The determination of the coordination of lead(II)in the yellow river water

Guo Boshu, Jiao Xiaobao, Zhang Yi
( Chemistry﹠Environment Science College of Inner Mongolia Normal University, Huhhot 010022, China )

3. RESULTS AND DISCUSSION
3.1 The complexation capacity (C.C) of Pb
Lead is a special ion its complexation capacity is 1.6-60 n mol·L^-1, which is lower than Cu, Zn, Cd, (Cu is 130-560 n mol·L^-1, Cd is 47-99 n mol·L^-1, Zn is 48-250 n mol·L^-1). But its conditional stability constants (lgK_(PbL)8.1-9.4) are higher than those of Cu, Zn, Cd, (lgK_(CuL)7.7-8.0, lgK_(ZnL)7.5-8.3, lgK_(CdL)7.8-8.9). It shows that some ligands are contended little in the Yellow River, but it is able to combine with Pb solidly. It is agreement with determination result of Pb complexation capacity of the Yellow River mouth. In comparison with the north of the south China sea, their lgK are similar, but its C.C is a litter larger, because the DOC is relatively small in the north of the south China sea (the DOC of the surface water is 1.79 mg·L^-1)
    The Yellow River water has high pH (8.3-8.8), high carbonate system ([HCO₃^-]=1.1×10^-3mol/L, [CO₃^2-]=2.1×10^-3mol/L), HCO₃^- CO₃^2- have important complex function to heavy metal and can form stable inorganic complex. So to speak, Pb exists with PbCO₃ & Pb(CO₃)₃^2- and after correction Pb’s carbonate complex stable constants are among 10⁵ to 10⁶ . Most carbonate complex of heavy metal are lower toxic. Carbonate's contribution to C.C is about a half or so. The C.C.I and lgK_(PbL) of rives with high content in the world have never been reported before.
    Complexation capacity index (C.C.I.) has a close relation with C.C and K. Through studying I, we know the complexation ratio of heavy metal with ligand and can judge the correction of C.C and K. Only when C.C and K are large, C.C determination is correct. When C.C are the same, K is large and I is large, too. When K is the same, I becomes large with C.C growing large. Although Pb’s C.C average is small (average 29 n mol·L^-1), lgK average 8.9, with certain I (75.32%). As soon as I in natural water is higher than 97%, K is about 10⁸, C.C must be at 10^-5-10^-4 mol·L^-1, and a conclusion can be drawn that this kind of natural water may be polluted by various organic level.
    Buckley and van den Berg ^[1] have determined and calculated organic copper with MnO₂ in all depths and found that the complexation function id between 89﹪and 99.8﹪. In addition, the data is between 98.8﹪and 99.4﹪got by using DPCSV method. However, I disagree with their findings in natural water, but in fact, the complexation function of organic copper under the MnO₂, method should be higher. Second, according to their determination the complexation function is too high. In that case, the concentration of organic should be more than 10^-5 mol·L^-1in the Atlantic Ocean. However, I think the concentration cannot be that high.
3.2 E (%)-pH of S- shaped curves

Fig.1 The spectra of powder X-ray with crystal diffraction


Table 1 Analyzing results of powder X-ray with crystal diffraction

    Because the Yellow River flows by two deserts and sediment has 35% SiO₂ (see fig 1and Table 1) which is 1:1 shape mineral and the basis structure is formed by-Si-O-Si-bond SiO₄. Because-Si-O-Si-bond is water repellent and layer charge is zero, and surface interaction is weak .The exchange process of surface sediment is mainly formed by clay minerals which are primarily crystalline aluminum or magnesium silicates with stacked-layer structure. Each unit layer is in turn a dimensional array. In illite, about 1/4 Si of is replaced by Al, which is TOT three layer. Chlorite is three-layer clay, too. Clay layer distance is a special chemistry reaction place, which is characterized by layer exchange, layer adsorbtion. In Yellow River water more than 80% heavy metal interacts with surface sediments. This process is S-shape.

Fig.2 The relationship between exchange ratio-pH of the sediment and Pb in the Yellow River water
1, 2, 3, (Transformed sediment)                       
1' 2' 3'(no transformed sediment)
[Pb²⁺](mmol.·L^-1): 1, 1'---2.4; 2, 2'---4.8; 3, 3'---9.6

Fig.3 Effect of cystine on the curve of  "E(%)-pH"(Transformed sediment)
Cystine (mmol.·L^-1): 1---0; 2---0.48; 3---2×0.48; 4---3×0.48; 5---4×0.48; 6---5×0.48; 7---10×0.48;
[Pb²⁺]＝4.8mmol.·L^-1

