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Important Ions

Important Ions. Discussion Approach: Ions of Major Interest in Soil Chemistry, Grouped According to Major Behavior Modes. Major Exchangeable Cations: Ca 2+ , Mg 2+ , K + , Na + , NH 4 + , Al 3+ ( H + ). Major Soluble Anions: NO 3 - ,SO 4 2- , Cl - , HCO 3 - , CO 3 2- .

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Important Ions

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  1. Important Ions

  2. Discussion Approach: Ions of Major Interest in Soil Chemistry, Grouped According to Major Behavior Modes. • Major Exchangeable Cations: Ca2+, Mg2+, K+, Na+, NH4+, Al3+( H+ ). • Major Soluble Anions: NO3-,SO42-, Cl-, HCO3-, CO32-. • Weakly Soluble Anions: H2PO4-, HPO42-, H2AsO4-, H2BO3-, MoO42-. • Transition Metals and Aluminum: Fe(OH)2+, Fe2+, Mn2+, Cu2+, Zn2+, Al3+, AlOH2+. • Toxic Ions: Cd2+, Al3+, Pb2+, Hg2+, AsO43-, CrO42-. • Active in oxidation-Reduction Reactions:

  3. Major Exchangeable Cations Ca2+, Mg2+, K+, Na+, NH4+, Al3+ (H+) ---- Occur predominantly as exchangeable cations in soils ---- These ions are relatively easily manipulated by liming, irrigation, or acidification. ---- Exchangeable Al3+ is characteristic of acid soils. ---- The exchangeable cations in productive agricultural soils are almost always present in the order Ca2+>Mg2+>K+ = Na+.

  4. Major Exchangeable Cations (續) Ca2+, Mg2+, K+, Na+, NH4+, Al3+ (H+) ---- The NH4+ ion occasionally appears in low concentration as a result of ammonium fertilization. ---- Na+ can predominate in drainage water from irrigated arid-region soils. It is a soil chemical concern when it occurs in excess. When ESP is > 5-15 %, water movement into and through many soils is inhibited.

  5. Major Soluble Anions NO3-, SO42-, Cl-, HCO3-, CO32- ---- Present in considerably lower concentrations than the major cations in all but the most coarse-textured and strongly saline soils, where they essentially equal. ---- Sulfate and NO3- are important nutrient sources for plants. ---- Cl- and HCO3- salts accumulated in saline soils. ---- Carbonate ions are present in appreciable amounts only in soils of pH > 9. ---- The major soluble anions are retained weakly by most soils.

  6. Weakly Soluble Anions (≦10-5 M in most soil solutions) H2PO4-, HPO42-, H2AsO4-, AsO2, H3BO3, H2BO3-, Si(OH)4, MoO42- ---- Strongly retained by soils. ---- Borates are the most soluble of the group. ---- Retention or fixation by soils is pH-dependent; molybdate and silica are most soluble at high pH; phosphate is more soluble at neutral or slight acid pH.

  7. Phosphate ---- The H2PO4- and HPO42- ions are predominant in acid and basic soil solutions, respectively. ---- Soil solution concentrations of phosphate are of the order 10-6 to 10-7 M (0.01 to 0.1 mg/L). ---- In acid soils, most solid phase phosphate is associated with iron and aluminum. In basic soils, phosphate is associated with calcium.

  8. Boron ---- Boron exists in solution as boric acid (H3BO4, pK= 9.2) ---- Boron concentrations greater than a few mg/L in the bulk soil solution can be toxic to sensitive plants, and concentrations less than several tenths of a mg/L may indicate deficiency. The range between deficiency and toxicity in soils is narrower for boron than for any other essential element.

  9. Silicon ---- Soluble silicon seems exist in solution as Si(OH)4 (also written H4SiO4, although it is a weak acid, pK1= 9.1).

