环境水质学国家重点实验室知识库

    在金属冶炼、化工制酸等工业生产过程中产生大量酸性含砷废水。其特点是酸性强、砷浓度高、产生量大，此类废水处理困难。目前处理此类废水使用最广的方法为中和沉淀法，但此方法产生大量的含砷废渣，且酸液被中和后不能回收利用，造成资源浪费。五硫化二磷（P2S5）遇水水解为 H3PO4和  H2S，H2S则与溶液中的 As(Ⅲ)/As(Ⅴ)反应生成 As2S3沉淀，使砷得以去除，大大减少沉渣的产
生，且含有硫酸的磷酸可以回收利用于磷肥的生产，有望实现酸液的零排放。本研究选取 P2S5为除砷药剂，主要研究 P2S5用于酸性废水中砷去除所需的药剂量、除砷时间、生成絮体的粒径变化以及除砷机理；进一步研究了紫外光照对P2S5的水解作用、砷的去除速率以及反应生成絮体的聚集作用的影响，并对 P2S5应用于实际酸性废水中砷的去除做了进一步研究，取得的研究成果如下：
（1）研究了不同 P2S5投加量、不同酸浓度、不同初始砷浓度对 P2S5净化酸性含砷废水的影响。研究结果表明，由于 As(Ⅲ)/As(Ⅴ)与 P2S5反应生成的 As2S3包裹于 P2S5表面，并且在 As(Ⅴ)去除过程中产生的单质硫也会包裹于 P2S5表面，阻碍 P2S5的水解，因此需按过量系数为 0.66投加 P2S5才可使砷的去除率达 99%以上。此外，在0.05mol•L-1 ~7.5mol•L-1硫酸浓度范围内、10mg•L-1  ~1000mg•L-1初始砷浓度范围内，P2S5均适用于酸性含砷废水中砷的去除。    

（2）研究了反应时间对 P2S5除砷效果的影响及生成絮体颗粒粒径的变化。结果表明，P2S5去除 As(Ⅲ)用时较短，但 As(Ⅴ)的去除需先转化为 As(Ⅲ)，而中间产物 H3AsO3S的形成及转化速率较慢，因此导致  As(Ⅴ)的去除速率极慢。此外，在 As(Ⅲ)-P2S5、As(Ⅴ)-P2S5体系中絮体颗粒的粒径增长缓慢，反应进行 30min后絮体颗粒的平均粒径分别为 88μm、41μm，不利于絮体的聚集与分离。
（3）研究了紫外光照对 P2S5水解作用的影响。研究发现，紫外光照对 P2S5的水解具有明显的促进作用。紫外光照条件下，酸性无砷溶液中，P2S5的水解率由 64.5%提升到 81.0%，且水解完成时间由 60min缩短到 30min。因此，紫外光照提高了溶液中 H2S的产量及产生速率，使得更少的  P2S5投加量及更短的反应时间可达到同样的除砷效果。
（4）研究了紫外光照对除砷速率的影响。研究结果表明，紫外光照对 As(Ⅲ)及 As(Ⅴ)的去除速率均有提高，对 As(Ⅴ)更为明显。对于 As(Ⅲ)，紫外光照提高了 P2S5的水解速率，提高溶液中  H2S的产生速率，缩短  As(Ⅲ)的去除时间。对于 As(Ⅴ)，在紫外光照条件下，光诱导产生的 HS•自由基和•H自由基促进 As(Ⅴ)去除过程中 H3AsO3S的形成和转化，且加速 S(-II)和 As(V)之间的电荷转移过程，大大缩短 As(Ⅴ)的去除时间。
（5）研究紫外光照对 P2S5-As(Ⅲ)、P2S5-As(Ⅴ)体系中絮体颗粒聚集作用的的影响。研究发现，紫外光照条件下，生成絮体颗粒的平均粒径得到明显提升。通过对 P2S5-As(Ⅲ)、P2S5-As(Ⅴ)两体系反应完成后上清液的浊度变化测试发现，紫外光照条件下，絮体的聚集作用明显提高，更有利于后续的固液分离处理。

