Contamination Characteristics Of Iodide In Typical Fish Pond Farming Areas In The Pearl River Delta And Study On The Adsorption Pattern Of Effective Iodine In Soil

With the rapid economic development,people gradually began to pay attention to the exploitation of groundwater for production and living needs.However,groundwater is easily affected by industrial pollution,agricultural pollution and residential sewage pollution.At present,groundwater environment in China is often faced with various types of organic pollution,ammonia nitrogen,nitrite,iron,manganese and other heavy metal pollution,as well as iodide pollution,of which iodide content in groundwater is crucial to the safety of people’s drinking water and has received extensive attention from domestic scholars.Iodine is an indispensable element for human beings and plays a vital role in human health development.Insufficient and excessive intake can cause thyroid diseases.In recent years,with the accelerated urbanization process,the discharge of large amounts of iodine-containing wastewater and domestic sewage from industrial and agricultural production has caused great pressure on the shallow groundwater environment.Previous studies on the sources of iodide in groundwater in China have mostly focused on the Yellow and Huaihai plains and inland basins in China,but less attention has been paid to the distribution,sources,water quality status and health risks of iodide in shallow groundwater in coastal areas,especially in the Pearl River Delta under the influence of urbanization and aquaculture activities.In this study,different types of water sources in the typical aquaculture area of the Pearl River Delta were selected as the research objects to study the spatial distribution characteristics and sources of iodide in water sources,and to analyze the current water quality and assess the potential health risks of iodide intake in local residents’drinking water,and then further investigate the adsorption pattern of effective iodine in soil by indoor simulation experiments of povidone-iodine.The results of the study provide scientific basis for improving groundwater management in typical aquaculture areas of the Pearl River Delta and reasonable iodine intake of rural residents in China,showing that:（1）The results of iodide contamination characteristics in the study area showed that:（1）The statistical analysis of each detection index showed that the water chemistry of shallow groundwater in the study area was generally weakly acidic and low in total dissolved solids.The average concentration of cations:Ca²⁺（45.08 mg/L）>Na⁺（11.47 mg/L）>K⁺（6.48 mg/L）>Mg²⁺（2.39 mg/L）,Ca²⁺is the main component of TDS in water.The average concentration of anions:HCO₃^-（121.06 mg/L）>SO4^2-（27.81 mg/L）>Cl^-（20.50 mg/L）>NO₃^--N（7.20 mg/L）.（2）Using Arcgis 10.8 using inverse distance spatial interpolation method,we learned that iodide concentrations in surface water and groundwater in the study area were consistent in spatial distribution,and both showed the spatial distribution characteristics of higher overall dispersion in the west.The iodide concentrations in shallow groundwater ranged from 2.0 to 343.0μg/L.J1,J2,J3,J6,and J7 were iodine-deficient water sources（ρ（I^-）<10μg/L）,accounting for 56%and uniformly distributed in the study area;J8 was a moderate iodide water source,accounting for 11.1%and distributed in the southeastern part of the study area;J4,J5,and J9 were high iodine water sources（ρ（I^-）>100μg/L）,accounting for 33%,mainly concentrated in the western part of the study area,showing an obvious aggregation.（2）Analysis of iodide pollution sources in the study area using Spearman correlation analysis and principal component analysis showed that:（1）mineral weathering and dissolution contribute to the enrichment of iodine in groundwater;the weak alkaline reducing environment is favorable to the fugacity of high iodine groundwater;organic matter deposited in the groundwater environment pools a large amount of iodine may become a source of iodide in groundwater.（2）The contribution of principal component 1（PC1）is 47.3%.Agricultural activities,industrial pollution and sea invasion factors have less influence on the enrichment of iodine in shallow groundwater.The leakage of septic tank effluent as well as domestic sewage in the village and town reductive sewage may be one of the sources of NH₄⁺,and I-in groundwater.The detection of ultra-high concentration of iodide at site J4 is likely to be the result of the combined effect of sewage treatment pond tailwater and waste leachate.In addition,the leaching of iodine-rich subsoil from fish ponds may also be an important source of shallow high iodine groundwater in the study area.（3）Gibbs plot of water chemistry controlling factors shows that J4,J5 and J9 are distributed in the rock weathering area,and the water chemistry driving factors are influenced by geological factors and mainly controlled by rock weathering;evaporation concentration also plays a role in the enrichment of I^-in water.（4）The composition characteristics of water chemistry Piper diagram shows that:the shallow groundwater water chemistry type in the study area is relatively simple,mainly HCO₃-Ca,locally HCO₃-Ca·Na type water,J4,J5 and J9 are all low total dissolved solids HCO₃-Ca type water.