酸性硫酸盐土的形成、 特性及其生态环境效应
2014-04-09黄巧义唐拴虎张发宝杨少海
黄巧义, 唐拴虎， 卢 瑛， 张发宝， 杨少海
(1 广东省农业科学院农业资源与环境研究所，农业部南方植物营养与肥料重点实验室，广东省养分资源循环利用与耕地保育重点实验室，广州 510640； 2 华南农业大学资源环境学院，广州 510642)
1 ASS的形成条件和过程
ASS形成过程中的铁(Fe)、 硫(S)生物地球化学循环，在全球物质循环过程中具有重要地位，一直是人们关注的热点[17]。ASS的成土母质为富含还原性硫化物的沉淀物，当硫化物被氧化后形成硫酸及一系列次生铁矿物。ASS成土过程中包含 还原性硫化物(硫化铁为主)沉淀物成土母质的形成，以及成土母质的氧化两个关键阶段[1, 18]。发育完全的ASS土壤剖面上，常呈现两种性质截然不同的土层， 下层富含黄铁矿的中性土壤(还原环境)，也称潜在酸性硫酸盐土(Potential Acid Sulfate Soils, PASS)，上层富含黄铁矿氧化产物的酸性土壤(氧化环境)，也称实际酸性硫酸盐土(Actual Acid Sulfate Soils, AASS)[18-19]。
1.1 ASS成土母质的形成条件与过程
Fe2++H2S→FeS+2H+
[1]
Fe2++2HS→Fe(HS)2→FeS+H2S
[2]
FeS1.1(代表亚稳定态硫化亚铁矿物)+H2S→FeS2+H2(完全厌氧条件)
[3]
[4]
1.2 ASS成土母质的氧化
形成于还原条件的ASS成土母质，因自然条件变化或者人为干扰等影响，使其环境氧化还原电位提高，ASS成土母质被氧化而形成ASS[1, 33]。例如，北欧地区因均衡性地壳上升及农用埋管排水的综合作用使全新世形成的富含还原态Fe-S矿物的成土母质被氧化形成了ASS[34]；泰国曼谷平原则因海岸线的变迁而形成带状ASS[6]。而干旱是导致塞尔维亚、 几内亚(比绍)等非洲国家ASS发育形成的主要因素[6]。
ASS成土母质接触氧气后，还原态硫化铁类矿物[亚稳定态硫化亚铁矿物(FeS1.1)和黄铁矿(FeS2)]发生氧化反应，其反应式为5和6[32, 34]。FeS1.1的反应动力学较快，而黄铁矿慢[34]。溶解态O2和Fe3+是该反应主要的氧化剂，最初氧化产物为元素硫(S0)[32, 35-36]。FeS1.1和FeS2被氧化的过程中，形成多种形态硫、 铁矿物，并产生硫酸、 H+，最终形成酸性极强、 生长障碍因素多及生态危害大的ASS[18, 34, 37]。
10FeS1.1(代表亚稳定态硫化亚铁矿物)+24O2+26H2O→10Fe(OH)3+11H2SO4
[5]
4FeS2+15O2+14H2O→4Fe(OH)3+8H2SO4
[6]
1.3 红树林与ASS
2 ASS中硫的演变
2.1 ASS中硫矿物的动态变化
4FeS+3O2+6H2O→4Fe(OH)3+4S0
[7](PAS)
FeS+S0→FeS2
[8](PAS)
[9](PASTZ)
[10](ASTZ)
[11](AS)
[12](AS)
CaCO3+H2SO4+H2O→CaSO4·2H2O+CO2
[13](AS)
