给排水专业英语翻译

Particle-Particle

Interactions Particle-particle interactions are extremely important in bringing about aggregation by means of Brownian motion .the theory that has been developed to describe particle-particle interactions is based on the consideration of interaction between two charged flat plates and between two charged spheres.As neither of these developments is directly applicable to the particles found in wastewater ,as described above,the analysis for two charged flat plates will be used for illustrative purposes.The two principal face involved are the forces of repulsion,due to the electrical properties of the charged piates,and the van der Waals forces of attraction.It should be noted that the van der Waals forces of attraction do not come into play until the two plates are brought together in close proximity to each other. The net total energy shown by the solid lines on Figure 3 is the difference between the forces of repulsion and attraction.The two conditions,with respect to the forces of repulsion ,are illustrated on

Figure 3.as shown for conditions 1, the forces of

attraction will predominate at short and long

distances.The net energy curve for condition 1contains

a repulsive maximum that must be overcome if the

particles,represented as the two plates,are to be held together by the van der Waals force of attraction.In

condition 2,there is no energy barrier to overcome .Clearly ,if colloidal particles are to be

removed by microflocculation,the repulsive force must be reduced.although floc particles can form at along distances as shown by the energy curve for condition 1, the net force holding these particles together is weak and the floc particles that are formed can be

ruptured easily. Particle Destabilization with Potential-Determining Ions and

Electrolytes To bring about particle aggregation through

microflocculation,steps must be taken to reduce

particle charge or to overcome the effect of this

charge.the effect of the charge can be overcome by

(1)the addition of potential-determining ions,

which will be taken up by or will react with the co lloid surface to lessen the surfaceto lessen charge and (2)the addition f electrolytes,which have the effect of reducing thethickness of the diffuse elec tric layer and, thereby,reduce the zeta potential

Use of Potential-Determining Ions.The addition of Potential-Determining ions to promote

coagulaion can be illustrated by the

addition of strong acids or bases to reduethe cha rge of metal oxides or hydroxides to near zero s o that coagulation can occur.The effect of adding potential-determining ions in a solution contain ing charged particles is illustrated on figu re 4 .

The magnitude of the effect will depend on the conc entration of potential-determining ionsadded .The f ollowing ratios,known as the Shultz-Hardy rule, can be used to assess theeffectiveness of potentia l-determining or counterions:1:1/2^6:1/3^6 or 100;1.6:0.13 (3) It is interesting to note that depending on the concent ration and nature of the counterions added,it is p

ossible to reverse the charge of the double layer a nd develop a new stable particle. The effect of adding counterions to a solution containing charged particles is illustrated on Figure

5.The upper curve on Figure 5 represents the surface charge of the particle as a function of the concentration of counterions added .The lines designated kT represent the thermal kinetic energy of the particle .The lower diagram is a plot of the turbidity that would result if the particles that have been destabilized and have undergone microflocculation were removed by settling .As shown ,when the surface charge (either positive or negative)is greater than the thermal kinetic energy of the particles ,the particles will not flocculate and original turbidity is observed.

Additional detail on the use of counterions may be found in Shaw(1966).The use of potential determining ions is not feasible in either water or wastewater treatment because of the massive concentration of ions that must be added to bring about sufficient compression of the electrical double layer to effect perikinetic flocculatin.

Use of Electrolytes . Electrolytes can also be added to coagulate colloidal suspensions .Increased concentration of a given electrolyte will cause a decrease in zeta potential and a corresponding decrease in repulsive forces as illustrated in condition 2 on Figure 3 and on Figure 4.The concentration of an electrolyte that is needed to destabilize a colloidal suspension is known as the critical coagulation concentration(ccc).Increasing the concentration of an indifferent electrolyte will not result in the restabitization of the colloidal particles .As with the addition of potential-determining ions ,the use of electrolytes is also not feasible in wastewater treatment .As discussed subsequently ,a change in the particle charge will occur when chemicals are added to adjust the pH of the wastewater to optimize the performance of hydrolyzed metal ions used as coagulants.

Particle Destabilization and Aggregation with Polyelectrolytes

Polyelectrolytes may be divided into two categories :natural and synthetic .Important natural

polyelectrolytes include polymers of biological origin and those derived for starch products such as cellulose derivatives and alginates .Synthetic polyelectrolytes consist of simple monomers that are polymerized into high-molecular-weight substances .Depending on whether their charge ,when placed in water ,is negative .positive ,or neutral ,these polyelectrolytes are classified as anionic ,cationic , and nonionic ,respectively .The action of polyelectrolytes may be divided into the following three general categories.

Charge Neutralization .In the first category ,polyelectrolytes act as coagulants that neutralize or lower the charge of the wastewater particles .Because wastewater particle s normally are charged negatively ,cationic polyelectrolytes are used for this purpose . In this application ,the cationic polyelectrolytes are considered to be primary coagulants .To effect charge neutralization ,the polyelectrolyte must be adsorbed to the particle .Because of the large number of particles found in wastewater ,the mixing intensity must be

sufficient to bring about the adsorption of the polymer onto the colloidal particles .With inadequate mixing ,the polymer will eventually fold back on itself and its effectiveness in reducing the surface charge will be diminished .Further , if the number of colloidal particles is limited ,it will be difficult to remove them with low polyelectrolyte dosages .

Polymer Bridge Formation . The second mode of action of polyelectrolytes is interparticle bridging (see Figure 6) .In this case ,polymers that are anionic (usually anionic to a slight extent when placed in water) become attached at a number of adsorption sites to the surface to the surface of the particles found in the wastewater .A bridge is formed when two or more particles become adsorbed along the length of the polymer .