陰離子及分子吸持
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陰離子及分子吸持. 緒論. 熱帶及強風化土壤中含有 Fe(Ⅲ) 及 Al(Ⅲ) 氫氧氧化物 (hydroxyoxides) ,尤其在低 pH 值下,其所帶之負電荷相對地少於所帶之正電荷。因此當在酸性條件下,這些 “ 可變電荷 (variable-charge) ” 之土壤吸持陰離子的量比陽離子多。. 緒論 ( 續 ).

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陰離子及分子吸持

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陰離子及分子吸持

緒論

  • 熱帶及強風化土壤中含有 Fe(Ⅲ) 及 Al(Ⅲ) 氫氧氧化物 (hydroxyoxides) ,尤其在低 pH 值下,其所帶之負電荷相對地少於所帶之正電荷。因此當在酸性條件下,這些“可變電荷 (variable-charge)” 之土壤吸持陰離子的量比陽離子多。


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緒論(續)

  • 土壤對不帶電荷分子的吸持與膠體表面所帶電荷無關,有機分子被土壤吸持之程度大多決定於其相對的揮發性 (volatility)、分子量、化學組成、物理構造及在土壤溶液之溶解度、土壤有機物含量和某些程度之土壤表面積等因子。假如有機分子含有 R-CO, R-COOH, R-CHO, R-PO4及R-NH2 等官能基,則土壤吸持量會增加;土壤對於無機分子之吸持;例如:未解離弱酸及氣態酸性分子。e.g. HF, SO2, NOx 等,隨著這些分子之水溶解度以及土壤 pH 之增加而增加;對於弱鹼分子,e.g. NH3,則較易為酸性土壤所吸持。


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陰離子吸持

  • 土壤體系中應探討之陰離子包括:Cl-, HCO3-, NO3-, SO42-, HPO42-, H2PO4-, OH-, F-, H2BO3-, MoO42-, CrO42-, HAsO42-,

    Dissociated phenoxyacetic acids (2, 4, 5-T, 2, 4,-D) etc.


Nonspecific anion reactions

非特異性陰離子反應(nonspecific anion reactions)

  • 包括陰離子排斥 (Anion Repulsion) 或稱負吸附(Negative adsorption) 、陰離子之靜電吸引(Electrostatic attraction of anions) 。

  • 陰離子排斥

    • 當一稀的 KCl 中性溶液加入乾的蒙特石中,在平衡之總體溶液中 Cl- 之濃度會大於原先加入之 KCl 溶液之 Cl- 濃度。同樣之現象會發生於當一鹽溶液加入一乾的膠體 (此膠體在一般 pH 下,沒有吸附陰離子之能力),這個過程稱為陰離子排斥或負吸附。


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非特異性陰離子反應(續)

  • 造成此種現象之原因,顯然的是由於不相等之離子分佈於帶電荷膠體表面之擴散電雙層(DDL)中。

  • 影響陰離子排斥的因子有:

    (1) 陰離子電荷數及濃度 (2) 交換性陽離子之種類

    (3) pH 值 (4) 其他陰離子的存在

    (5) 膠體表面之電荷及其特性


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影響陰離子排斥之因子

  • 陰離子電荷數與濃度:

    隨著陰離子之電荷 (價數) 增加,每單位面積固體排斥陰離子之莫耳數隨之增加。

    e.g. Na-Montmorillonite 之 Anion Repulsion

    Cl- ≈ NO3- < SO42- < Fe(CN)64-

    增加陰離子濃度也會增加陰離子之排斥量,但是擴散電雙層 (DDL) 中陰離子被排除 (排除體積,exclusion volume) 之體積是減少的。

  • 交換性陽離子之種類:

    Cl-在層狀矽酸鹽懸浮液中之排除體積 Ba2+ < Ca2+ < K+ < Na+


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PO43-

影響陰離子排斥之因子(續)

  • pH值

    土壤中若含有 pH-dependent charge,則降低 pH 會減少膠體表面之 net negative charge  Anion repulsion↓

  • 其他陰離子之存在

    磷酸根離子被土壤膠體吸附,而遮蔽膠體之正電荷,並且這些吸附之陰離子對排斥後來加入之陰離子。


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影響陰離子排斥之因子(續)

