(2011.07)、 枯水期(2014.05)和平水期(2014.10)表层沉积物样品, 使用标准测试程序(SMT)和钼锑抗分光光度法测定样品中磷赋存形态, 并在实验条件下模拟表层沉积物对磷的等温吸附及吸附动力学过程。 研究发现: (1) 相比于国内主要河流表层沉积物中磷的形态特征, 黄河甘宁蒙段表层沉积物中有机磷(OP)和铁/铝结合态磷(NaOH-P)含量相对较低, 钙结合态磷(HCl-P)含量较高; 各形态磷含量的平均值均在丰水期最高, 表明丰水期表层沉积物中磷的积累程度最高, 黄河甘宁蒙段水环境受到沿程农业发展的冲击较大。 因此, 合理使用含磷化肥和优化灌渠退水水质是未来降低黄河水体磷污染风险的发展方向。 (2) 基于低磷浓度下等温吸附中各采样点表层沉积物吸附-解吸平衡浓度(EPC0)与判断水体发生富营养化的磷浓度阈值的比较发现, 研究区域所有水期大多数采样点表层沉积物发挥着“磷源”的作用, 存在向上覆水释放磷的趋势, 且枯水期中多数采样点的EPC0值较高, “磷源”作用更加明显; 基于L模型和F模型对高磷浓度下等温吸附的拟合参数, 显示丰水期表层沉积物对磷的持留能力最强, 枯水期次之, 平水期最小, 所有采样点表层沉积物对磷的吸附过程均易发生; 基于吸附动力学曲线的变化趋势可知, 所选择的各采样点磷吸附量在反应开始的12 h内迅速增大, 12~48 h内吸附量逐渐增加并趋于稳定; 基于伪二级动力学方程对吸附动力学过程的拟合结果, 表明表层沉积物对磷吸附过程的反应速率受化学吸附控制为主; 从同一水期不同采样点吸附过程中的限速步骤不同及不同水期邻近采样点吸附过程中限速步骤均为微孔扩散的结果发现, 表层沉积物组成和理化性质的差异对磷吸附速率的影响大于不同水期条件下上覆水流速和流量的变化。

Phosphorus (P) is the major controlling factor for the eutrophication in the water. After the external P pollution is gradually controlled, the influence of surface sediments as the main source of endogenous phosphorus on the water quality of the Yellow River can not be ignored. It is of great significance to master the accumulated degree of various P fractions in surface sediments and exchange ability of phosphorus at the water-sediment interface for administering the water environment and regulating the P load. In this study, surface sediments of high-water period (2011. 07), low-water period (2014. 05) and normal-water period (2014. 10) from Gansu, Ningxia and Inner Mongolia sections of the Yellow River were collected, respectively. The contents of various phosphorus fractions were determined by using standards measurements and testing (SMT) method and molybdenum antimony spectrophotometry, and the isothermal adsorption and adsorption kinetics processes of P in surface sediments were simulated in the laboratory. This study found: (1) compared with the characteristics of phosphorus fractions of surface sediments in major rivers of China, the content of OP was as low as that of NaOH-P, and the content of HCl-P was higher in surface sediments. The average phosphorus content of all fractions was the highest at high-water period, indicating that the accumulated degree of phosphorus in surface sediments was the highest at high-water period and the water environment of the study area was greatly impacted by the development of agriculture along the route. So, reasonable use of phosphorus fertilizers and optimizing irrigation return water quality will be the developing direction to reduce the risk of phosphorus pollution in the Yellow River in the future. (2) comparing the equilibrium phosphorus concentration (EPC0) of surface sediments at all sampling sites from isothermal adsorption for low phosphorus concentrations with the criterion about the phosphorus concentration threshold of the eutrophication in the water, we found that surface sediments at most sampling sites played a role as “phosphorus source”, and there was a trend of phosphorus release from sediments to overlying water, especially for most sampling sites with high values of EPC0 in low-water period, the release trend was more obvious. Based on the fitting parameters of L model and F model from isothermal adsorption at high phosphorus concentrations, the retention capacity of surface sediments to phosphorus in high-water period was the strongest, followed by low-water period and the minimum in normal-water period, the adsorption processes of surface sediments on phosphorus in all sampling sites were easy to occur. Based on the changed trend of kinetics curves of phosphorus adsorption, we found phosphorus adsorbent contents of all selective sampling sites increased rapidly in the first 12 h, increased gradually and tended to be stable during 12 h to 48 h. According to fitting results of the pseudo-second-order kinetics for adsorption kinetics processes, the reaction rate of phosphorus adsorption on surface sediments was mainly controlled by chemisorption. According to results that different sampling sites at the same water period had different rate-limiting steps and the pore diffusion was the rate-limiting step in nearby sampling sites at different water periods, we inferred differences of composition and physicochemical properties of surface sediments on phosphorus adsorption rate had a greater influence than variances of flow rates and flow fluxes of overlying water among different water periods.