Background

Cationic polyacrylamide copolymers (PAMs) are used for sludge dewatering in municipal waste water treatment and might enter the environment by spreading of the sludge on agricultural land. Concern has been expressed since little is known about the degradation of PAMs in soils. To obtain detailed information on the polymer’s fate in the soil compartment, the degradation of ¹⁴C-radiolabelled PAM in an outdoor lysimeter was studied.

Results

No plant uptake and leaching of radioactivity was observed. There was practically no vertical movement of polymer and no transformation products found at the end of the study. For the top 10 cm soil layer, a mass balance was established throughout the study. About 10% of applied radioactivity was not extractable from soil even with a matrix destructive method, and this was concluded to be bound residue. Characterization of extractable radioactivity by means of GPC-analysis showed a significant decrease of the molecular weight of the PAM with time. The decrease in molecular weight indicates a breakdown of the polymer backbone (the C–C-chain), and is assumed to be primary degradation. The total radioactivity content in the 10 cm top soil layer was quantified every 6 months over a period of 3 years. The results show a significant decrease of the total radioactivity over time and this is defined as ultimate degradation following the definition of OECD and EPA. Based on the data, a half-life time of 2.0 × 10³ days and a rate constant of 0.00035/day were calculated. With a χ² of 12.0 the results of the calculation are thus valid and reliable. The rate constant indicates a mineralization of 22.5% within a period of 2 years based on the total recovered radioactivity. This half-life time is solely based on mineralization and does not take into account the degradation of the polymer backbone, hydrolysis of the side chains, incorporation into the soil matrix, and thus is a conservative approach.

Conclusions

¹⁴C-PAM degrades very slowly in soil after land-spreading as a component of sewage sludge. Even in a very conservative evaluation which only considered the loss of radioactivity, a half-life time of 5.4 years was determined.

中文翻译：

---

阳离子聚丙烯酰胺共聚物 (PAM)：污泥处理土壤中的环境半衰期测定。

背景

阳离子聚丙烯酰胺共聚物 (PAMs) 用于城市污水处理中的污泥脱水，并可能通过污泥在农田上的扩散进入环境。由于人们对土壤中 PAM 的降解知之甚少，因此人们对此表示担忧。为了获得有关聚合物在土壤隔间中的命运的详细信息，研究了¹⁴ C-放射性标记的 PAM 在室外蒸渗仪中的降解。

结果

没有观察到植物吸收和浸出放射性。在研究结束时，几乎没有聚合物的垂直运动，也没有发现转化产物。对于顶部 10 厘米的土壤层，在整个研究过程中建立了质量平衡。即使使用基质破坏方法，也无法从土壤中提取约 10% 的应用放射性物质，这被认为是结合残留物。通过 GPC 分析对可提取放射性的表征表明 PAM 的分子量随时间显着降低。分子量的降低表明聚合物主链（C-C 链）的断裂，并被认为是初级降解。在 3 年的时间里，每 6 个月对 10 cm 表层土壤中的总放射性含量进行量化。结果显示总放射性随着时间的推移显着下降，这被定义为按照 OECD 和 EPA 定义的最终降解。根据数据，半衰期为 2.0 × 10^计算了3 天和 0.00035/天的速率常数。因此，当χ ²为 12.0 时，计算结果是有效且可靠的。速率常数表明基于总回收放射性在 2 年内矿化率为 22.5%。该半衰期仅基于矿化，不考虑聚合物主链的降解、侧链的水解、掺入土壤基质，因此是一种保守的方法。

结论

¹⁴ C-PAM 作为污水污泥的成分在土地蔓延后在土壤中降解非常缓慢。即使在仅考虑放射性损失的非常保守的评估中，也确定了 5.4 年的半衰期。