Important Ions

• Discussion Approach:Ions of Major Interest in Soil Chemistry, Grouped Accord

ing to Major Behavior Modes.

Major Exchangeable Cations: Ca2+, Mg2+, K+, Na+, NH4

+, Al3+( H+ ).

Major Soluble Anions: NO3-,SO4

2-, Cl-, HCO3-, CO3

2-.

Weakly Soluble Anions: H2PO4-, HPO4

2-, H2AsO4-, H2B

O3-, MoO4

2-.

Transition Metals and Aluminum: Fe(OH)2+, Fe2+, Mn2+,

Cu2+, Zn2+, Al3+, AlOH2+.

Toxic Ions: Cd2+, Al3+, Pb2+, Hg2+, AsO43-, CrO4

2-.

Active in oxidation-Reduction Reactions:

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+.

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.

Major Soluble Anions

NO3-, SO4

2-, Cl-, HCO3-, CO3

2-

---- 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.

Weakly Soluble Anions (≦10-5 M in most soil solutions)

H2PO4-, HPO4

2-, 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.

Phosphate

---- The H2PO4- and HPO4

2- 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.

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.

Silicon

---- Soluble silicon seems exist in solution as

Si(OH)4 (also written H4SiO4, although it is a

weak acid, pK1= 9.1).

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.

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.

Active in Oxidation-Reduction ReactionsC (organic to HCO3

-)O (O2- to O2)N (-NH2 to NO3

-)S (-SH to SO4

2-)Fe (Fe2+ to FeOOH)Mn (Mn2+ to MnO4)Se (organic to SeO4

2-)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.

植物吸收

土壤溶液

離子交換與吸附

氧化還原

沉澱與溶解

水解或錯合

土壤中之移動

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.

0.0E+00

1.0E+04

2.0E+04

3.0E+04

4.0E+04

5.0E+04

6.0E+04

7.0E+04

8.0E+04

0 2 4 6 8 10 12

Retention Time (min)

Inte

nsity

(cp

s)HPLC/ICP/MS Chromatogram of a mixture of

As(III), DMA, MMA, and As(V) (50μg L-1)

As(III)

DMA

As(V)

MMA

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

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。

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 (NH2OHHCl)‧ 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。

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。

序列抽出法 ( 續 )