Abstract:

Iron (Fe) and manganese (Mn) oxides are important factors affecting the speciation and bioavailability of arsenic (As) in soil. In this study, a pot experiment was conducted to investigate the effects of manganese content on the speciation of iron, manganese oxides, and arsenic in soils. Five soils with manganese contents of 580 mg∙kg^-1 (Mn1), 980 mg∙kg^-1 (Mn2), 1900 mg·kg^-1 (Mn3), 3030 mg∙kg^-1 (Mn4), and 4230 mg∙kg^-1 (Mn5) were obtained by mixing two natural soils with similar soil physical and chemical properties and different manganese contents. After planting rice (Oryza sativa L), pore water, rice, and soil samples were collected at the tillering, heading, and maturity stages of rice, respectively. The results showed that Eh, mass fraction of amorphous Fe oxides, amorphous Mn oxides, and free Mn oxides in soil were increased with the increase of Mn mass fraction in the soil at each growth stage of rice. These results indicated that a higher mass fraction Mn in soil can delay the decrease of Eh and further promote the formation of amorphous Fe oxides, amorphous Mn oxides, and free Mn oxides in soil. With the increase of soil manganese content, the proportion of non-obligate adsorbed As and obligate adsorbed As in soil decreased, and the dominant As in soil was amorphous iron oxide bound As, crystalline iron oxide bound As, and residual As. The mass fraction of crystalline iron oxide bound As and residual As in Mn1 soil decreased by 2.7% from the tillering stage to the maturity stage, and the mass fraction of crystalline iron oxide bound arsenic and residual As in Mn2-Mn5 soil increased by 6.3%-14.0%. Meanwhile, the mass concentrations of Fe (II) and As (III) in soil pore water decreased. These observations reflected that an increase of Mn content can promote the transformation of dissolved and adsorbed As to iron oxide bound or residual As, leading to a decrease of the availability of As in soil. As a result, with the increase of soil Mn contents, the As concentrations in roots, stems, leaves, and grains of rice decreased and the As enrichment coefficient in each part decreased. In summary, findings suggested a new insight on As inactivation in soil: a higher Mn concentration in soil can promote the formation of amorphous Fe oxides, amorphous Mn oxides, and free Mn oxides in soil and further increase the immobilization of As in soil.

Key words: iron-manganese oxide, arsenic speciation, soil, rice, arsenic, uptake

摘要：

土壤中铁锰氧化物是影响土壤砷形态及其生物有效性的重要因素。通过盆栽试验法,利用土壤理化性质相近而锰质量分数相差大的两种自然土壤混合得到土壤锰质量分数（mg·kg^-1）分别为580（Mn1）、980（Mn2）、1900（Mn3）、3030（Mn4）、4230（Mn5）的5个土壤,种植水稻（Oryza sativa L.）后,分别在水稻分蘖期、抽穗期和成熟期采集土壤孔隙水、水稻和土壤样品进行分析,探讨锰质量分数对土壤中铁、锰氧化物形态和砷形态转变及其对水稻吸收砷的影响和作用机制。结果表明,随着土壤锰质量分数增加,土壤Eh呈升高趋势;土壤中无定形铁氧化物、锰氧化物和游离态锰氧化物质量分数增加,成熟期Mn1和Mn5处理土壤无定形态铁氧化物、锰氧化物分别比分蘖期增加11.9%、40.5%和3.6%、35.7%;随着土壤锰质量分数增加,土壤中非专性吸附砷和专性吸附砷比例呈下降趋势,土壤砷更大比例以无定形铁氧化物结合态、晶质铁氧化物结合态和残渣态存在,从水稻分蘖期到成熟期处理Mn1土壤晶质铁氧化物结合态砷和残渣态砷的质量分数共减少了2.7%,Mn2至Mn5处理晶质铁氧化物结合砷和残渣砷的质量分数共增加了6.3%—14.0%,土壤孔隙水Fe(Ⅱ)、As(Ⅲ) 质量浓度降低;随着土壤锰质量分数增加,水稻根、茎、叶和籽粒砷质量分数减少,各部位的砷富集系数减小,水稻各部位砷质量分数与土壤非专性吸附态砷质量分数呈现极显著正相关,与无定形铁氧化物结合态砷质量分数显著负相关。综上所述,土壤中高质量分数锰可以延缓Eh下降,促进土壤中无定形态铁氧化物、锰氧化物和游离态锰氧化物的形成及对砷的氧化和吸附固定,降低土壤砷的有效性,从而减少水稻对砷的吸收。

关键词: 铁锰氧化物, 砷形态, 土壤, 水稻, 砷, 吸收