Environmental Behavior And Coupling Mechanism Of Sulfur And Typical Heavy Metals During Sediment Aging In Mariculture Areas

The rapid development of mariculture has greatly met the demand for marine aquatic products.However,with the increasing density and scale of aquaculture,the overall efficiency of aquaculture areas is declining,and the aging of sediment is an important reason.Sediment deterioration is a phenomenon in which the pollution load in the sediment exceeds the self-purification capacity of the sediment due to the pollution of the culture itself,thus deteriorating the environment of the sediment.Aging sediment in aquaculture areas has become one of the important factors limiting the sustainable development of mariculture,and the restoration of aging sediments is urgent.Sulfur and heavy metals are important factors leading to the aging of sediments,among which sulfide accumulation is a typical characteristic of sediment aging,while heavy metals such as Cd,Pb,and Cr have strong biological toxicity and can be coupled with sulfur to produce compound ecological effects,which directly determine the suitability of sediments for aquaculture.However,the environmental behavior of sulfur and heavy metals in the aging process of sediments and their driving mechanisms are not yet well understood.This study focused on the aging process of sediments and carried out a simultaneous study on the environmental behavior of sulfur and heavy metals through a combination of field measurements and indoor simulations.The migration and transformation processes of sulfur and heavy metals under different conditions of dissolved oxygen,organic matter,disturbance intensity and water change frequency were quantitatively characterized to explore the coupling mechanism of sulfur and heavy metals in the sediments to provide a scientific basis for identifying the causes of aging and preventing the aging of sediments in mariculture areas.The main findings are as follows:(1)Environmental behavior and coupling mechanism of sulfur and heavy metals in sediments of mariculture ponds: The study area is located in closed ponds(DY1,DY2)and semiopen culture pond(DH).The distribution characteristics and environmental risks of inorganic sulfide(RIS)and heavy metals in different mariculture ponds were studied,along with the coupling relationships between sulfur and heavy metals.The studies found that acid volatile sulfur(AVS)accounted for a higher proportion(over 50%)in DY1 and DY2,while chromium(II)-reducible sulfur(CRS)dominated in DH(over 70%).The conversion of AVS to CRS in the sediments of closed ponds was hindered,and the risk of aging was much higher than that of semiopen pond.The values of Pb,Cd,Zn,Cu,and Cr in sediments in the study area were superior to the corresponding standard levels of heavy metals in the State Marine Sediment Quality Standard.The dominant fractions of Cd,Zn,Cu,and Cr were the residual fraction(F4),while Pb was dominated by the reducible fraction(F2).The risk levels of heavy metals in the three ponds increased according to the following sequence: DH1<DH2<DH.Analysis of the correlation between sulfur and biologically effective forms of heavy metals in the sediments revealed a significant positive correlation between AVS and biologically effective forms of Cu and a significant negative correlation between CRS and biologically effective forms of Pb.(2)Environmental behavior and coupling mechanism of sulfur and heavy metals in sediments of open sea mariculture areas: The study area is located in Sishili Bay and its adjacent area in the North Yellow Sea.The environmental behavior and coupling relationship between sulfur and heavy metals in sediments of open sea mariculture areas were studied.The studies found that RIS was dominated by CRS(over 45%),and mariculture activities promoted the accumulation of AVS in sediments.The AVS values at 35% of the sampling sites were inferior to the first level of sulfide in the State Marine Sediment Quality Standard.Pb,Cu,Zn,and Cd exhibited a homogeneous distribution in the study area,and the values were superior to the corresponding standard levels of heavy metals in the State Marine Sediment Quality Standard.Principal component analysis(PCA)and correlation analysis showed that Cu and Zn were mainly from mariculture activities.Analysis of the correlation between different forms of sulfur and heavy metals revealed that the accumulation of AVS was conducive to the formation of F2 and the oxidizable fraction(F3)of Pb and F3 of Zn,conversely to the formation of F4 of Pb and the acid-soluble fraction(F1)of Cu.Increased Pb stability and decreased Cu stability led to a reduction in the mariculture sediment aging process.(3)Influence of different factors on the environmental behavior of sulfur and heavy metals in mariculture sediments: The environmental behaviors of sulfur and heavy metals and their coupling mechanisms under different mariculture styles were found to be significantly different through the studies in the former two chapters.Compared to different mariculture environments,factors such as dissolved oxygen,disturbance intensity,water exchange frequency and organic matter content were significantly different.To verify whether the above differences were caused by these four factors,indoor simulation experiments were conducted to study the effects of these four factors on the environmental behavior of sulfur and heavy metals in mariculture sediments.The studies found that organic matter has the most significant impact on the sediment aging process(AVS accumulation process),followed by dissolved oxygen.The disturbance intensity and water change frequency had less impact on the sediment aging process.The higher the organic matter content and the lower the dissolved oxygen content are,the faster the sediment aging.Unlike the effects on the aging process,disturbance intensity and water change frequency had a greater impact on the total amount of heavy metals.Disturbance and high-frequency water exchange increased the total amount of heavy metals in the surface sediments.While organic matter and dissolved oxygen had less impact on heavy metals,organic matter only had a greater impact on Cd.The average content of Cd in sediments increased with increasing bait.Changes in dissolved oxygen concentration did not significantly change the release or adsorption of heavy metals from the sediments.In terms of the forms of heavy metals,disturbance and high-frequency water exchange increased the activity of heavy metals in the surface sediments and promoted the conversion of potential biologically effective forms of heavy metals(F1,F2,F3)to F4.Organic matter mainly affected the F1 of Pb,Cr,and Cu(CV>15%).The dissolved oxygen concentration had a significant impact on the F3 of Cu.(4)Influence of different factors on the coupling mechanism of sulfur and heavy metals in mariculture sediments: Based on the analysis of the environmental behavior of sulfur and heavy metals in the sediments under the factors of dissolved oxygen,disturbance intensity,water exchange frequency and organic matter content,the coupling relationship between sulfur and heavy metals was investigated.The studies found that different factors have significant differences in the migration and transformation of sulfur and heavy metals in mariculture sediments,resulting in significant differences in the coupling relationship between sulfur and heavy metals in the sediments under different factors.The aging process of sediments under different dissolved oxygen levels was closely related to Cu.The aging process of sediments was closely related to Pb under different water change frequencies.Under different disturbance intensities and organic matter contents,the increase in F3(mainly metal sulfides)generally hindered the conversion of AVS to CRS and aggravated the aging of sediments,while the increase in F2 generally promoted the conversion of AVS to CRS,which would reduce the degree of sediment aging.