土壤溶液中砷物種分佈及轉變與其對水稻之毒害

Abstract

土壤中砷形態的轉變受氧化還原電位、pH值和微生物的影響。而As(Ⅲ)的毒性較As(Ⅴ)大，在溶液中易移動。因此本研究目的在於探討砷在土壤浸水孵育後砷物種型態及濃度之變化及其對水稻毒害之評估。 本研究以三種土壤性質差異大的土壤－平鎮、太康和將軍系土壤，添加As(Ⅴ)製備為0至480 mg As(Ⅴ) kg-1等六種不同濃度處理之砷污染土；另取三個不同砷濃度之關渡平原土壤，為供試土壤。以水土比1：1的比例作0至42天的浸水孵育試驗。土壤溶液抽出後以0.01 M磷酸溶液作保存，以HPLC/ICP/MS測定As(Ⅲ)、DMA、MMA 和As(Ⅴ)含量。結果發現，浸水後，土壤溶液中As(Ⅲ)濃度隨孵育時間增加而增加，而總砷濃度以關渡土壤增加最多，且土壤溶液中砷與鐵濃度改變趨勢相似，可能因土壤浸水期間，土壤溶液中鐵還原導致砷溶出增加。且浸水後鐵、砷和硫因浸水逐漸還原，砷與鐵、硫形成難溶之化合物，會造成土壤溶液中砷和鐵含量減少。但在人工製備之砷汙染土，砷在土壤溶液相及固相間未達平衡，導致土壤浸水孵育後，因砷被土壤固相吸附使溶液相中砷濃度減少。另外發現在供試土壤之酸性土壤溶液中以As(Ⅲ)佔總砷比例較高，而中性偏鹼性之土壤溶液則以As(Ⅴ)佔總砷比例較高。浸水孵育後，As(Ⅲ)佔總砷的比例會增加。各供試土壤種植之水稻，在人工添加480 mg As(V) kg-1處理下皆出現毒害情形。由於浸水孵育前之土壤溶液中As(Ⅲ)濃度與水稻植體株高有顯著負相關，推估使水稻株高降低20 %之土壤溶液中As(Ⅲ)濃度為0.02 mg L-1；且以NaH2PO4抽出之總砷濃度與植體株高百分比之劑量反應關係有較顯著的負相關，推估其EC20為78.7mg kg-1。因此，此兩種抽出方法可有效評估土壤中砷對水稻毒害之影響。

The distribution of As species in soils is influenced by redox potential, pH, and microbial activity. Arsenite is more toxic and mobile than arsenate. Therefore, to understand the As species distribution is very important to evaluate the mobility and phytotoxicity in As-contaminated soils. In this study, the As speciation in soil solutions of flooded As-contaminated soils were determined and the relationship between the As speciation and toxicity to rice plants was investigated. Three Guandu soils containing different levels of As and three soils spiked with various levels of As(V) were used in this study to determine the As speciation of soil solutions under flooded incubation condition for 42 days. The soil solutions were taken and preserved in 0.01 M H3PO4 immediately and then the concentrations of As(Ⅲ), DMA, MMA and As(V) were determined using HPLC/ICP/MS. The results showed that in all studied soils As(Ⅲ) concentrations increased with flooding time and the extent of increase was higher in Guandu soils than that in other studied soils. In addition, the change of As concentration had the same pattern with that of Fe. It may result from that As release in soil solution due to Fe reduction or As precipitation with Fe. Moreover, total As concentration in soil solutions also increased with flooding time and the extent of increase was higher in Guandu soils than three As(V)-spiked soils. It may be due to that spiked-As(V) was adsorbed by soil solids during the flooding incubation period. Among the studied soils, As(Ⅲ) percentages of total As in solutions of acid soils were higher that As(V). On the contrary, As(V) percentages of total As in solutions in alkaline soils were higher that As(Ⅲ). For the relationship between As species concentration in soil solutions with rice seedling growth, As(Ⅲ) concentration before flooding was significantly and better correlated with plant height of rice seedlings than other As species. The effective toxicity concentrations of decreasing 20% of rice plant height were 0.02 mg As(Ⅲ) L-1. The results also showed that there was a significant relationship between the plant height of rice seedlings and soil arsenic extracted by NaH2PO4, and the effective toxicity concentrations of decreasing 20% of rice plant height were 78.7 mg As(Ⅲ) kg-1. Therefore, soil solutions As(Ⅲ) and NaH2PO4 extracted As could be used for the assessment of availability and phytotoxicity of arsenic in soils.