PASS的氧化还原电位低，只有热力学稳定性较差的FeS1.1能被氧化，生成氢氧化铁和元素硫S0，尚无酸形成，土壤pH仍较高，反应式为7[46]。在该pH条件下，元素硫S0又能与土壤中残余的FeS1.1反应，形成黄铁矿(反应式8)[29, 47]。接近PASS的TZ区域，氧化还原电位仍较低，只有少量黄铁矿被完全氧化(反应式9)，生成硫酸根及氢氧化铁，但该反应速度缓慢，不能使土体酸化[48-50]。而接近上层AASS的TZ区域，土壤pH下降到4.5以下，Fe3+溶解度提高，成为黄铁矿氧化反应的主要氧化剂，进一步加快黄铁矿氧化(反应式10)[32, 50]。同时，在该pH条件下，嗜酸氧化亚铁硫杆菌催化Fe2+氧化形成Fe3+，加快催化该氧化反应；但当pH下降到3.5以下时，大部分释放出的Fe3+被水结合，形成氢氧化铁沉淀[3, 51]。AASS中黄铁矿氧化产生大量硫酸，但大部分被淋溶出土体，当土壤进一步酸化(pH低于4.0)，硫酸根与Fe、 K、 Na等元素形成黄钾铁矾[KFe3(SO4)2(OH)6][52]和施氏矿物[Fe8O8(OH)6SO4][53]，尤以黄钾铁矾最普遍，其形成过程为反应式11[52]。黄钾铁矾和施氏矿物最终被水化形成针铁矿(FeOOH)，同时释放出硫酸和酸(反应式12)[53-55]。开垦耕种的ASS土壤上，常表施石灰，能中和土壤部分酸并生成石膏(反应式13)[32]。
2.2 ASS中各种硫形态含量
3 ASS中铁(Fe)的地球化学动态
3.1 ASS中铁矿物的转化
[14]
FeS1.1(s)+H2S(aq)→FeS2(s)+H2(g)
[15]
FeS1.1(s)+Sn+1(aq)2-→FeS2(s)+Sn(aq)2-
[16]
[17]
Fe8O8(OH)6SO4+2.5H2O→8FeOOH+
[18]
[19]
图1 黄铁矿氧化途径及可能产物Fig.1 Steps in pyrite oxidation and possible secondary Fe minerals that may form as weathering products
3.2 ASS中各种铁矿物含量
4 ASS的酸特性
5 ASS的生态环境效应
淹水还原环境下，由于存在硫化物、 有机物质，有利于ASS形成金属硫化物和金属含量较高的有机复合物，将游离的金属固定下来，实现海水净化[86-87]。在氧化环境下，ASS严重酸化，Al、 Cd、 Mn、 Ni、 As等有毒金属及类金属大量活化[88-93]，另一方面，P、 Ca、 Mg、 Zn、 Cu、 K、 B等生物生长必需营养元素被固定或流失[58, 94-96]，严重毒害实地植物、 动物生长。同时，酸根离子和有毒金属及类金属随水经渗漏、 侧流等途径进入周边水系，酸化污染周边水体环境[15, 97-99]，及水系周边生态系统[73, 100]。另外，淹水环境中因潮汐、 作物根系作用以及生物扰动等影响，也会发生氧化反应，使还原性硫化物发生氧化，释放出酸根离子和金属或类金属，进而污染海水环境、 危害滨海生物[101-104]。因此，ASS生态系统在不同环境条件扮演着环境金属和类金属净化者和污染者的双重角色，而从土壤学家的角度出发，ASS主要是环境污染源。
5.1 ASS中有毒金属和类金属的活性及流失风险
ASS产生的酸性离子以及活化的有毒金属和类金属随地表径流、 侧渗、 淋溶等途径进入地下水、 周边河流，污染水体质量[11, 51, 94, 97, 100, 108-109]。ASS影响周边水体生态，一方面因水体本身受污染，导致水生生物生长受害，影响水产养殖[13, 78, 97]，且以受污染水灌溉、 饮用等用途的农户，其身体健康也将受到威胁[6, 7, 110]；另一方面，污染水的流动、 利用，进一步将污染扩大到周边环境，加上金属的移动性及生物累积效应，致使ASS的污染区域不断扩大。因此，ASS作为一个潜在或现有的环境隐患，若开发不当将引发严重的生态环境问题[6]。