  • 土壤膠體之電荷及其特性

    土壤固體帶有較大之負電荷會有較大之陰離子排斥。e.g. Montmorillonite > Kaolinite

  • 當溶質在土壤中傳輸,陰離子排斥有重要之影響。當陰離子被排於圍繞土粒之某些體積外,這些陰離子在土壤中之傳輸會快於原先溶有鹽之水。


Electrostatic attraction

OH-1/2

OH-1/2

+ H+ +Cl-

H+1/2

O

Cl-

H

OH-1/2

陰離子之靜電吸引(Electrostatic Attraction)

  • 在層狀矽酸鹽及水化氧化物礦物表面正電荷吸引陰離子之行為,與陽離子被土壤膠體表面負電荷吸引之行為相類似。

  • 陰離子被土壤表面吸持,可單獨以靜電吸引來描述,稱此反應為“非特異性吸附(nonspecifically adsorbed)” 。


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OH-1/2

OH-1/2

+ NO3-

+ Cl-

H+1/2

H+1/2

O

O

Cl-

NO3-

H

H

陰離子非特異吸附

  • 非特異吸附之陰離子是可被其他陰離子交換取代的,就如同陽離子交換反應一樣。

  • Cl-, NO3- 和 SO42- 一般均被認為是非特異性吸附之陰離子。

  • 降低土壤 pH,會增加土壤之陰離子吸附容量。特別是含高嶺石之土壤。

  • Cl-, NO3- 和 SO42- 在土壤系中均是屬於很重要的陰離子並且被廣泛的研究。

  • Cl- 常被用來作為土壤中 NO3- 移動之指標,因為 Cl- 並沒有像 NO3- 有複雜的生物反應特性,而除此之外,Cl- 之行為與 NO3- 類似。


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陰離子之特異性吸附

  • 配位基交換可以在帶有淨的 (net) 負、正或中性電荷的表面發生,此不同於非特異性陰離子吸附(其只能發生於帶有淨的正電荷的表面)。

  • 配位基交換可以解釋為何弱酸陰離子之最大吸附量在 pH = pKa 時發生。

    原因:當 pH = pKa 可用來做為配位基交換之解離酸 (dissociated acid) 和可用來做為質子捐贈者(proton donor) 而中和配位基交換產生之 OH-,其量均為最大值。


Specific anion reaction

陰離子之特異性反應(Specific Anion Reaction)

  • 鐵、鋁氧化物含量多之土壤其陰離子吸附容量(對於某些陰離子)會大於由“電中性” (或單獨靜電吸引)來預估之吸附容量,原因:特異性吸附反應發生。

  • 鐵及其他氧化物對於 arsenate, phosphate, molybdate 和其他相似之陰離子有很強之吸附力。

    原因:配位基交換 (Ligand exchange) or 陰離子穿透 (Anion penetration) 。


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  • 配位基交換或陰離子穿透:

    在水化氧化物表面之氧離子有時可以被陰離子取代,譬如:含氧酸根(磷酸根或過氯酸根離子)及氟離子,這些陰離子可以進入 Al3+ 及 Fe3+ 離子六個配位位置。此即為配位基交換或陰離子穿透。

    e.g.


Reactions of phosphates in soils

在土壤中磷酸根之反應(Reactions of phosphates in soils)

  • 磷酸根離子大概是特異性吸附陰離子最重要的例子,許多土壤固定大量可溶性的磷而使其轉變成植物較無法利用之型態的磷。

  • 磷酸根離子之Ligand exchange


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  • 磷被層狀矽酸鹽吸附可分為二個階段:

  • 起初之快速階段(大概在一天內即可完成反應)及後續之較慢之反應階段(可連續幾週或更長的時間)

  • 快速階段:非特異性吸附或礦物邊緣之配位基交換

  • 較慢之反應階段:可能是礦物溶解和加入的磷和交換性陽離子或在晶格中之陽離子產生沉澱等複雜反應之綜合結果。

  • For example:P 被 Kaolinite 吸持之反應,隨著時間和 P 之濃度增加而吸持量也隨之增加,同時矽在總體溶液中之濃度也隨之增加。


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Kaolinite

Variscite (磷酸鋁)

Variscite

Kaolinite

在土壤中磷酸根之反應(續)

或全反應:

  • 此反應表面上看來為磷酸四面體取代矽之四面體,但一般更容易被接受的是由於鋁離子之溶解再沉澱生成磷酸鋁。


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  • 在低 pH 時,P 與 Fe3+, Al3+ 形成不易溶解化合物;在 pH 接近中性時,P 與 Ca2+ 及 Mg2+ 形成較易溶解之化合物,而在更高的 pH 時,P 形成不易溶解之磷酸鈣化合物。

