The levels of metals in sediments of urban river ecosystems are crucial for aquatic environmental health and pollution assessment. Yet little is known about the interaction of nutrients with metals for environmental risks under contamination accumulation. Here, we combined hierarchical cluster, correlation, and principal component analysis with structural equation model (SEM) to investigate the pollution level, source, toxicity risk, and interaction associated with metals and nutrients in the sediments of a river network in a city area of East China. The results showed that the pollution associated with metals in sediments was rated as moderate degree of contamination load and medium-high toxicity risk in the middle and downstream of urban rivers based on contamination factor, pollution load index, and environmental toxicity quotient. The concentration of mercury (Hg) and zinc (Zn) showed a significant correlation with toxic risks, which had more contribution to toxicity than other metals in the study area. Organic nitrogen and organic pollution index showed heavily polluted sediments in south of the study area. Though correlation analysis indicated that nutrients and metals had different input zones from anthropogenic sources in the urban river network, SEM suggested that nutrient accumulation indirectly intensified toxicity risk of metals by 13.6% in sediments. Therefore, we suggested the combined consideration of metal toxicity risk with nutrient accumulation, which may provide a comprehensive understanding to identify sediment pollution.
Toxicity rate of metals in sediments from urban river network indirectly intensified by nutrients accumulation
Ozonation of the azo dye Cationic Red X-GRL was investigated in a bubble column reactor at varying operating parameters such as oxygen flow rate, temperature, initial Cationic Red X-GRL concentration, and pH. The conversion of dye increased with the increasing of pH and oxygen flow rate. As the reaction rate constant and the volumetric mass transfer coefficient increase while the ozone equilibrium concentration decreases with the temperature, there is a minimum conversion of dye at 25 degrees C. The increasing of initial dye concentration leads to a decreasing conversion of dye while the ozonation rate increases. The formation of intermediates and the variation of pH, TOC, and nitrate ion during ozonation were investigated by the use of some analytical instruments such as GC/MS, GC, and IC. The intermediates of weak organic acids lower the pH value of the solution. The probable degradation mechanism of the Cationic Red X-GRL in aqueous solution was deliberated with the aid of Molecular Orbital calculations. The N(12)-C(13) site in Cationic Red X-GRL, instead of the N(6)-N(7) site, is found to be the principal site for ozone cycloaddition in the degradation processes. During the degradation process, among the six nitrogen atoms of Cationic Red X-GRL, one is transferred into a nitrate ion, one is converted into an amine compound, and the remaining four are transformed into two molecules of nitrogen. 相似文献