How to remove heavy metals from sludge?

The removal methods of heavy metals in sludge include chemical method, biological leaching method, vegetation restoration method, solidification method and stabilization method. Chemical method is effective in removing heavy metals from sludge, but acidification of sludge and neutralization of acid in leachate require a lot of chemical reagents, which are not only expensive, but also troublesome to operate. At present, phytoremediation is limited to small-scale planting. If it is applied on a large scale, it will not only take too long, but also be uneconomical. Moreover, vegetation restoration method is selective to heavy metals, so it is difficult to completely remove all pollutants, and it is also a problem to treat plants rich in heavy metals. Solidification technology is generally applied to hazardous waste, which requires a lot of immobilized materials. For a large amount of sludge, firstly, the consumption of immobilized materials is too large, and secondly, the final disposal of immobilized waste is also a problem. Stabilization technology is a new research and development field. In recent years, high-efficiency chemical stabilizer technology has been proposed internationally, but this stabilization technology also has the defects of consuming a lot of stabilizers, increasing sludge volume and reducing sludge fertilizer efficiency. Bioleaching has attracted more and more attention because of its advantages of less acid consumption, low operating cost, high removal efficiency of heavy metals and strong practicability.

Mechanism of bioleaching

Bioleaching technology uses Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans to dissolve insoluble heavy metals in sludge from solid phase to water phase, and then removes them through solid-liquid separation. It is generally believed that Thiobacillus ferrooxidans has two mechanisms to dissolve heavy metals in sludge.

1. Direct mechanism: The bacteria directly adsorb the metal sulfide (MS) on the sludge surface through the extracellular polymer (EPS) secreted by themselves, and directly oxidize the metal sulfide through the unique oxidase system in the cell to generate soluble sulfate.

Second, indirect mechanism: mainly using the metabolite of Thiobacillus ferrooxidans-ferric sulfate and metal sulfide for redox reaction, ferric sulfate is reduced to ferrous sulfate and generates elemental sulfur, and the metal is dissolved in the form of sulfate, while ferrous is oxidized to trivalent iron by bacteria, and elemental sulfur is oxidized to sulfuric acid by bacteria, forming a redox circulation system. Through bioleaching, the pH value of sludge was reduced to about 2.0, which greatly promoted the dissolution of heavy metals in sludge. The reaction equation is as follows:

2fe2++1/2o2+2h +→ 2fe3+H2O (ferrous oxidizing bacteria such as thiobacillus ferrooxidans and thiobacillus ferrooxidans).

MS+2Fe3+→M2++2Fe2++S (chemical oxidation)

2S+3O2+2H2O → 2H2SO4 (sulfur oxidizing bacteria, such as Thiobacillus ferrooxidans and Thiobacillus thiooxidans)

Steps of bioleaching method

In order to obtain higher heavy metal removal effect and shorten residence time, the following five steps are often adopted in practical application or continuous operation process design:

Firstly, the sludge (solid content is 2% ~ 10%) is pre-acidified to pH4.0 (it has been reported recently that it can be successfully achieved without pre-acidification);

Secondly, adding energy substances, such as FeSO 47 H2O (5 g/L ~ 20 g/L sludge) and elemental sulfur (5 g/L ~ 10 g/L sludge);

Thirdly, adding inoculum, stirring and refluxing sludge (reflux rate is 10% ~ 20%), and culturing for several days to weeks under aerobic conditions;

Fourthly, filtering and dehydrating the detoxified sludge; Fifth, the dehydrated sludge is neutralized with lime and then used in agriculture.