ASS导致的Fe、 Al、 Mn、 Cd等重金属的污染已有目共睹[13, 89, 90, 111-112]，而且有些改良措施虽然中和了土壤酸性且固定Al、 Mn、 Ni和Zn等潜在的有毒金属，但却导致As和Fe的活化[13, 113]。同时，Fe-S矿物对痕量元素的生物有效性影响显著，已有研究表明，ASS中Fe矿物的转化影响土壤中稀土元素的含量及分布[114]，同时，受ASS影响的江河沉淀物中，痕量元素的含量及活性明显较高[115-116]。
5.2 ASS对实地及周边生物的影响
ASS对植物生长的毒害不仅因低pH的直接影响，还与Al、 Fe等金属离子活化的毒害作用有关[117]，同时，P、 Ca、 Mg、 Zn、 Cu、 K、 B等营养元素有效性低、 土壤结构差等因素均限制了植物的生长[6, 118]。我国受ASS影响的水稻产量远低于全国平均水平，印度尼西亚、 泰国、 几内亚比绍、 斯里兰卡等国家ASS稻区的产量也较低[6]。更有甚者，因长期酸害影响，植被无法生存，土表裸露[119]。据调查，ASS上牧草、 橡树的Co、 Ni、 Mn含量均较常规值高[97, 120]。但Fältmarsch等发现ASS中重金属淋溶流失的比例较大，仅少量被土壤上生长的包菜吸收，因此，重金属对实地植物的影响相对较小[121]。有研究表明，受ASS影响的奶牛的牛奶中Al含量明显偏高，但动物体内金属含量与ASS的关系尚不清晰[97]。
受ASS影响的水域中水生苔藓体内Al、 Cu、 Fe含量显著高于常规值[97]。澳大利亚东部受ASS影响的流域常发生大规模鱼类死亡事件，其原因主要归结于强酸和金属离子含量高[122]。另一方面，鱼类蛋黄、 卵子形成和产卵的过程受到该恶劣环境干扰而导致繁殖失败，使部分鱼类绝种灭亡[122]，同时，离子调节系统和器官呼吸作用受到破坏使鱼类行为失常[97]。Faltmarsch 等[97]认为ASS可能还影响区域的老人痴呆症、 帕金森等神经性疾病和心血管疾病的发生率。有学者对澳大利亚ASS土地上的房地产开发后的表土、 粉尘及水样调查表明，其金属含量及活性均在生态临界值以内[7]。Hinwood 等[110]调查ASS区域居民尿液、 脚趾甲及头发中重金属累积情况，结果表明Al、 As、 Cd、 Pb、 Cu及Zn的含量并不高。ASS对人类身体健康的影响已逐渐引起学者的关注，但目前相关证据尚不确凿。
6 展望
1) ASS中Fe-S演变、 循环一直是人们关注的热点。因沉积微环境下各种条件的不同，黄铁矿形成途径、 形态不一致，不同来源、 形态的黄铁矿发育的过程也有所差异。我国前期学者对发育于红树林景观的ASS研究较多，忽视了非红树林海滨沼泽地带Fe-S矿物的形成和累积，亟需加强该方面探讨，以更好规划该问题土壤。该方面应进一步探讨ASS发育过程中不同来源、 形态的黄铁矿对土壤各种理化性状的影响。
2) ASS的形成及发育过程中，Fe-S矿物的转化显著影响着其他金属元素的固定、 活化、 溶解等一系列地球化学过程。其中黄铁矿形成过程中重金属的富集、 黄铁矿氧化过程中重金属的活化是国内外研究热点。近些年，国外有学者开始关注ASS对稀土元素、 痕量元素的影响，但国内尚无相关报道。
3) 尽管我国ASS的面积高达0.11 M hm2，但是从学术界到社会上，对其了解、 认识仍处于表面的危害性。曾有部分学者进行过探讨，但均没有深入到机理层面上，而社会上对该类土壤概念模糊，只注重其生产应用，忽视了其潜在的环境危害、 及产品安全性的考量。在我国大量的ASS被改造成水稻田，采用适当的改良、 排水措施能保证相当的产量收成，被认为是ASS利用的有效途径。然而，ASS开垦种稻后，相应的农艺措施对土壤酸、 重金属的影响如何，其对周边河流、 环境、 居民的影响如何，甚至稻谷的安全性均有待进一步评估。
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