底泥硫化金屬的溶解:溶氧、pH、天然有機物吸附之影響

Abstract

底泥中的硫化金屬在厭氧環境下，可使重金屬穩定化並且降低其生物可及性。然而，因暴雨造成之河川紊流或是生物擾動可能使硫化金屬暴露在耗氧環境中，進而導致其氧化溶解並釋放有毒重金屬至生態環境中。本研究之目的為探討溶氧pH值及天然有機物吸附對硫化鉛、硫化銅及硫化鋅溶解之影響。 本研究使用黃酸(fulvic acid)做為天然有機物，並利用連續曝氣裝置在中性及厭氧環境下進行10小時之等溫吸附實驗，研究結果顯示硫化金屬表面之天然有機物吸附量為：硫化鉛 > 硫化銅 > 硫化鋅。溶解實驗首先針對未吸附天然有機物之硫化金屬在不同溶氧(DO = 0 mg/L, 5 mg/L, 8.4 mg/L)及pH值(pH 5, pH 7, pH 10)下，進行3天之批次溶解實驗。硫化金屬在高溶氧(DO = 8.4 mg/L)及低pH值(pH 5)環境下釋出最多金屬離子，一般而言溶解量之序列為：硫化鉛 > 硫化銅 > 硫化鋅。在厭氧情況下(DO = 0 mg/L)，硫化金屬的溶解主要由氫離子導致 (proton-induced dissolution)，而在氧氣存在條件下，氧化溶解(oxidative dissolution)及氫離子導致之溶解則須同時考慮。其中，氧化溶解對硫化銅及硫化鋅的金屬離子的釋出影響較為顯著; 在pH 10，三種硫化金屬都屬穩定，從pH 7 降到 pH 5，硫化鉛及硫化銅金屬量都顯著增加，而硫化鋅則無明顯變化。 針對天然有機物吸附對不同硫化金屬溶解的影響，實驗以不同黃酸吸附量之硫化金屬在飽和溶氧(DO= 8.4 mg/L)及中性(pH 7)環境下進行。黃酸吸附對硫化金屬溶解的影響有二：因黃酸吸附導致之配位基引起之礦物溶解(ligand-induced dissolution)，及因黃酸吸附於硫化金屬表面形成一保護層，避免溶氧及氫離子之攻擊，而降低溶解量。結果顯示，黃酸能有效抑制硫化鉛及硫化鋅的溶解，且抑制效果隨其表面黃酸吸附量增加而增強，顯示黃酸吸附並不會引起顯著之配位基礦物溶解，相反的，其吸附可有效抑制溶氧及氫離子對硫化鉛及硫化鋅的攻擊。但對硫化銅而言，天然有機物吸附對其溶解隨吸附量增加呈現先促進後減緩之趨勢，然而氧化溶解仍為其主要溶解機制。

Metal sulfides in sediment can immobilize and decrease the bioavailability of heavy metals under anaerobic conditions. However, storm events and bioturbation may suspend metal sulfides and expose them to aerobic conditions, causing oxidative dissolution and release of harmful heavy metals to the ecosystem. The objective of this research is to investigate the influences of dissolve oxygen (DO), pH value and natural organic matter (NOM) adsorption on the dissolution of PbS, CuS, and ZnS. In this research, fulvic acid was used as a NOM model compound, and NOM adsorption isotherm experiments were conducted using a continuous aeration setup at neutral and anaerobic conditions. Results showed that NOM adsorption capacity of the three metal sulfides was in the order of PbS > CuS > ZnS. Dissolution experiments were firstly conducted using metal sulfides without NOM adsorption under different DO concentrations (0 mg/L, 5mg/L, 8.4 mg/L) and pH values (pH 5, pH 7, pH 10) for 3 days. The obtained results demonstrated that metal sulfides generally release more metal ions at high DO and low pH conditions. The metal release showed the following sequence: PbS > CuS > ZnS. Under anaerobic conditions (DO = 0 mg/L), the dissolution of metal sulfide is mainly resulted from proton-induced dissolution. Under aerobic conditions, on the other hand, both oxidative dissolution and proton-induced dissolution should be simultaneously considered. It was found that oxidative dissolution contributed significantly to metal release of CuS and ZnS. At pH 10, the three metal sulfides were relatively stable. A decrease of pH 7 to 5 induced significant metal release from PbS and CuS but no obvious difference for ZnS. To understand the impact of NOM adsorption on the dissolution of metal sulfides, experiments were conducted using metal sulfides adsorbed with different levels of fulvic acid at saturated DO (DO = 8.4 mg/L) and neutral conditions (pH 7). The influences of NOM adsorption on metal sulfide dissolution could be attributed to two distinct impacts: 1. Enhanced metal release resulting from fulvic acid adsorption via ligand-induced dissolution and 2. Formation of a barrier to protect the surfaces of metal sulfides from DO and proton attacks, in turn, reducing dissolution. Fulvic acid was found to effectively inhibit PbS and ZnS dissolution and the inhibition could be enhanced by the increasing level of NOM adsorption. This result indicated that fulvic acid adsorption could not cause significant ligand-induced dissolution for PbS and ZnS but effectively protect their surfaces from DO and proton attacks. As for CuS, its dissolution was mainly caused by the oxidative dissolution and the rate of dissolution was found to increase then decrease with the increasing level of NOM adsorption.