  • Lindsay&Moreno 發展出磷酸根在含有 Variscite (Al(OH)2H2PO4) 、Strengite (Fe(OH)2‧H2PO4) 、Fluoroapatite (Ca10(PO4)6F2) 、Hydoxyapatite (Ca10(PO4)6(OH)2) 、Octacalcium phosphate (Ca4H(PO4)3) 、Dicalcium phosphate dihydrate (CaHPO4‧2H2O) 之系統中的溶解度圖,此溶解度圖描述在各種不同的 pH 值,平衡磷沉澱反應。


P compounds solubility diagram

P-compounds之溶解度圖 (solubility diagram)

  • Example:

  • a (Dicalcium phosphate dihydrate) b (Fluoroapatite)

  • c (Variscite)  d (strengite)


P solubility diagram

P solubility diagram (續)

  • Soil pH 和土壤溶液中 P 之濃度在 solubility diagram 上,若數據點位於任一化合物之等溫線(isotherm) 上,即代表此溶液對於此 P-化合物是過飽和的,也就是這個 P-compound 之沉澱可能生成;若數據點位於一 P-化合物等溫線之下方,則代表此土壤溶液之 P 對於此一 P-compound 是未飽和的,也就是此 P-compound(如果存在的話)會溶解。兩條等溫線之交叉點,代表此土壤溶液之 P 是和此兩種 P-固體達成平衡。


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  • Lindsay & Moreno's P-solubility diagram 解釋了下列現象:

    (1)在酸性土壤中,因 Fe3+ 及 Al3+ 之活性高,P 被固定成大量的磷酸鐵和磷酸鋁。

    (2)在鹼性土壤中,因 Ca2+ 之活性高,P 被固定成不同之磷酸鈣化合物。

    (3)在微酸至中性的土壤,P 之有效性最大,因在pH 值之下,磷酸鋁及磷酸鈣之溶解度同時為最大。

  • 由 p-solubility diagram 來探討 P 在土壤中之變化,其只是由“平衡” 之觀點,並未考慮轉變之動力學 (Kinetics) ,因而此 solubility diagram 大致可定性解釋 P 在土壤中之反應。


Breakthrough curve

陰離子在土壤中之突破曲線(Breakthrough curve)


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H

Cl

C

Cl

DDT

H

Cl

2,4,5-T

H

O

(2,4,5-trichlorophenoxy acetic acid)

Cl

O

CH2

C

OH

H

Cl

O

Cl

O

CH2

C

OH

Cl

分子吸持

緒論

NH3, undissociated weak acids (H3BO3, H4SiO4), undissociated pesticides (nonionic pesticides, e.g. DDT, 2,4,5-T, 2,4-D) 。

2,4-D (2,4-Dichlorophenoxy acetic acid)


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H

H

H

C

C

C

H

H

Br

Cl

Br

Sources and classification of organic chemicals released into soils

Sources

  • Agricultural practices

    (a) Fumigant

    e.g. DBCP(1,2-Dibromo-chloropropane)

    Carcinogen. Suspended from use in 1979 in USA.


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H3C

CH3

CH3

C

O

C

H3C

=

C=C-C-C-C-O-CH

C-CH2-CH=CH-CH=CH2

H3C

H2C

C=O

(b) Insecticides

  • Natural organic insecticides

    ---- They are derived for the most part from plant extracts.

    ---- Characterized as containing complex chemical groups subject to rapid chemical and microbial attack.

    ---- With the advent of the cheaper synthetic pesticides, the natural organics declined in popularity.

    e.g. Pyrethrin


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S

H5C2O

HC

O

NO2

P

Cl

Cl

H5C2O

CCl3

  • Synthetic organic insecticides

    • Organochlorine insecticides

      ---- inexpensive, readily available, did not require repeated applications as frequently as the natural organics.

      ---- persistent, most of them have been banned.

      e.g. DDT

    • Organophosphorous insecticides

      ---- less persistent

      ---- high mammalian toxicity

      e.g. Parathion


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Cl

N

N

N

H7C3-N

NC2H5

H

H

(c) Herbicides

e.g. Atrazine


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Cl

C

CH3

Cl

Cl

(ii) Organic from wastes

---- Industrial wastes

---- Municipal wastes

Released into soils from landfill, buried waste disposal site and land application.

e.g. TCE (1,2,3-trichloroethane)


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(iii) Leaking of underground fuel storage tanks

e.g. BTX&E (benzene, toluene, xylene, and ethylbenzene)


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Cl

EPA pollution priority list

e.g.

