Cortina Jose-Luis, Lagreca Isabella, De Pablo Joan, Cama Jordi, Ayora Carlos
Department of Chemical Engineering, Universitat Politècnica de Catalunya, Diagonal 647, 08028 Barcelona, Spain.
Environ Sci Technol. 2003 May 1;37(9):1971-7. doi: 10.1021/es026018m.
Passive remediation consists of a permeable system that enables the water to pass through while retaining metals by means of biogeochemical reactions. Conventional passive treatments are based on calcite dissolution. This increases the pH to values between 6 and 7, which are insufficiently high to precipitate divalent metals. Alternative treatments are based on sulfate reduction with organic matter in order to precipitate metal sulfides. However, redox reactions are usually too slow to treat large groundwater flows as currently found in gravel aquifers (>50 m/a). Caustic magnesia obtained from calcination of magnesium carbonate was tested as an alternative material to devising passive remediation systems. Caustic magnesia reacts with water to form magnesium hydroxide, which dissolves, increasing the pH to values higher than 8.5. Then zinc and lead are mainly precipitated as hydroxides, copper is precipitated as hydroxysulfate, and manganese(II) is oxidized and precipitated as manganese(III) oxides. Thus, metal concentrations as high as 75 mg/L in the inflowing water are depleted to values below 0.04 mg/L. Magnesia dissolution is sufficiently fast to treat flows as high as 100 m/a. The new precipitates may lead to a permeability drop in the porous treating system. Mixtures of caustic magnesia and an inert material such as silica sand (approximately 50% of each) have been shown to be as reactive as pure magnesia and permeable for a longer time (more than 10 months and 1000 pore vol).
被动修复包括一个渗透系统，该系统能使水通过，同时通过生物地球化学反应截留金属。传统的被动处理方法基于方解石溶解。这会使pH值升高到6至7之间，这个值不够高，无法使二价金属沉淀。替代处理方法基于利用有机物进行硫酸盐还原，以沉淀金属硫化物。然而，氧化还原反应通常太慢，无法处理砾石含水层中目前发现的大流量地下水（>50米/年）。对由碳酸镁煅烧得到的苛性氧化镁作为设计被动修复系统的替代材料进行了测试。苛性氧化镁与水反应生成氢氧化镁，氢氧化镁溶解，使pH值升高到高于8.5的值。然后锌和铅主要以氢氧化物形式沉淀，铜以羟基硫酸盐形式沉淀，锰（II）被氧化并以锰（III）氧化物形式沉淀。因此，流入水中高达75毫克/升的金属浓度可降低到0.04毫克/升以下。氧化镁的溶解速度足够快，能够处理高达100米/年的水流。新的沉淀物可能会导致多孔处理系统的渗透率下降。苛性氧化镁与惰性材料（如硅砂）的混合物（每种约占50%）已被证明与纯氧化镁具有相同的反应性，并且在更长时间内（超过10个月和1000孔隙体积）具有渗透性。