    The non-transition sediment is the natural sediment in the Yellow River and its interface function is good for the environment. As for the transition, they are mixed exchange agent with its organic and metals on the surface removed, and they also sediments transformed form the Ca-type into Na-type. In addition, its interface function is significant in theory.
    In Fig.2, either transformed sediment or non-transformed sediment has a maximam adsorption exchange value to Pb at pH=6.5, that is critical value in Pb of the Yellow River water. Since transformed sediment has cleared up organic carbonate and metal ion on surface, therefore it has many adsorptions active station and E (%) is greater than that of non-transformed sediment. [Pb²⁺] has an effect on ion exchange ratio, too. So to speak on transformed sediment, [Pb²⁺]=4.8mmol.·L^-1, E(%)=71.5%; [Pb²⁺]=9.6mmol .L^-1, E (%)=63.0% non-transformed sediment is in the same case.
3.3 "peak-shaped"E(%)- pH curve
From Fig.3, we can see that after cystine is added, E(%) of Pb is growing at first, but when the pH reach above 6, E (%) of reduces, depression, cystine restrains E(%). The concentration of cystine become greater, become stronger. It explained that cystine does not adsorb on the surface sediment, but to stay in the water of Yellow River.
    Under pH of the Yellow River water, we determined that the lg b_(oA) value is 8.37 of cystine+Pb, but the lgK_(oA) of ternary surface complexes (surface sediment+cystine+Pb ) is 3.14^[3] (Yang Hongwei et al,1998 Pb) For lgK_(oA)<<lg b_(oA), the stable chelate formed in cystine with Pb of the Yellow River water, the result is Pb from surface sediment to water, leads to E(%) degraded. Therefore, lgK_(oA) and lg b_(oA) determine shape of E(%)-pH curve, are important factors.
    In Fig3, [Pb²⁺] has an effect, when pH is less than 6, the concentration of cystine has no effect to curve; pH is more than 6, E(%)-pH curve changes from s shape to peak shape. We can explain it like that, the reaction took place between cystine and Pb.


    As pH increases, the reaction moves to right, the concentration of chelate became larger, so did lg b_(oA), there, when it is at high pH, Pb²⁺ goes into the solution from above the interface and E(﹪) reduces b S-shape curve changes to"peak-shaped"curve. When [Pb²⁺] is constant (4.8mmol.·L^-1), the concentration of cystine increases from 0 to 5×4.8mmol.·L^-1, (curve 1, curve 2 …… curve 6), the E(%)-pH curve goes upward, but when pH is above 6, the E(%)-pH curve goes downwards. When the concentration of cystine increases to 4.8mmol.·L^-1, (see curve 7) and when pH is about 4, the E (%)-pH curve goes downwards. So that means that when the concentration of cystine increases ten times and when the pH is about 4, ternary surface complexes decompose in advance and chelates come into existence in the Yellow River water.
3.4 Ternary surface complex
From Equation 1,we can see that the H in hydroxy on the solid surface is replaced in chelation. That show the following type of complex (ligandlike ternary complex) is impassible to come into existence.

    And when the pH is below 6, the metallike ternary complex comes into existence as follows.

    Because the molecular weight of cystine is large, R is also large, and the stability of Pb four-membered ring is relatively small, the lgK_(oA) is only 3.14 and ternary surface complexes are easy to get a way from the surface of solids and get into the solution as chelate sometimes, the S-S-bond in cystine can even break, forming cysteine ternary surface complex. The reference presents a new usuful method to study ternary surface complexes at liquid-solid interface, that is the method of E(%)-pH curve. The peak-shape curve experiment using this method can also determine the type of ternary surface complex.

REFERENCES
[1] Buckley.P.J.M. and van den Berg.C.M.G.1986, Copper Complexation profiles in Atlantic Ocean. Marine Chemistry, 19: 281-296.
[2] Sun Jinzhu,1994，The Early Warning and Dredge Countermeasure of Inner Mongolia Life and Relation to the Environment. Inner Mongolia peoples press, Huhhot, P26-30.
[3] Yang Hongwei, Jiao Xiaobao, Guo Boshu et al,1998, The curve of Ion Exchange Ratio(%)-pH of the Interaction between Suspended Particles with Cd (Ⅱ) in the Yellow River. Journal of Environmental Sciences.10 (2):252-256.
[4] Yang YuYing, Wu Di, Hong Xia, Guo BoShu, 1999. Determination of Complexation Capacity of trace Metal-Organic in natural Water, Journal of Environmental Sciences, 11(1):124-128.
[5] Zhang Zhengbin, Liu Liansheng, Zhao Hongbin, Fu Youjun and Wu Zhijian, 1996. Studies of Ternary Surface Complexes at Liquid-Solid Interfaces in Seawater by the E(%)-pH curve Method. Journal of Colloid and Interface Science. 182:158-165.

黄河水中铅(Ⅱ)配位作用的测定
郭博书 焦小宝 张毅
（内蒙古师范大学化学与环境科学学院，内蒙古呼和浩特010022）
摘要 用阳极溶出伏安法测定了黄河水体中铅(II)的络合容量，计算了条件稳定常数和络合容量指数，数据表明：C.C_(PbL)较小(16-60nmol·L^-1)但lgK_(PbL) 却较大(8.1-9.4)。报道了E(﹪)-pH S形曲线和峰形曲线，研究了液-固界面三元表层络合物。也研究了半胱氨酸对Pb(Ⅱ)和表层沉积物之间相互作用的E(﹪)-pH曲线上离子交换率的影响。结果表明三元表层络合物为“类金属”络合物。
关键词 络合容量；液-固界面作用；E(﹪)-pH曲线；三元表层络合物

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