（3）The results of water quality analysis and evaluation and health risk assessment show that:（1）the percentage of Class IV water in the surface water of the study area is 16.7%,and the water quality is in the poor category;COD and TN pollution in the surface water of the study area is the most serious.High breeding density,high feeding volume,accumulation of metabolites and residual bait are the main reasons for the exceedance of COD and TN.The excessive use of povidone,the depolymerization product of povidone iodine,which is strongly reductive,may also be one of the important reasons for the high COD index of fish ponds.（2）The percentage of Class III water in the groundwater of the study area is 33.3%,and the water quality is in the better category;the p H and iodide pollution in the groundwater of the study area is more serious,among which the p H of J2 and J6 is 4.85,which may be influenced by acidic rainfall making the p H of the recharge area as a whole more acidic.（3）Iodide health risk assessment results show that:the highest risk entropy（HQ）population in the study area occurs among children,and the risk entropy（HQ）of children ranges from 0.01 to 1.60,with the HQ of point J4 exceeding the allowed limit（HQ=1）,and there is a non-carcinogenic risk for children in the drinking water source at this point,and there is no non-carcinogenic risk for adult women and adult men.（4）The experimental results of the effect of stability of povidone-iodine（PVP-I）solution showed that the stability of PVP-I solution was affected by various factors such as concentration,temperature,light and p H value.Among them,the stability of PVP-I solution is influenced by its own concentration,the greater the concentration the better the stability of the solution,the lower the concentration the worse the stability of the solution;temperature is the main factor affecting the stability of PVP-I solution;light intensity has less influence on the stability of PVP-I compared with other conditions;and the stability of PVP-I solution is closely related to p H value,the stability of the solution is good under acidic conditions,p H The stability of PVP-I solution is closely related to the p H value.The solution stability under acidic condition is good,and the p H value changes in a wave pattern with the storage time,and the rise and fall of p H value is negatively correlated with the rate of decrease of effective iodine.（5）The experimental results of adsorption of effective iodine in PVP-I showed that:（1）The kinetic characteristics of p H on the adsorption of effective iodine in PVP-I showed that the adsorption value of effective iodine in soil under alkaline conditions was higher than that under neutral and acidic conditions,which was mainly due to the influence of p H on the stability of PVP-I,which was more stable under acidic conditions and the free iodine released was Under acidic conditions,PVP-I is more stable and the free iodine released is limited:I₂+3H₂O?I^-+IO₃^-+6H⁺,and the IO₃^-generated is also limited,so the soil soon reaches adsorption equilibrium,while under alkaline neutral conditions PVP-I is less stable and the free iodine continuously released can be used as a source of soil adsorption of effective iodine:3I₂+6OH^-?5I^-+IO₃^-+3H₂O,and the molarity of IO₃^-increases,leading to a continuous increase in soil adsorption of effective iodine.The content of effective iodine continues to increase,so it has been for reaching the adsorption equilibrium state,and the adsorption value is greater than the adsorption value under neutral and acidic conditions.（2）The kinetic characteristics of temperature on the adsorption of effective iodine in PVP-I showed that the soil Q_e value at high temperature 37°C>the soil Q_evalue at room temperature 25°C.（3）The kinetic results of solid-liquid ratio on the adsorption of effective iodine in PVP-I showed that the content of effective iodine per unit time was limited and the soil Q_e value decreased with the increase of the solid-liquid ratio.25.4 mg)>1:10（13.30 mg）>1:50（5.71 mg）,the reason is that keeping the adsorbent mass constant and increasing the soil adsorbent content makes the total amount of adsorption sites increase,therefore,the larger the solid-liquid ratio value the higher the total amount of soil effective iodine adsorption.（4）The kinetic characteristics of NH₄⁺adsorption on effective iodine in PVP-I showed that the addition of NH₄⁺destabilized PVP-I and promoted the adsorption of effective iodine by soil.（5）The thermodynamic isotherm fitting of soil adsorption of effective iodine showed that both Langmuir and Freundlich equations could better describe the thermodynamic process of soil adsorption of effective iodine in the experimental concentration range.As the equilibrium concentration of effective iodine in solution increased,the adsorption of effective iodine by soil also increased.The results of the survey in the study area showed that the p H value of fish ponds ranged from 6.52 to 9.44,with an average value of 7.99 and an overall weak alkalinity,the water temperature of fish ponds in the culture area ranged from 33.2 to 38.6°C,there were differences in the size of the culture water area and the soil and water environment varied,the ammonia nitrogen content of fish ponds in the culture area ranged from 0.178 to 2.292 mg/L,and there was a problem of excessive ammonia nitrogen pollution,and by visiting It is known that fishermen often use PVP-I in fish pond disinfection activities.Therefore,it can be assumed that the adsorption of iodine by fish pond soil is most likely to increase under the condition of alkaline,high temperature and ammonia nitrogen pollution in fish pond water bodies,and the iodine adsorbed in the soil accumulates over time and is likely to produce the risk of groundwater iodide pollution after natural leaching.