  10. Molybdenum ---- Molybdenum is present in soil solution as the molybdate anion (HMoO4-, pK2= 5) ---- Molybdate reacts strongly with iron hydroxyoxides. ---- Its solubility and plant availability increase with increasing with pH.

  11. Transition Metals and Aluminum(≦10-5M in the soil solutions) (I). Al3+, AlOH2+, Al(OH)2+, Fe(OH)2+, Fe2+, Mn2+ ---- Insoluble hydroxides tending to accumulate in soils as silica and other ions weather; iron and manganese are more soluble in waterlogged or reduced soils. (II). Cu2+, Zn2+ ---- More soluble than the above cations in all but very acidic soils. ---- Availability increases with increasing soil acidity . ---- Complexed strongly by soil organic matter.

  12. Active in Oxidation-Reduction Reactions C (organic to HCO3-) O (O2- to O2) N (-NH2 to NO3-) S (-SH to SO42-) Fe (Fe2+ to FeOOH) Mn (Mn2+ to MnO4) Se (organic to SeO42-) Hg (organic to Hg0 or Hg2+) ---- Soil biochemistry revolves around the oxidation state changes of soil carbon, nitrogen, and sulfur compounds; molecular oxygen is the main electron acceptor; Fe(Ⅲ), Mn(Ⅲ-Ⅳ), nitrate, and sulfate are electron acceptors when the oxygen supply is low.

  13. 植物吸收 離子交換 與 吸附 沉澱與溶解 土壤溶液 水解或錯合 氧化還原 土壤中之移動

  14. Chemical Speciation • Use chemical speciation model, such as GEOCHEM, to predict the distribution of chemical species in soils. • Use various methods to determine the distribution of chemical species in soil solutions or soil solids.

  15. HPLC/ICP/MS Chromatogram of a mixture of As(III), DMA, MMA, and As(V) (50μg L-1) As(III) DMA MMA As(V)

  16. XANES spectra to differentiate Cr ( Cr(III) or Cr(VI)) species in soils

  17. Sequential extraction (序列抽出法) for differentiate the forms of element present in soil solids 1.可交換相(Exchangeable) 以 16 毫升的 1M 硝酸鎂，pH 為 7 的溶液加入裝有上述樣品的離心管中，在室溫下、160 rpm的條件下振盪 1 小時。在 3000 rpm 下離心 10分鐘，上清液以 Whatman 40 濾紙過濾後收集，紀錄離心瓶重量。 2.碳酸鹽結合相(Carbonate bounded) 加 16 毫升以醋酸調整到 pH5 的 1M 醋酸鈉溶液於上步驟殘餘固相之離心管中，在室溫，160 rpm 下振盪下 5 小時。固液相分離及處理方式同前步驟。

  18. 序列抽出法 (續) 3.鐵錳氧化物結合相(Iron and manganese oxides bounded) 加 30 毫升含有 0.04 M 胺鹽酸鹽 (NH2OH‧HCl)之 25 % (v/v) 醋酸溶液於含有上一步驟殘餘固相之離心管中，在 96±3 ℃下間歇振盪，萃取 5 小時。固液相的分離及其處理方式同前。

  19. 序列抽出法 (續) 4.有機物結合相(Organic matter bounded) 加 6 毫升 0.02 M 硝酸及 4 毫升 30% 雙氧水 (pH 2 with HNO3) 於含有上述步驟殘餘固相之離心管，將此混合物置於 85±2 ℃的水浴中 45 分鐘，再添加 4 毫升 30% 雙氧水 (pH 2 with HNO3) 後置於水浴中 45 分鐘，此步驟重覆三次，整個步驟歷時 5 小時。冷卻後，加入 8 毫升 3.2 M 醋酸銨的 20 % (v/v) 硝酸溶液，振盪 30 分鐘，固液相分離及其處理方式同前。 5.殘餘相(Residual) 以微波消化法所得之土壤總量減去以上四步驟抽出的量即定義為殘餘量。

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