A large  amount  of  strongly acid  arsenic  wastewater  is produced  by  mineral processing industry and sulfuric acid production industry. It is characterized by strong acidity, high arsenic concentration and large  output, and the treatment of this kind of wastewater is very difficult. At present, the most widely used method to treat this kind of wastewater  is  neutralization-precipitation. However,  this method  produces  large amount of arsenic-containing waste residue, and the acid solution can not be recycled after neutralization,  resulting in the  waste of  resources. Phosphorus sulphide  (P2S5) hydrolyzes to H3PO4 and H2S,  and H2S can react with As(Ⅲ)  /As(V) in the solution and  produces  As2S3  precipitation,  so  as  to  remove  the  arsenic  and  reduce   the production of  the sludge, at  the same time,the phosphoric  acid-containing sulphuric acid can be recovered and used in the
production of phosphate fertilizer. In this study,P2S5 was employed as arsenic removal agent. Effecting  factors including P2S5  dosage and arsenic removal  time were studied. Particle size  of the formed flocs  and arsenic removal mechanism  were  investigated. Furtherly,  UV irradiation  was employed  to enhance were carried on the removal of  arsenic from actual acid wastewater by P2S5.The following research results have been obtained:
(1)  The effects of  different P2S5  dosage, acid  concentration and initial  arsenic concentration on the purification of  acid arsenic-containing wastewater by P2S5  were studied. The results show that the As2S3 produced by As(Ⅲ) /As(V) with P2S5  wraps the surface of P2S5,  and the elemental sulfur produced during the  removal of As(Ⅴ)can also wrap the surface of P2S5, which hinder the hydrolysis. The removal efficiency of above 99%  for As(Ⅲ)  /As(V) can be  achieved, when a high  P2S5  stoichiometric coefficient of 0.66 is employed. In addition, within the
range of 0.05mol.L-1 ~ 7.5mol.L-1  acid concentration  and 10mg.L-1~1000mg.L-1 initial arsenic concentration, P2S5 can be applied to remove of arsenic from acidic wastewater.

(2)  The effect  of  reaction  time  on the  removal  of  arsenic  by P2S5  and  the characteristics of particle  size of the  floc were studied.  It is found  that the removal time of P2S5  for As(Ⅲ) is relatively  short, but
the removal  of As(V) is very slowly, due to  the slow transform  of As(Ⅴ)  to  As(Ⅲ),  in which,  the slow  formation and transformation   of  intermediate   product   H3AsO3S   is  the   controlling   step.   In As(Ⅲ)-P2S5 and As(V)-P2S5  systems,  the particle  size  of  the floc  increased  very slowly. The average particle size of floc was 88μm and 41μm respectively after 30min reaction, which was not conducive to the aggregation and separation of flocs.
(3) The  effect of  ultraviolet light  on the  hydrolysis of  P2S5 was  studied. It  is found that UV  irradiation has a promoting  effect on the hydrolysis  of P2S5, because ultraviolet light can penetrate the sulfur and As2S3 on
the surface  of P2S5, making the P2S5  more hydrolyzed.  The hydrolysis  rate  of P2S5  was  increased from  64.5%  to 81.0%, and  the time of  hydrolysis was shortened  from 60min  to 30min. Therefore, ultraviolet light improves the yield and production rate of H2S, making less dosage of P2S5 and shorter reaction time can achieve the same effect of arsenic removal.
(4) The effect of ultraviolet light on the rate of arsenic removal was studied. The results showed that  UV irradiation increased the  removal rate of As(Ⅲ)  and As(V),especially for the As(V). For As(Ⅲ),  UV irradiation
increased  the hydrolysis rate of P2S5 and increased the production rate of H2S in the solution. Shortening the removal time of As (Ⅲ). For As(V), light induced HS?free radical and H? free radical, and then the formation and transformation  of H3AsO3S in the process  of As(V) removal were promoted by  UV irradiation. The  charge transfer process  between S(-II)  and As(V) was accelerated, greatly shortening the removal time of As (V).
(5)  The  effect  of  UV  irradiation  on  floc  aggregation  in  P2S5-As(Ⅲ)  and P2S5-As(V) systems  was studied. It was  found that the  average diameter of the  floc particle size was obviously improved under UV  
irradiation.Through the turbidimetric test of the supernatant after the reaction of P2S5-As(Ⅲ) and P2S5-As(V) systems, it was found that  the aggregation of  the floc is  obviously improved  under UV irradiation, which may facilitate the subsequent solid-liquid separation.