PCBs (polychlorinated biphenyls)

209 congeners


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2+

2-

H3C

H3C

N

N

Br

Classification by chemical properties

i) Ionic organic chemicals

(a) cationic organic chemicals

e.g. Paraquat (Bipyridium quaternary salts)


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(b) Anionic Organic chemicals

e.g. Surfactant alkyl benzene sulfonate

(ABS)


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O

O

[RCOO-]

pH - log

pKa =

[RCOO-] [H+]

Cl

Cl

O-CH2-C-OH

O-CH2-C-OH

[RCOOH]

Ka =

[RCOOH]

Cl

Cl

-

+

+

H

(c) Acidic organic chemicals

e.g. 2,4-D (2,4-dichlorophenoxy) acetic acid


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[RCOO-]

If pH=2.80, pKa=pH, 0= - log

[RCOOH]

[RCOO-]

log

[RCOO-] = [RCOOH]

=1,

[RCOOH]

[RCOO-]

pH increases,

increases

[RCOOH]

2,4-D pKa=2.80

  • [RCOO-] and [RCOOH] percent ratio depends on pH. Therefore, need to consider the behavior of both [RCOOH] and [RCOO-] in soils in order to evaluate the rate of acidic organics.


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Cl

Cl

N

N

H+

N

N

+

H+

N

N

H7C3HN

NHC2H5

H7C3HN-

NHC2H5

BH+

B

[B] [H+]

Ka =

[BH+]

(d) Basic organic chemicals

e.g. Atrazine

For organic bases, the Ka refers to the dissociation of the conjugated acid BH+.


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[B]

pKa = pH - log

[BH+]

[B]

increases.

For basic organics, pH increases,

[BH+]

Atrazine, pKa = 1.68

If pH = 1.68, [B] = [BH+]


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[RCOO-] × 100

[RCOOH] +[RCOO-]

[RCOO-]

[RCOO-]

[RCOO-]

[RCOOH]

[RCOOH]

[RCOOH]

X

=10 1.20

= 101.20 

1-X

% ionized = 94%

pH=6.00

% ionized ≌100%

  • For 2,4-D (acidic)

    pKa=2.80

    pH=2.80 % ionized = = 50%

pH=4.00 pH-pKa=log

4.00-2.80 = 1.20 = log

X  0.94


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[BH+]

% ionized =

×100 = 82.7 %

pH=1.00

[B]

% ionized =

% ionized =

50 %

4.8 %

pH=3.00

pH=1.68

[B]

[B]

[B]

pH-pKa =

log

pH=4.00

[BH+]

[BH+]

[BH+]

1-X

=102.32

 X=4.76×10-3

X

4.00-1.68=2.32=

=102.32

log

% ionized =

0.48 %

  • For atrazine (basic)

    pKa = 1.68


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%

ionized


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Classification by chemical properties (continued)

(ii) Nonionic organic chemicals

classification by hydrophobicity (water solubility)

(a) Hydrophobic compounds e.g. DDT

(b) Hydrophilic compounds.


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  • Each chemical has its own characteristic properties which affect its fate in the soil environment.

    Important properties

    e.g. Water solubility

    Vapor pressure (vapor density)

    Henry’s law coefficient (Kh)

    Octanol-water partition coefficient (Kow) Organic carbon partition coefficient (Koc)

    Degradation rate (half life, t1/2)

    Vapor diffusion coefficient in air (Dg)

    Liquid diffusion coefficient in water (Dl)


Sorption

Sorption

  • Definition: any process by which a chemical species is lost from an aqueous solution phase to a contiguous solid phase.

  • Precipitation : the growth of solid phase comprising a primitive molecular unit (complex) which repeats itself in 3 dimensions.

  • Adsorption : the accumulation of matter at the interface between an aqueous solution phase and a solid adsorbent without the development of 3-dimensional molecule arrangement.


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  • Absorption : the diffusion of an aqueous chemical species into a solid phase.

  • Partition – the sorbed organic chemical permeates into the network of an organic medium by forces common to solution.


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Supernatant soln.

Soil slurry

Method of determining the extent of sorption

Generally, using batch equilibrium method

q= x/m = [(Ci-Ce)] V/m

Where

q: amount of chemical sorbed by unit mass of sorbent (surface excess)

Ci: initial concentration of chemical in the solution before applied to soils

Ce: equilibrium chemical concentration in the solution after equilibrium sorption

V: volume of solution

m: mass of sorbent


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  • Assumption:

    1. No other processes except sorption occurring result in the lost of the chemical species from an aqueous solution. For example, no degradation and volatilization occurring.

    2. Equilibrium between solid phase and solution phase.


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  • Sorption isotherm

  • A graph of surface excess against equilibrium concentration in the solution phase at fixed temperature and applied pressure.

  • The sorption isotherm can be classified into four categories based on the initial slope of the curve: S-curve, L-curve, H-curve, and C-curve.


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K Ce b

q =

1+KCe

Isotherm equation

  • Langmuir equation

    Where K: coefficient related to the binding strength

    b: maximum amount of sorbate that can be sorbed

    Ce/q = (1/Kb) + (Ce/b)

    Plot Ce/q vs. Ce, obtaining a straight line which has a slope of 1/b and the intercept of 1/Kb.


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  • Freundlich equation

    q = Kf CeN

    Where Kf: Freundlich sorption coefficient

    N: nolinearity between q and Ce

  • Constant partition equation

    q =K Ce

    Where K :partition coefficient


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(C-1)P

P

1

+

=

V (P0-P)

VmC

VmCP0

  • BET (Brunauer, Emmett, Teller) Eq.

    C: a constant

    P: partial vapor pressure of the adsorbate.

    P0: saturated vapor pressure of the adsorbate.

    V: vol. of adsorbed gas at a given relative pressure,P/P0

    Vm: vol. of adsorbed gas required to cover the surface with the layer.


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1

Intercept:

 Vm

VmC

C-1

Vm

Slope:

A (Surface area of the adsorbent) =

(N)(Am)

VmC

V0

Vo: vol. of one mole of adsorbate

N: Avogadro constant

A: Cross sectional area of adsorbate


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  • S-curve: initial slope that increases with the concentration of a substance in the soil solution --- suggest that the relative affinity of the soil phase for the substance at low concentration is less than the affinity of the soil solution.

  • L-curve (Langmuir) : initial slope that does not increase with the concentration of a substance in the soil solution ---- suggest a high affinity of the soil solid phases for the substance at low concentrations coupled with a decreasing amount of adsorbing surface as the surface excess of the adsorbate increases.


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  • H-curve (high affinity): extreme version of the L-curve isotherm. Its characteristic large initial slope (in comparison with the L-curve isotherm) suggests a very high relative affinity of the soil solid phases for an adsorbing substance.

  • C-curve (constant partition): initial slope that remains independent of the concentration of a substance in the soil solution until the maximum possible adsorption. This kind of isotherm can be produced either by a constant partitioning of a substance between interfacial region and an external solution or by a proportional increase in the amount of adsorbing surface as the surface excess of an adsorbate increases.


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General comments about the sorption isotherm equation

1.One cannot infer an sorption process by fitting data to a sorption equation.

2.The adherence of experimental sorption data to an sorption isotherm equation provides no evidence as to the mechanistic aspects of a sorption process in soil.

3.Although the analysis of equilibrium sorption isotherm data can provide relevant information to suggest the mechanism of sorption, a molecular explanation is ultimately required in order to describe fully the mechanism of sorption.


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  • Partition (or linear sorption)

    Koc (organic carbon distribution coefficient)

    For nonionic organic chemicals

    Koc = K/foc = (K/%OC)100

    where foc: organic carbon fraction of soil or sediment

    %OC: % of organic carbon of soil or sediment

    K: partition coefficient or linear sorption coefficient

  • Numerous studies have shown that values of Koc obtained in this manner (for a specific chemical ) are relatively constant and reasonably independent of the soil or sediment used.


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  • Estimation of Koc

    ----Handbook of Chemical Property Estimation Methods, McGraw-Hill Book Company, New York.

    1. From empirical relationships between Koc and water solubility (S).

    e.g. log Koc = -0.55 log S + 3.64 r2=0.71

    No. of chemicals studied = 106, S: mg/L


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Conc. in octanol phase

Conc. in aqueous phase

2. From empirical relationships between Koc and Kow

e.g. log Koc = 0.544 log Kow +1.377 r2=0.74

No. of chemicals studied = 45

where Kow (octanol-water partition coefficient)

=

Octanol C8H17OH

Kow also can be estimated from fragment constants or from other solvent/water partition coefficients.


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